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A TREATISE ON
PHO T O G R A PHY
(LAPTKINZ IHBVVIVIHJESI JE) IYBBHEY
R.E. , F .R.S.
FIF TH EDI TION
RE VI SED A ND ENL ARGE D
LONDON
L O N GM A N S, G R E EN, A ND CO.
AND NEW YORK : 15 EAST 16th STREET
1 888
A ll r igh ts r e s e r ved
PRINTED BY
SPOTTISWOODE AND CO.,NEW-STREET SQU ARE
LONDON
LIBRARY
THE 1 v
FRAN KL IN
INSTITU TE
PREF A C E.
THE numerous advances in the art ofPho tographyhave made it incumbent on the autho r to make a
tho ro ugh rev is ion of the prev io us editions of th is
wo rk . This has not only necessitated the addit ion of
new matter, but also the excision ofsome wh ich it
was fe lt might be om itted without detriment to the
completeness ofthe subject. The addit ions have,
however, considerably o verbalanced the exc isions,w ith
the resu lt that the pages have been cons i de rablyincrease d in number.
The aim ofthe autho r has been to gi ve a rat ional
explanation Ofmo st ofthe d ifferent phenomena to be
met with in Pho tography,and at the same t ime to
give suffic ient practical directions to enable the student
to produ ce a pi cture wh ich shou ld be techn i cal lygo od,
and also to show how Pho tography may be made an
aid to research .
The theo ries wh ich we re put fo rward in the o lde r
ed itions have so far stood the test oftime , and the
author has no t had to mo dify o r change them in the
/Af7o ?f
Preface.
present one . It is a matter ofregret to him that
amongs tpho tographers,bo th amateurandprofessional ,there are but few who take what may be called a
sc ientific interest in the art they practise , and it wou ld
be a sou rce ofgratificat ion to him ifth is wo rk shou ld
be the means Ofaddingeven a few earnestwo rkers in
the wide fie ld which is st il l Open fo r investigat ion .
Some ofwhat may be cal led the comme rc ial
appl ications ofPhotography have no t been dealt with
in th is work, as it was fo und imposs ible to intro duce
them into the avai lable space . For these the reader
is referred to‘Instruction in Pho tography,
’
by the
same author, o r to some of the various manuals
which are to be met with.
C O NT ENT S.
CHAPTER I .
H ISTORICAL SKETCH OF THE D ISCOVERY AND PROGRESS
OF PHOTOGRAPHY .
Scheele andRitter’s ExperimentsWedgwood and
'
Niépce
Dagu'
erre’s ' DiscoveryTalbo t ’s Photogenic DrawingPo sitive andNegative PicturesThe Calotype ProcessThe Co llodion ProcessPro cesses with Gelatine, &c .
CHAPTER II.
EXPERIMENTS WITH LIGHT.
r
The Prismatic SpectrumLight, Heat, and A ctinic RaysLimits ofthe Spec trumWork caused bj the Absorption ofRays
CHAPTER III.
THEORY OF SENSIT IVE COMPOUNDS.
Arrangement ofMo leculesChemical Effects no t an absolute measure ofWork expended
v i i i s lerzls.
Comparison ofPho tographic Compounds with ExplosivesOscillations ofa Mo leculeThe co -existence ofLonger Waves with Shorter WavesThe Visible and Invisible Photographic ImageMethods ofDevelopmentFixingthe Photographic Image
CHAPTER IV .
THE ACTION OF LIGHT ON VARIOUS COMPOUNDS.
Action ofLight on Silver ChlorideAction ofLight on Silver IodideAbsorbents ofthe Liberated AtomsAction ofLight on Silver BromideAction ofLight on Organic Compo unds ofSilverAction ofLight on Ferric and U ranic CompoundsAction ofLight on Chromium CompoundsAction ofLight on Asphaltum,
Dyes, and a Mixture ofChlorineandHydrogen
Light causingaCombination between Sulphur andAntimoniurettedHydrogen
CHAPTER V.
ON THE SUPPORT AND THE SUBSTRATUM .
Essentials ofa SupportThe Vehicle Ho ldingthe Sensitive Compounds on the Support
CHAPTER VI.
THE DAGUERREOTYPE.
Preparation ofthe Sensitive SurfaceDevelopment ofthe ImageStrengtheningthe Image by Go ldReproductions ofDaguerreo types
s lem‘s
CHAPTER V II .
COLLODION.
Action ofSulphuric Acid on Organic MatterAction ofAnhydrous Nitric Acid on CottonPreparation ofPyro xylineThe So lvents ofPyro xylineFormula for Plain Co llodionOrder ofSensitiveness ofSensitive Silver Compounds prepared
from die rent Metallic CompoundsFormulae for Iodiz ed CollodionTestingPlain Collodion
CHAPTER VII I.
PREPARATIONS NECESSARY FOR ' WORKING THE
PROCESS.
Cleaningthe PlateThe Sensitiz ing BathDistillingWaterFormulae fo r tha Sensitiz ingBathKeepingthe Bath in Order
CHAPTER IX.
ON THE DEVELOPMENT OF THE PHOTOGRAPH IC IMAGE.
Theoretical Considerations ofDevelopmentOn Different Strengths ofDevelopersOn Viscid developingSolutionsFormulae for Developers
CHAPTER X .
GIVING INTENSITY To AND FIXING THE IMAGE.
Different Methods ofgivingIntensityFormulae for Intensifiers
x Cements.
Action ofthe So lvents in Fixingthe ImageFormulae for FixingSo lutionsVarnishingthe Co llodion Film
CHAPTER XI.
MANIPULATIONS IN WET-PLATE PHOTOGRAPHY.
Cleaningthe PlateCoatingthe Plate with Co llodionSensitiz ingDevelopmentIntensifyingthe ImageFixingthe ImageVarnishingthe Negative
CHAPTER XII .
DEFECTS IN NEGATIVES.
Defects due to the Chemical ProcessesDefects due to the LensIrradiation
CHAPTER XIII.
POSITIVE PICTURES BY THE WET PROCESS.
Formulae for Sensitiz ingBathFormulae for Developer and Co llodion
CHAPTER XIV.
DRY PLATE OR ALKALINE DEVELOPMENT.
DevelopmentExperiments with Alkaline DevelopersComparison ofthe Methods ofDevelopment
CHAPTER XV.
DRY PLATE PROCESSES WITH THE BATH .
Contents.
Sensitiz ingApplyingthe PreservativeD ryingthe PlateDetails ofthe Gum Gallic Pro cessFormulae for DevelopersA lbumen-beer Process
CHAPTER XVI;
COLLOD ION-EMULSION PROCESSES.
Experiments with Silver Bromide EmulsionCanon Beechey
’s U nwashed-Emulsion Process
CHAPTER XVII .
WASHED -COLLODION EMULSIONS.
Washed-EmulsionsPreparation ofEmulsion
CHAPTER XVIII.
THE GELATINO-BROMIDE PROCESS.
Preparation ofPlatesEmulsification ofthe Silver HaloidsAddingthe GelatineWashingthe EmulsionCoatingand Dryingthe Plates
CHAPTER XIX.
EXPOSURE AND DEVELOPMENT OF GELATINO-BROM IDEPLATES.
Exposure ofDry PlatesDevelopmentIntensification
Defects
xi
Contents.
CHAPTERxxPAPER NEGATIVES.
Preparation ofIodiz ed PaperSensitiz ingDevelopingWaxingthe PaperLe Gray
’s ProcessGelatine Bromide Paper
CHAPTER XXI.
SILVER PRINTING.
Experiments with Sensitiz ed PaperTheory ofToningFixingthe Print
CHAPTER XXII.
MANIPULATIONS IN SILVER PRINTING.
Preliminary Preparation ofthe PaperThe Sensitiz ingBathPrintingthe P ictureToningthe PrintFixingthe PrintWashingthe PrintTestingfor Sodium HyposulphiteDefects In Prints
CHAPTER XXIII.
COLLODIO AND GELATINO -CITRo -CHLOR IDE PROCESSES.
Formula for CollodiO~Chloride
Paper fo r Collodio—ChlorideFormula for Gelatino-Chloride
CHAPTER XXIV.
PRINTING WITH IRON AND URANIUM COMPOUNDS.
PrintingPro cesses with Salts ofIronChryso type
CHAPTER XXX.
PHOTO -COLLOTYPE PROCESS.
General Principles ofthe Photo -co llotype Pro cessesAlbert’s ProcessProcess by Capt. Waterhouse
CHAPTER XXX I.
ELEMENTARY PHOTOGRAPH IC OPTICS.
Laws ofRefractionDispersion and its CorrectionSpherical AberrationU se ofthe Diaphragm or StopDistortion ofthe ImageThe Optical CentrePortrait LensesLandscape LensesFurther U ses ofthe StopDefinition ofthe Focus ofan Object .
CHAPTER XXXII.
APPARATUS.
CamerasHare ’s ChangingBox~Warnerke
’s Ro ller slide
The Camera StandDrop ShutterRouch’
s Dark TentThe Dark Room
'
CHAPTER XXXIII .
ON THE PICTURE.
Principles to be followed in Choosinga Point ofViewExamplesillustratingLandscape PhotographyExamples illustratingGroupsEpitome ofSimple Rules to be fo llowed in Landscape Com
position
Contents. xv
Placingthe CameraArchitectural PicturesExposure
CHAPTER XXXIV.
PHOTO-SPECTROSCOPY.
Herschell’s ExperimentsArrangement for a Pho to -Spectroscopic ApparatusCameraHeliostatLockyer
’s Pho to -Spectroscopic Apparatus .
Method ofExaminingMetallic VapoursMiller’s Researches in Absorption Spectra
CHAPTER XXXV.
EFFECT OF THE SPECTRUM ON THE HALO ID SALTS
OF SILVER.
Influence ofthe Spectrum on Silver IodideInfluence ofthe Spectrum on Silver BromideInfluence ofthe Spectrum on Silver ChlorideThermal end ofthe Spectrum Mixtures ofSilver Haloids
CHAPTER XXXVI .
ON THE APPARENT DESTRUCTION O F THE ACTION OFL IGHT ON ‘
THE PHOTOGRAPH IC IMAGE.
Destructionofthe Daguerrean ImageReversal ofImages
CHAPTER XXXVII .
ORTHOCHROMATIC PHOTOGRAPHY.
Efi'
ect ofDifl'
erent DyesAmmoniacal So lutions ofDyes .
Luminosity ofthe SpectrumLumino sity ofthe Spectrum through Y ellow GlassTheory ofthe Action ofDyes
CHAPTER XXXVIII .
LIGHT FOR THE DARK ROOM .
Absorption Spectra ofDyes and GlassesArtificial Lights
CHAPTER XXXIX.
ACTINOMETRY.
Draper’s Experiments
Bunsen and Roscoe ’s Silver-
Chlo ride ActinometerRoscoe ’s ActinometerReadingthe Tints ofthe ActinometerDiscussion ofthe Truth ofLight and Shade in PhotographsSpinge
’s Sensitometer
Density ofDeposit in Negatives
CHAPTER XL.
CELESTIAL PHOTOGRAPHY.
So lar Photography with a FixedTelescopeThe Pho to -HeliographSolar Eclipse Pho tographsLunar Pho tographyStellar Photography .
CHAPTER XLI.
PHOTOGRAPHY WITH THE MICROSCOPE.
Apparatus requiredMonochromatic Light
APPEND IX.
Table ofElementsComparison ofthe Metrical with the Common Measures
INDEX
A TREATISE
PHO T O G RA PHY .
CHAPTER I.
H ISTOR ICAL SKETCH OF THE D ISCOVERY AND PROGRESSOF PHOTOGRAPHY.
To the alchemis ts of the six teen th century belongs thehonour ofhaving first no ticed the change that took placein Si lver chlo ride (known to them as Luna co rnua ’
) by ex
posure to light , but they regarded the darkening as a specie s
oftransmutation ofme tals, and i t remained for Scheele , the
Swedish chemi st, in 1777, to investigate the propertie s of
this compound, though his researche s led at the time to‘
no practical end. Scheele found, when he expo sed silverchlo ride to the action ofl ight beneath water, that in the
fluid was disso lved a substance which, on the application
ofsi lver n itrate , gave once mo re silver chlo ride, and that,after applying ammonia to the blackened body, an inso luble
residue ofme tall ic Si lver remained behind. These were the
only facts e l ici ted at the time , and a delay ofmo re than half
a cen tury o ccurred before they were put to really go od purpo se. In I 8O I R i tter, ofJena, repeated the experimen ts
ofScheele, and disco vered that the chlo ride darkened rapidly
in tho se rays ofthe spectrum which lie beyond the extremeviolet. To him also 15 due the announcemen t that the red
B
V io let, though he attributed the effect to the wrong cause .
In 1 802 Thomas Wedgwood read a paper befo re the
Royal Institut ion, entitled ‘ An account ofa Me thod of
Copying Pain tings on G lass 1 and ofmaking Profi le s by theagency ofL ight upon Nitrate ofSilver. ’
Wi th these experiments ofWedgwood’s Sir HumphryDavy was asso ciated, and in the ir recordWe find i t s tated thatmuriate ofsilver was mo re readily acted upon by l ight thanthe n itrate
,and that white leather used as a basis gave be tter
images than paper; Images obtained by the so lar microscope were impressed without any serious difficulty, but nomeans was discovered ofrendering them anything but transitory when expo sed to daylight. For Charles, a Frenchman, has been claimed the credit ofemploying at an earl ierdate the same method of obtaining black p rofiles by the
action oflight,but there seems to be no authen tic proof
extant that thi s claim should be allowed. Dr. Wo llaston ,
in 1 803 , discovered that gum guaiacum,when expo sed to
the action ofthe blue rays of light, became changed in
co lour, and that on expo sing tho se altered portions to thered rays
, the o riginal tint was restored.
In 18 14 Pho tography was to rece ive a n ew vo tary in
the person of Jo seph Nicéphore de Niepce. Leavingthe salts ofs ilver, he devo ted himselfto the study oftheaction oflight on resins. After several years ofresearch,he at length completed the pro cess known as he l iography,which consis ted in the production ofa picture in bitumen
on a po l ishedmetal plate. The discovery hemade in regard
to this re s in was that, after insolation, i t became inso luble
in its o rdinary so lvents. An expo sure ofmany hours in acamera obscura was necessary to produce the required effect ;hence , as may be imagined, the views taken by this means
I A mistake often o ccurs in the readingofthis sentence . Wedgwood did no t make the copies on glass, but copied paintings whichwere drawn on glass.
3
were wanting In Vigour, owing to the shifting directIon of
the sunl ight, and,as we shall see later on in this wo rk,
from o the r causes, were ofnecessity deficient in del icatel ights and Shades. In 1827Niepce came over to England,wi th the intention ofdrawing the atten tion ofthe Royal
Society to his discovery, but his pro cess being secret, hiscommun ication was not rece ived, andhe re turned to France .
In 1 824 Daguerre , a French painter, began a series ofex
periments in the same direction, and in 1 829 he and Niepce
entered in to a partnership , and pre sumably it Was the
knowledge ofthe latter’s method ofworking Whl(h gave the .
fo rmer the idea ofthe daguerreo type . Niépce had employeds ilver plates covered with asphaltum, which, after exposure and application ofthe so lvent, left the me tal bare in
parts. The image thus formed was brown 5 the shadows
be ing represented by the metallic surface. In o rder toproduce a proper effect, it was necessary that the partscovered by the bitumen should be whitened and the bareparts darkened. After various experiments
,he appl ied iodine
to the picture, subsequently removing the bitumen . I t is tobe presumed that Daguerre no ticed the action that the light
produced‘
o n tho se portions ofthe plate which had beenconverted into iodide. At any rate tO
'
Daguerre be longs theglo ry ofthe discovery that an image could be produced on asi lvered plate which had been expo sed to the vapour ofiodine ,though i t was by fo rtui tous Circumstances that he hit on the
method ofdeveloping an invisible image .
In January 1 83 9 the discovery ofthe daguerreo type process was first announced, and in August ofthe same yearthe details ofproduction were given to the wo rld, DaguerreandNiépce the younger (the successo r ofNice
’
phore ), obtaining a pension from the
‘
Governmen t of France . WhilstDaguerre was wo rking in France, we find that . one ofo ur
own coun trymen,Fox Talbo t, had been experimenting In
ano ther direction. Bearing in mind the wo rk ofScheeleand
‘Wedgwood, he devo ted himselfto the production of,
B 2
4 Historical Skete/z.
drawings, &c.
, on silver chlo ride, and in January 1839 he
read a paper befo re the Royal Society on‘ Pho togenic
Drawings . ’ His method ofprocedure was somewhat asfo llows : Writing paper was coated with a so lution of
common salt, and after drying was brushed over with silve rn i trate , by this means si lver chloride
’
was obtained,with
a slight excess ofthe n itrate, in which condi tion i t proved
excessive ly sensi tive to l ight. Various bodies, such as laceandferns, were laid on this paper, and a reversed facsimileofthem in black andwhite was produced, andhe fixed the
F IG . 1 . FIG . 2 .
6 Experiments wit/t L tgitt.
ofthe paper was oxidised. Ge lat ine, gum, starch,'
albumen,
we re all found to become inso luble when expo sed in
contact with it ; and Po itevin utilised thi s fact in the pro
duction of pictures in powdered carbon by a pro cess
analogous to that subsequently to be described in thesepage s . Swan, Johnson,
Woodbury, and o thers, have mo rerecently extended its application by the production ofimagesfo rmed in gelatine, co loured with pigmen ts ; whil s t a Stillwider field has been opened by Albert, Edwardes, ando the rsin the production by mechanical means ofprints in printersink from a gelatine image, founded on the fact that oxidisedgelatine i s incapable ofabsorbing water.
CHAPTER I I.
EXPER IMENTS WITH LIGHT.
BEFORE entering into the theory ofpho tography,i t will be
conven ient to enter briefly into some ofthe phenomena oflight ; for it is with this fo rm ofenergy that the pho tographer has to deal. I t will be as we ll first to try one praet ical experiment with l ight, in o rder to clear up certaindifliculties which may present themse lves .Darken a room ofsome 1 2 or 14 feet in length -bymeans
8 Experiments wit/i I. zg/zt.
about 6 inches from A,in such a manner as to cause the
beam to fall upon the prism, B . The floating dust in the
room will immediately Show that the original beam ofwhitelight has been spli t up into a series of co loured rays ,and a position for the screen may then be found whichwill cause the top and bo ttom edge ofthe spectrum (as thi sglo rious band ofco lour is termed) to be Sharply defined ; andifthe cut in the tinfo i l be fine enough
,a serie s ofdark line s
wil l traverse i t vertically . With these, however,'
we have nothing to do at present. Now experiments
,the results of
which have formed the groundwork ofmathematical reasoningon the theo ry oflight, have conclusive ly proved that lightas light is merely a sensation. Permeating all known spacei s assumed to be an imponderable and e lastic fluid knownas e ther, and in i t a luminous or heat source i s able togenerate a series of ripples or waves
,flowing unbrokenly
and continuously from i t. What the prime fo rm oftheseundulations may be we cannot tel l. They may be, and
mo st 'probably are, compounded ofan almo st infinite numberofdifferent undulations , when o rdinary l white l ight i s theimpression given to the eye , and each ofthese serie s of
undulations vary in length from crest to crest. Tho se of
certain lengths are able to affect the nerve s which line the
retina ofthe eye ; whilst some ofthese are able to affecto ther nerve s lying in our bodies , producing the sensationofheat o thers again, though incapable ofproducing the sen
sation ofl ight or heat, exhibit themselves by their effect oncertain compounds, causing chemical combination or
'
decom
po sition. Oftho se waves who se'
impact on the eye produc esthe sensation oflight the shorte st i s about 600mi ll ionths ofa
mill imetre , whilst the longest is about 350mi ll imetres . The
fo rmer give the sensation ofa vio le t co lour, the l at terofa brillian t red. Examining the. spectrum thrown on the
screen ,the intermediate co lours ofblue, green. yellow,
and
o range are seen, and the wave-lengths producmgtheir effe cts1z’
.e. not polarised.
Wanos . 9
On the eye lie intermediate be tween the l imits given above.
There i s uncertainty as to the lower limi t to which the heat
producing rays are refracted, but probably to a length equalto that ofthe visible spectrum, whil st the range in length ofthe chemically active waves o ther than those Si tuated in
l
the
visible portion ofthe spectrum,and which lie beyond the
vio let (being called the ultra-vio le t or fluo rescent rays),i s, ifanything, s ti ll more uncertain. I t will be evident onreflection that it must be accidental that, between certainl imi ts
, the waves should be capable ofproducing a sensation ofl ight or ofheat. The exact upper limit ofthe thermalspectrum i s unknown, but from theo ry it must be tso-terminous with the chemi cally act ive rays
,as will be seen further
on, the inferio r limit‘
ofthe capaci ty Ofany waves to produce decomposi tion i s as yet unascertained. All tho seSeries ofwaves which effect decompo sition in any compoundare called aetinie rays
, and,as will be seen, the range of
'
these vary for every ordinary photographic compoundIt may help us in a right comprehension ofour subject
ifreference 13 made here to one quality ofthese undulations .The interstellar ether In which these waves ripple Is assumedto permeate every body, so lid, l iquid, and gaseous ; and itdepends upon the disposition ofthe u l timate mo lecules ofthe body whether it is opaque or transparent to any ofthe
Visible or dark rays oflight. I t must be bo rne in mind thatthe mo lecules ofevery substance are presumably in a stateofvibration , the extent and velo city ofwhich depend partlyupon the temperature, and partly upon the nature , ofthesubstance, and that this must ever be so unless the purelytheore tical condition ofabso lute cold be arrived at. Suppo sing, then, we have a glass, which with white l ight fallingon it allows only the transmiss ion ofred l ight, andwe lookthrough it at the spectrum fo rmed by white light, we shouldfind that it cuts offthe who le ofthe co lours excepting thered, obliterating them mo re or less perfectly, that Is, in technical language, i t abso rbs them. Now, according to all
IO Experiments wit/z L eg/"fi t.
ideas ofthe con servation ofenergy, this abso rption mustIndicate the perfo rmance ofsome kind ofwo rk.
‘
I t may bethat it cause s the already vibrating mo lecules ofthe glass totake up and swingin some complicated manner with tho serays particuiarly abso rbed, and thus to cause a rise in
‘
tem
perature in the body, so small indeed, perhaps, as to be
indist ingu ishable, owing to the rapid coo l ing due to radiation ;or i t may b e that work is p erfo rmed in effecting chemicaldecompo s ition, for even glass is thus affected by l ight. The
rays which simply pass through the glass produce no effecton it— the ir energy is unimpaired.
I t should also be no ted that where light is no t entirelyabsorbed, but is only reduced in intensity, even thenalso wo rk must be performed by it
'
; for the intensity . of
any co lo ured or white l ight is dependent on the extent, oramplitude, as i t i s termed, ofthe wave or waves ; and anydiminution ofthe amplitude indicates that a po rt ion ofi tsavailable energy has been exhausted, and that therefo re atransference ofthe portion so expended must have beenmade to the body through which it passed. This exchange
or tran sference of energy is an impo rtant subject in allpho to graphic matters i t explains many ofthe phenomenain pho tography which often present a great difficulty to the
beginner o r to the rule-of- thumb pho tographer, whilst i t i sall-impo rtant in the right unders tanding ofthe revelations
which are made by the spectro scope.
I t may then be laid down as an unal terable law, thatm/zere time is absorption ofage: (zonetner ofdark or visible
rays) by any body ,work ofsomedescriptionmust nave beenper
formed in t/zat body. An accoun t of the valuable experi
mental research ofJoule on the mechanical values . ofl ightand heat is given in the Philo sophical Magaz ine ’ for 1 843 ,
and i s deserving ofSpecial study.
T/zeory"ofSensitive Compoana
’s.
‘
I I
CHAPTER I I I .
THEORY OF SENSITIVE COMPOUNDS.
EVERY particle ofmatter may be cons idered to be made upofmo lecules, each mo lecule consisting ofconstituent atoms.Thus a particle (andWhen we say particle we mean to con
vey the idea ofthe smallest visible quantity ofmatte r) Ofs ilver iodide is compo sed ofmo lecules of a like definitecompo sition, the components be ing two atoms ofiodineand two ofs ilver, or multiple s ofthese numbe rs. The
physical aspect'
ofm atter often c onveys to the mind an idea
ofa certain kind ofarrangement In the molecule s as does
the analysi s ofa compound, ifno t ofthe abso lute arrangement of the atoms, at all events of the arrangementswhich they cannot take . Oxalic acid, for instance, weknow is compo sed ofcarbon,
hydrogen,and oxygen ,
havingthe fo rmula C2 H2 04 , or the exact equivalents of water,H , O, carbon monox ide C O,
and carbon dioxide C O2,
yet the compound is to tal ly different in itS'
physical charac ters and. chemical reactions from any ofthese .
‘Fromthis we can argue that the atoms ofi ts mo lecules must beseparated in such a manner thatthe oxygen -m01ecules
'
cann0t,seiz e
upon the hydrogen to fo rm water,or on carbon to fo rm carbon
'
mo
noxide or dioxide . When the
atoms are so arranged as to be
Incapable offo rming a mo leculeofa simpler type , they o ccupy apo sition ofexcessively stable equilibrium
, and i t would -be
necessary to expend a large amount ofwo rk to separatethem. On the o ther hand, where the atoms ofthe mo lecu1e are so grouped that by rearrangemen t they may fo rm
perhaps mo re than one mo lecule , each ofwhic h may be
T/zeory ofSensitive Compounds} .
ofless complex character, i t often happens that a ll the
atoms are in state ofstable, though verging on indifferent ,equilibrium. We may take as an illustration ofth is stateof equilibrium the frustum of a pyramid standing baseuppermo st, On a narrow section parallel to the base. I ti s apparent that the wo rk expended in o rder to cause the
frustum to fi nd a new po s ition ofmore stable equil ibrium ,
(or, in o ther words, to fal l on to one ofi ts sides), may bemade as small as we please by dimini shing the area ofthesection on which i t stands . Whils t falling, the body can doa certain amount ofwork, which will be quite independent
ofthe amount ofwork expended to Cause i ts fall. So with
the atoms ofa mo lecule which are in'
thi s state ofalmo stindifferent equil ibrium a very small amount ofwork need
be expended in o rder to cause them to take up mo re stable
po sitions but the kinet ic energy they may po ssess whils tpassing to this new state, need be no measure ofthe wo rkperfo rmed U pon them. A measurement ofthe work per
formed by their re-arrangement would principally te ll whatamount ofwo rk had been expended in some chemical process, in
'
order to place them in that state bordering on
indifferent equil ibrium. I t is possio/e, however, under cer~
tain circumstance s, to compare two or mo re energies wi thone ano ther, by comparing the effects they produce on suchmo lecules. Extending our previous illustration,
suppo singwe had a row ofsuch frusta ofpyramids, and that i t wasfound that one pelle t ofa number (all be ing ofequal weight)when striking one frustum w i th a certain velocity, was ableto cause i t to fall , and also that in every case the accuracyofaim was undoubted
,and that in fall ing one frustum did
no t strike i t s n eighbour ; then at any in terval after the
commencement of a bombardment the amount ofwo rkexpended in proj ecting the pellets could be compared bys imply coun ting the number offrusta which had fallen.
I t is in a manner akin to'
this that the comparat ivevalues ofthe inten sity oftho se rays which produce chemical
I4 T/zeory ofSensitive Componna’s .
though the excursions they take will be the greater,the
higher the temperature ofthe body ; and from analogy itmay be assumed-that the agitation IS really a definite o scillat ion
,though the paths described may be ve ry complex .
Now, o rdinary white light, as has already been po in ted out
in the last chapter, mo st probably consis ts of.an almo st .infin i te series ofundulations ofvarying length , traversmgamedium,
and i t is qui te conceivable that the mo lecules ofabody, Who se o scillations synchron i se with one ofthese serie s
ofe thereal waves , may have the ir paths altered in fo rm,and
their amplitude increased to such . a degree,
FI G 5. that a rearrangement ofthe atoms must ensue . In o rder to il lustrate the effect ofoneo scillation upon another, the late Professo rRankine employed the fo llowing contrivance .
A i s a lath to which is suspended a leadenbob, B, some six or seven pounds in we ightC i s a string attached to B, by Which is suspended a wooden bob
, _
D. The who le i scaused to o scillate on an ax is placed atWhen the length ofthe string is such as to .
cause the heavy and the light pendulum to
synchronise accurately,a slight ho riz ontal
displacement of B will cause the length“
ofamplitude
'
ofthe o scillations .ofD to increaseto such » an extent that the latter will passthe semicircle and tumble. When .the . syn.
chronism i s only nearly perfect, the amplitude ofD Will at '
firs t increase, gradually stopping the o scillation ofB,whe n
i t will diminish , and finally come . to rest and bring B intoo scil lation once more , and so ou
'
. Ifwe take the swingD as
'
the type ofthe o scillation mo lecule, and that
'
ofB asthe o sci llation ofthe e thereal medium
,it wil l be seen how
perfect and nearly perfect synchroni smiwil l increase the'
oscil lat ion of. the molecule . The same illust ration appliesto,
a'
. part ofthe theo ry ,
:ofexplo sives, w hether caused to
Waves ofVaryingL engt/i .I
f 5_
explode by the energy of radiant heat, or by that of
atmospheric or gaseous waves. This is in acco rdance .
with what we have already advanced : i t i s only tho sewave s which are entirely or partially absorbed, and who se:amplitude i s consequently annihilated or reduced
,which can
do wo rk on a body therefo re, in choo sing any particular rayof light with which to cause this class Ofdecompo sition in acompound, i t i s a sine qua non that it must be abso rbed;in addition to which , some atoms must be less loo se lybound to the mo lecules than are o thers. I t i s found praetically that the bodies employed for. pho tographic purpo ses
are affected principally by the waves ofshort length, and thatas a rule tho se ofgreater
‘
length are inoperat ive and herewe come to a great distinction existing between the re
arrangement ofthe atoms ofthe mo lecules in explo sive s andin pho tographic compounds. The short wave lengths dono t affect the former, though the longer one s
,which we
call radian t heat, can do so . Now the energy transmi t ted
from a hot and luminous body by the medium l iesprincipally in tho se wave s which are capable of pro
ducing what we cal l heat (in fact the energy can onlyproperly be estimated by ascertaining the heating effect due
“
to the radiations ) and as the heat produced in a body bythe waves oflengths such as 450 m
‘ ill ionths ofa'
mi l limetre i s insign ificant, and when they are ofa length of200
mi ll ionths of a mill imetre i s at present immeasurable,i t
i t is evident that the energy expended on the productionof these last wave-lengths is small ; at
'
the same time It
happens that the ir production , as a rule, necessitates the
existence oftho se ofgreater length . Thus,a platinum wire
inserted in an electrical circu i t may be heated, and yet on lyradiate dark rays by increas ing the current i t may become
cherry-co loured, and a spectro scopic examination wil l .demonstrate that only red rays are emi tted, whilst at thesame time i t may be shown that the intensity ofthe darkrays
'
is increased. Byfurther increasing the curren t, the
16 Tneory ofSensitive Compounds.
yellow, green,blue, V io le t, and ultra Vio let rays may in suc
ce ssion be caused to radiate from the wire ; all the first
em itted rays increasing in intensity. Fig. 6 shows the
relat ive ly greater increase in an indigo ray, compared with a
ye llow ray, emitted from a carbon fi lament heated in vacuo
by an e lectric current.F IG. 6.
In o rder, therefo re, to displace the molecules ofsmal lstabili ty ofthe pho tographic compound which are in equili
brium, it i s as a rule necessary to produce waves ofgreat
length as we l l as wave s ofsho rt length, and this may meanthe exi stence ofa great heat energy at the primary sourceofradiation, though no t necessarily at a reflecting surface.
Now, the usual result ofthe displacement ofan atom from
what we may call the sensitive molecule i s . to fo rm affresh.
M olecnlar’
Vibration‘
. I7
solid body, and consequently the potential energy ofthe
mo lecule is smal l, also the number ofthese mo lecules actedupon in agiven time i s small in comparison with the to talhence the kinet ic energy (which may take the fo rm ofheat)that may be generated by the chemical decompo sitionand recombination falls far short ofthat required to produce
even red l ight, much les s waves ofstill shorter length. We
thus see that although one mo lecule ofan explo sive per so,
after its po tential energy has become kinetic, can causevibration s ofsuch a character as to effect a di sruption ofthe
ne ighbouring mo lecules, yet a similar disturbance produced Inamo lecule ofa pho tographic compound ISno t capable ofcausingan extension ofthe action beyond the mo lecule itse lf, andthat i t requires a renewed action ofthe disturbing force to doi t. At first sight this seems unfortunate, but when we con
sider what . wo uld happen were such an event po ssible,i t i s
apparent that the production ofa pho tographic image insuch a case would be impo ssible.
In a succeeding chapter it will be found that a mo leculeofchlo ride ofsilver responds principally to the swing oftheultra-vio le t waves in the spectrum,
and that it undergoesa change, owing to the throwing offofone ofi ts cons tituent atoms , yet the same body may, by the aid of.an
artifice . be fused by the dark rays ofheat, which arecomparative ly ofgreat wave length, and though it in i tselfbecomes luminous, emitt ing the very same rays that, whenfalling on i t, can cause one ofits atoms to be shaken off
,
yet i t remains unaltered. In this last case the vibrations ofthe mo lecules are mo t '
ot the definite character needed tocause the change. A small fo rce, appl ied at defini te intervals, may cause a body to attain a great amplitude of
vibrat ion . A boymay cause a vio lent oscillation ofa churchbell ifhe t ime his pulls at the rope properly
,and the
accumulated energy may be such that i t may drag the ringerup, though the work he mayhave executed at each pull ofthe rope may be very small. .Ou the o ther hand
, the ringe r
may expend the same amount ofenergy at the wrongtime ,and the effect on the bell will be insign ificant. The ex
periment given at p . 14, fig . 5, il lustrates this effect.As befo re s tated, the number ofmo lecules affected in a
short interval oftime by light may be so small that theirchange in atomic compo sition maybe invisible to the eye, or
inp/zysical appearance may be ofa similar nature to the com
poundfrom which they are derived, in which case even aprolonged expo sure to the actinic rays would produce no visibleeffect. When the sensitive compound isformed in a thin layerheld in sitn on some substratum, such as paper, glass, &c . ,
the light reflected and radiating from an object after passingthrough a lens may be caused to fall upon i ts surface and
fo rm an image . When the rays are ofsuch a nature as tocause the equilibrium ofthe consti tuent mo lecule s to bedisturbed
, the change will take place only In such partsofthe thin lamina as are illuminated and thus an invisible‘
image fo rmed by the shaken compound may be impressed ifthe time ofexpo sure be short, or the change produced
_
be
such as no t to be within the scope ofourvision. Otherwiseupon long expo sure a visible image may be produced, theresulting compound be ing different in appearance from the
o riginal .AS the po int i s ofgreat importance, we must again
direct attention to the fact, that the two images are exactlyalike in chemical compo sition, one differing from the o therso lely in the number ofmo lecules altered . Fo rtunately“
,
methods exis t ofrendering visible to the eye what i s o rdinarily and primarily invisible, and this operation i s termed
the deve lopment of the image. The invisible image is
frequen tly termed latent, an appellation which, thoughconven ient, i s yet open to some criticism. We will now
discuss the vario us ways in which development may be
effected.
1 st M etborb— The new compound may po ssess an attrae
tive fo rce. Ifa rod or wire ofz inc be placed in a so lu
Tne Efl eets ofn /zt. 19
tion’
Oflead acetate, chemical operations immediatelycommence . The outs ide particles ofthe z inc enter into com
b ina tion with the acetic acid ofthe lead ace tate , and particles oflead are depo sited upon the rod in the i r stead.
AS the action continues the lead further reduced is, by acertain wel l-ascertained law, attracted to the lead already '
depo sited. Spangles ofthe metal in a crystall ine fo rm at
tach themselves to the rod and then to one ano ther, untilwhat is known as a lead tree results
,j us t as a magne t will a
string ofnails suspended from one ofi ts po les. In a s imilar
way a silver tree may be formed from a so lution ofi ts salts,provided the reduction be slow. So the action ofl ight on cer
tain sensit ive compounds, especially amongst which may be
mentioned tho se ofsilver, i s to cause the fo rmation ofa bodywhich is capable ofattracting the metal (ofwhich iris i tselfasalt ), when slowly depo sited from a so lution . This first de
po sit is capable ofattracting -s till more ofthe metal, and
thus an image i s completely built. This action i s mo re fully
treated at p. 64.
2nd M etbocb— The al tered compound may be ab le to
effect a reduction to a
'
metallic state of a metal from a
so lution of its salt, which the o riginal compound may be
incapable ofdo ing. In thi s case the metal would be naturally precipitated ou the altered compound, and the attrae
tive force ofthe freshly-depo sited metal would de terminethe attraction ofany o ther that might be caused by extraneons cause s to depo sit i tself. In this me thod, as in the last,i t i s evident that the minutest po rtion ofthe altered com
pound is able to effect a building up ofthe image.
3rdM etbocl. —The image may be formed by the partialreduction, to a more
'
elementary state, ofthe altered com;
pounds, when treated with certain so lutions, which reduction
in the o riginal compound was impracticable ; also in thisreduced state i t may exercise the same attractive fo rce as
above . We shal l have an
'
example ofthis in alkaline ' de
velopment.
C 2 . I
20 Sensitive Compounds.
4t/z M tbod— The altered compound may be“ capable of
fo rming a co loured body when treated with metall ic or o therso lutions. In this case i t is man ifest that the image must be
due so lely to the amount of the’
sensitive salt o riginallyaltered in compo sition, and its vigour must consequen tly..
depend upon the time the light has acted. Ofthis method
ofdeve lopmentwe shall have examples in the more sensi tiveferric salts.
5t/z M etbod— The attractive fo rce ofan al tered mo lecule may be util ised by causing me tall ic or o ther vapour tocondense upon i t in prefe rence to the ne ighbouring mo lecules which may not have been changed by light. Thisfirst condensation may determine the fo llowing condensation. Ofthis we have an example in the development of
the daguerreo type plate.
6tli M t/zod.— The alteration in the compound may be
shown by its incapacity to abso rb mo i s ture.
7tb M etbod— The new.
compound may be incapable of
entering into so lution, though the o riginal compound may
be readily solubleThe chemical agents which are util ised in o rder to allow
the development ofthe latent image to take place wil l bediscussed as each method is brought under particular c ons ideration . I t is to be r emarked that these agents are
technically called deve lopers, a term which,critically Speak
ing, i s a misnomer, as in the majo rity ofcases the part theyplay 15 a secondary one, and one which they fill whe therapplied to development or no t. The term is conven ient,however, and will be adopted in this wo rk, though the studen tmust in his own mind make the reservation indicated whencoming acro ss the term .
Inten sifying an image already developed or Visible i s aterm appl ied to a pro cess whereby the image i s (I ) renderedmore visible to the eye, or
‘
(2) rendered more absorben t of,and-therefore less transparent to , some particular kind of
l ight, be i t white, blue, red, yellow, &c . Bo th ofthese re sults
22 Tne Action ofL eg/it.
blackened body with ammonia, metallic silver was leftbehind. There i s no t much need to carry the investigation
further than Scheele, only the conclusion that he accepted,viz . that metall ic silver was separated at the t ime ofex
posure,‘ should
,be Viewed with much doubt, part icularly
when it is found that the darkening action ofthe chloridetakes p lace even when immersed in the strongest n itric acid.
The accepted theo ry seems to be that exposure to lightreduces any chlo ride to the state ofsubchlo ride, thus
Silver Chloride Silver Subchloride ChlorineAn l2 An l Cl
When the same compound is mo istened the reaction appearsto be different, as chlorine decompo ses the water withwhich it i s in contact, fo rming hydro chlo ric acid (HCl)whilst the o ther atom ofhydrogen and the oxygen atom in
the mo lecule ofwater combine with ano ther atom ofchlo rineto fo rm hypochlo rous acid (HClO). If, instead ofexpo sing
the sil ver chlo ride in a dry state or in the pre sence of
mo isture, it'
i s exposed in presence offree silver n i trate,fresh s i lver chlo ride i s fo rmed, and this same compound ofchlo rine and o xygen l iberated and i t i s found generallythat the darken ing takes place much mo re rapidly when anybody which will take up the chlo rine i s in contact wi th it.Thus, stannous chlo ride will cause more rapid darkening,from the readiness with which it absorbs chlo rine . The
s tuden t would “
do well to repeat the experiments ofScheele
and tho se subsequently indicated,in order to convince him
se lfthat these reactions really occur. The easiest 'me thodofpro curing pure silver chlo ride i s to precipitate i t froma so lution of silver n i trate by an excess ofpure hydro ~
chlo ric acid,and to wash it thoroughly by decantation,
re
peating the washing to such a po int that the supernatantwater shall no longer Show acidity when tested with blue l itmuspaper. This method ofprocedure prevents the po ssibi li tyofcontamination by the o rganicmatter offil ter paper. The
Silver Clitoride and Iodide.
s ilv‘
er’
chlo ride, ifrequired in a dry state, should“
be driedin the dark over a water~bath, in a watch-glass or po rce laincapsule. A test-tube i s a convenient vesse l in which to givethe expo sure to the light, and the subsequent washings areconveniently carried out by simple agitat ion and pouring off
the l iqu id. It may be no ted here that perfectly dry s i lverchlo ride when expo sed to light in vacuo remains white.
This is probably due to the absence ofa second substancein the tube
,and a consequent inability to be decompo sed.
It must ever be remembered that there isi
no experimentproperly carried out, with a set object in View, which is no two rthy ofrecord. The mo st trivial deviation from the
expected results ofan experiment Often causes some new
l ine ofthought to be'
taken'
up, andmay suggest impo rtantinvestigations.
The next silver salt that requires a careful studyis theiodide and i t i s owing to certain peciIliarities in i ts behaviour when expo sed to light that so much difficulty has arisenin defining the true changes that take place in i t.
Silver iodide may be produced in two or more ways.The mo st common is by treating
.
a silver n itrate so lutionwith a so luble iodide, such as ammonium. Ifthe fo rmerbe in excess
,even in minute proportions, after mo s t careful
washing, i t will be found that the compound darkensslightly on expo sure to light, whilst it the latter be in excess,there is no apparent change in co lour. To explain this lastphenomenon i s somewhat difficult, but it must be remembered that even with the mo st thorough washing any saltwhich may have been in excess canno t be really elim inated .
The iodide ofthe alkali is i tselfsensitive to l ight, li beratingiodine, and i t seems probable that, bo th being acted uponby light, there i smerely an interchange ofio dine atoms . Alsoit must be borne in mind that, whilst chlo rine and bromineare gaseous at o rdinary temperatures, iodine i s so l id, andcannot, therefore, so readily escape. There is also reason to
bel ieve that the molecules ofiodine are mo re complex than
24
those ofthe o ther halogens, andwe can thus understand thata difficulty exists in causing it to change . The Sho cks whichwi ll break up the chlorides or bromides are insufficient toproduce any alteration when no thing but pure iodide i spresent.When, however, there i s an excess, however slIght, of
silver ni trate, the conditions are quite altered ; for then
there i s a compound at hand which is ready to sei z e any
iodine which may be brought near it. Thus, when the S ilvern itrate i s present, the mo lecule ofiodide i s at once changedin chemical compo si tion, and a subiodide is fo rmed in asimilar way to the fo rmation of subchloride from the
chloride.
Silver Iodide Silver Subiodide Iodine
Ag,I2 Ag,1 I.
I t may here be remarked that in“
one respect iodine is
unlike chlo rine in behaviour i t i s incapable offorminghypo iodous acid (HIO), though chlo rine, as alreadypo intedout, forms HClO hence there i s some diffi culty in ascertain
ingtheo reti cally the exact reaction which takes place between
the l iberated iodine and the silver n itrate which is neces
sarily pre sent to produce the change.
A simple experiment, however, which i t i s well to repeat,throws l igh
'
t upon i t. Take washed Silver iodide, and place
i t ina test tube contain ing in so lution silver n i trate which haspreviously been thoroughly bo iled in order to expel any airwhich i t may contain. Ifan air-pump or an exhaustingsyringe be at hand
,the bo iling may be dispensed with , and
the same end attained by creating a'
vacuum in the tube.
Now expo se to light in a sho rt time bubbles ofgas wil l befound co llecting in the so lid iodide, andwith care these maybe co llected
,and on testing by the ordinary means wi ll be
found to con tain oxygen. From thi s we may suppo se thatthe liberated iodine decompo ses the water in
'
contact with i t
(as does chlorine) , and produces; hydro iodic acid (HI) and
oxygen.
Absorbents. 25
The fo rmer combine s with the Surrounding silver n i trate ,andwe have a to tal reaction , as fo l lows
Silver Silver Silver Silvei IodideIodide Nitrate Water
Subiodide+ Oxygen
(newly formed)+ AgNO, + H,O Ag,I O AgI
Nitric AcidHNO,
Ifany iodine‘
absorbent be placed in contactWi th washedsilver iodide, prepared with an excess ofso luble iodide , thereaction that take s place is apparently mo re simple, theiodine atom combin ing directly with such a body . I t may
“
thus be'
stated as a law that in order to produce a elzange bytbc action ofleg/it in silver iodide, some body must be present
var/tick can absorb iodine. l
There are one or two suggestive experiments whichmay impress this on the mind. The first is to silver aglass plate as iffor a mirro r, and then to expo se i t to
the action ofiodine vapour (as in the daguerreo typepro cess) to such a
‘
degree , that the who le ofthe extremelythin film ofme tal
.
i s converted into iodide . On expo s ingsuch a plate to sunlight no change i s vis ible
,nor can
one be brought to the cognisance ofthe senses by bringingdeveloping agen ts
‘
in contact with it . Ifthe film be
no t who l ly converted‘
into iodide , thi s result will not o ccur,as the metall ic silver is an iodine absorbent. Ano ther ex
periment, which is very conclusive, i s as fo llows : Preparea film '
of Silver iodide, as in the wet process, and im
merse It In po tassium iodide so lution t ill any exce ss Ofsi lver n itrate i s converted into silver iodide , and wash
.
tho roughly for an hour, and dry. Next take a small square
piece'
ofsilver leafand apply i t to one po rtion ofthe io didesurface
,brushing it wel l down
,in o rder that real con tact may
be obtained. T o ano ther small po rtion apply a so lution of
tann in in alcoho l, and after drying expo se the plate to theI This law seems to have been first emphatically enunciated by
Vogel , though a claim has been made by Poitevin.
26 T/ze Action ofLig/zt.
l ight. On developing, as indicated at page 71 , a darkeningaction will be apparent after a short interval oftime ontho se po rtions ofthe plate treated with the silver leafandthe tann in.
The action will be mo st intense in the latter, as mightnatural ly be expected, the who le thickness ofthe iodidebe ing in the one case brought in contact with the absorbent
,
whils t only tho se particles which form the surface are
brought in contact with it in the latter. The experiment i s
more tell ing ifthe plate be expo sed behind a negative,with
the uncoated side next the image.
Ifinstead ofthe silver leafa thin silvered plate be pressedinto firm contact with a sensitive co llodion film, prepared as
Fm. 7.above
,it will be found that even a fair
exposure i s sufficient to cause the formation ofan image on bo th, which,though unrecogni sable to the senses
,
i s yet capable ofbe ing developed by
the p roper methods. This experimentthus serves to show conclusively thatiodine i s liberated by the impact of
light ; for, were the change one merely ofmolecular arrangement (as many enquirers have held to be the case), no imagecould be formed on the metal plate, the possibility ofdevelopingan image on it be ing dependent on the presence ofsi lver iodide (see the daguerreo type pro cess).
The behaviour of_
silver bromide is similar to that ofsilver chlo ride hypobromous acid being fo rmed undersimilar circumstances to those in which hypochlorous acid isproduced. The chlo ride and bromide are both so luble inammon ia (which is an important po int when dryplate pro .
cesses are considered), whilst the iodide is not. I t may behere reco rded that with the chlo ride and bromide, as withthe io dide , the presence ofsilver ni trate increases sensitiveness to a very high degree .
Bes ides the salts already mentioned there are o ther in
Organic Salts ofSilver. 27
organi c compounds of silver, such as the fluo ride, pho sphate, sil icate , which are a ltered by the action of l ight, butthese are comparatively un impo rtant. There are , however,certain o rgan ic compounds formed, the action ofl ight uponwhich can only be briefly no ted here , though a fuller description ofthe phenomena will be given in a subsequentchapter.When
,
o rganic matter is brought into contact with asoluble salt ofsilver, a defini te compound is often fo rmed,and the effect ofimpact oflight upon this is somewhat complex to trace . Thus, ifwe form an albuminate of si lverby bringing a so lution ofsilver n i trate in . contact with one
ofalbumen, and expo se i t to light whether there i s an excessor defect ofthe silver salt present, a darkening ofthe com
pound results .The blackened compound is not a true s i lver o xide
,
though chemical considerations lead us to infer that theco louration i s dependent on the fo rmation ofs ilver o xide,in combination with o rganic matter. The same results are
obtained ifgelatine or o ther kindred body is substituted for
albumen. I t will be as we ll ifthe student experimentallycompare the effect oflight on an o rganic s ilver salt withthat on silver chloride, as bo th are employed in the si lverprinting pro cess.
The fo l lowing experiments will naturally suggest themselves. Take sodium chloride and dissolve in water and
add an excess ofsilver n itrate to i t, by'
which we haveprecipitated silver chlo ride fo rmed ; also take the sameso lution and allow the sodium chlo ride to be in excess.
Carefully spread the mo is t chlo ride on pieces ofglass, and
expo se to l ight: Bo th will readily darken,more especially
the fo rmer, which will gradual ly assume an inky blacktin t
,whilst the latter remains a pale vio le t. Fro m what
has already been said, the cause ofthis phenomenon willbe apparent, the chlorine l iberated in the firs t
.
case is
rapidly abso rbed, whilst in the second it is merely he ld in
‘
28 TileAction ofL'
igni.
so lution, cunging as i t were to the silver subchlo ride, and
ready to reduce i t back to the same state as befo re . If
to the silver chloride, in which the sodium chlo ride i s inexcess, we now add a little stannous chlo ride which
“
is
ready to abso rb chlo rine, the blacken ing will proceed asrapidly in the one case as in the o ther. Now treat all theseresidues with ni tric acid, and they will all be found to
remain“ unattacked by it, but
’
instantly yie ld to a strong
solution‘
ofsodium hypo sulphite, leaving metall ic silver insmall quan ti tie s behind. Next precipitate albumen in ex
cess , « o r o therwise ofsilver,and expo se to l ight thedarken
ingwill pro ceed mo re rapidly and to a greater depth m the“
one case than in the o ther. Treated with ammon ia butl it tle alteration i s visible , but on applying nitric acid, the o xideat once di sappears . If
, however, it be treated with sodiumhypo sulphite, i t will remain
'
nearly unaltered in appearance .
Next treat the undarkened albuminate ofs ilver with hypoSulphite
,and i t will disso lve, leaving a milkiness in the so lu
tion'
on further addingammon ia to the so lution, however,this will disappear entirely. Ifbo th the darkened bodie s aretreated, after the sodium hyposulphite has been applied, withasolution ofhydrogen sulphide (H2S) , the former will blackenfrom the formation ofsilver sulphide, the latter wil l b leachfrom the fo rmation ofa new o rganic compound ; the bearingofthis experiment will be seen when we consider the fadingofsilverprints.
’
Again,to a s1m11arly treated precipitate ofchlo ride and
‘ albuminate add po tassiumcyan ide ; the one will be but slightlyacted on, whil st the
'
o ther will be speedily attacked. In determining the fix ing agent to employ in-
si lver printing, this po in thas to be taken into consideration. Ifexperimen ts with o thero rganic bodie s be carried on in a simi lar manner, it will -be
found that the same phenomena will be observed ; the distinction be tween the nature ofthe reduced o rgani c c om:
pound will be seen -in the different co lours they assumeFrom the se simple experiments
-then, we learn,'
that'
the
30 The Action ofn nt.
act1on ofl ight on,
allferric salts, under certain conditions, is
to reduce them to the ferrouss tate. I t may be remarkedthat in o rder to produce the requisite reduction, the presenceofo rganic matter, such as the siz e ofthe flpaper, with some ofthese iron salts seems a necessity ; ifthis be absent, the actionis
’very slow. And, again, the o rganic compound should beofsuch a nature that it is ready to combine with the atomsthrown ofi"
,in the same way as that already indicated for
silver iodide. There are a variety ofbodies which willcombine with these atoms ; but unfo rtunately, as a rule,they have a greater affinityfor the atoms than has the i roncompound with which they are only loo sely combined. The
o rganic matters with which they will combine without beingto rn away from the iron are rather slow absorben ts, and
therefore generally the sensi tiveness is no t great . For,as
with the s ilver iodide , the sensitiveness depends chiefly on
the readiness ofthe ne ighbouring matter to abso rb what isthrown off.
In o rder,then
,for iron salts to become as sensitive to
l ight as silver salts,some body must be found which,per se
,
will no t reduce them to the ferrous state or decompo se them,
yet which, when the atom i s liberated, will se iz e i t withgreater facili ty than any body with which we are as yet ac
quainted. As a rule, the development of these picturesi s carried out by either method 2 or 4 (p. the detail s ofwhich will be given in the section on printing with thesesalts . Since these compounds are comparatively but littlesensitive to light, they are chiefly used for obtaining posi tiveprints ; an expo sure in the camera to produce a deve lopableimage wo uld have to be very pro longed.
The same experiments carried out with regard to the
uranium‘
compounds give identical results . The uran iccompounds
‘
are reduced to uranous,and the methods of
development are similar.To the same class ofmetal s belongs vanadium,
the
interesting compounds ofwhich were investigated by . Pro
fessor Ro scoe. The reactions are similar to the above.
3 1.
Thelast me tall ic compounds to which we shall refer atlength are tho se ofchromium combined with the alkalis .
The salts found mo st sensi tive to light are the dichro
mates , though the chromates are also , to a certain extent,capable ofbe ing acted upon. Mungo Ponton first indicated
the principle which governs the ir employment. Ifa so lut ion ofa di chromate, such as that ofpo tassium, be brushed
over paper, and be allowed to dry, and be then expo sed
to l ight beneath an engraving, it will be found that in thoseportions corresponding to the white paper the orange co lourwil l gradually assume a delicate brown tint, whils t on the
parts shaded by the l ines the sal t remains unchanged.
The eye then at once tells that some chemical change hastaken place in
’
the chromium compound. Chemi sts areaccustomed to employ the dichromate to convert a ferroussal t into a ferric, and by having it in a so lution ofknownstrength, and ascertaining when the reaction i s comple te, theamount of iron in the ferrous so lution can be estimatedquantitatively. Thus We have, say, the amount offerrouschlo ride to test quantitativelv the amount is calculated byapplying the fo llowing equation
Ferrous Po tassium Hydro chloric Ferric Po tassiumChloride + Dichromate Acid Chloride Chlo ride6 F eCl2 KzCrZO, 14 HC], 3 FezCl6 2 KCl
Chromic Chloride WaterCr
2C16 7H20
I t will be seen that the po tass ium dichromate readily parts
with its oxygen and po tassium“
,and becomes converted into
a pure chromium compound. The change induced by the
light is analogous to this, there be ing every reason to be lieve
that the fo llowing equation is a type ofthe reaction, thoughcarbon dio xide may be a product to form potassiumcarbonate—a
Organic Matter Po tassium Dichromate Po tassium Hydroxidec , H, o , K
,c r,o , gKHOChromic Oxide Organic Matter
C1j202 C1.
3
32 Ttie TAction ofL ig/zt.
An analogous reaction of a chromium -sal t in ‘
the
presence ofan o rganic compound, without the impact of
l ight,i s found in chromium trioxide: Ifalcoho l be dropped
on these dry“
crystals, oxygen i s evo lved so rapidly that thespirit is ignited by the energy ofthe act ofcombination.
Now the dichromate contains less oxygen than the acid
(H2CrO4) fo rmed by the trio xide (CrOa), hence the evo
lation ofoxygen from i t i s l ikely to be less easi ly effected .
by organic matter thanfrom the latter. The swing causedby the waves ofl ight is sufficient to effect the change, that i sindicatedby the equation
‘
above . I t will be no ticeable that
not only i s the chromium compound altered in compo sition,
but that al so the organi c matter 18 deprived ofhydrogen ;and i t is the fact ofthis deprivation ,
or change in o rgani c
matter, that renders the dichromates valuable for pho togra
phic purpo ses . It'
will be found, after experiment, that“
the
dichromatised paper p repared as abo ve i s nearly insensi tivewhen mo i st, and that the image can be fo rmed mo st readily
when - it i s dry. The reason ofthis i s probably that whendry the organi c matter anddichromate fo rm a real compound,which i s , however, readily split .up on remo istening . If
,
however, the contact be long continued , an alteration in thepo sition ofthe atoms ofthe mo lecule s probably commences .Thi s might account for the insolubil ity of-
old carbont issue , and i t may
'
be presumed that the change which israpidly effected by light is much mo re slowly accomplishedby the longcontact even in the dark.
The deve lopment ofpictures taken on o rdinary siz edpaper is usually effected by metho d 4 (p. willbe no ticed when treating ofthe ani l ine proces s . When ,
however, the paper is coated with a layer ofo rgan ic matter,such as gelatine or albumen, the deve lopment ofthe picturemay be effected by methods 6 or 7. Co l lo idal bo die savailable for pho tographic purpo se s , when o xidised , are
changed in physical as wel l as in chemical propert ies .rst, they canno t after oxidation be disso lved by water, e ither
On Organic B odies. 33
ho t or co ld, though befo re oxidation they may be easi lyso luble . and, they will no t absorb water, and consequen tlywill no t increase in bulk
,if the impact of l ight be pro
longed . These modes ofdeve lopmen t will be entered intofully when treating ofthe carbon and co llo type prin tingpro cesses.
I t i s scarcely necessary to refer to the salts ofo thermetals ; they are mo stly too insensitive to the action of
l ight even for contact printing. Robert Hunt, in his excellent ‘Researches on Light
,
’ has en tered fully into the
phenomena observable with mo st ofthese compounds, andthe student should study that wo rk for further info rmation.
Ofo rgan ic bodies there are a varie ty wh1ch re spond to
the‘
chemicalvibrations . First andfo remost, as be ing ofprac
tical util ity,i s the substance known as asphaltum,
or bitumen
ofJudaea. I t is the substance which was first employed byNiepce for practical pho tography, and i t still retains itsplace amongst useful pho tographic compounds . I t is readilyso luble in a variety ofmenstrua, such as benz o le, chlo roform, and turpentine. After expo sure to light
,i t lo ses its
exce ss ive so lubil i ty, and it i s no t only poss ible, but prac ticable, to disso lve away from a thin layer ofi t all tho se portions which have no t been acted upon by light . For certainpho to -engraving and rel ief-printing pro cesses i t i s still em
ployed,on account ofits resistance to the action ofacids (see
p . It seems that during expo sure i t becomes oxidisedto a certain extent .
Amongst o ther sensi tive o rgan ic compounds may be
named the extracts offlowers and leaves and certain dyes .The sen31tiveness ofthe last has recently been found to beofuse in what is known as o rtho -chromatic pho tography.
Among gaseous bodies which are sensitive to light wemay name chlo rine, when expo sed in the presence ofhydro
gen. Ifin a dimly lighted room the proper combin ingvo lumes ofthese two gases be mixed in a glass bulb, or o therconven ient ho lder, and then expo sed to the direct rays of
D
34 T/ze Action ofL ig/zt.
the sun, or o ther strong source oflight which emi ts the shorterwave-lengths , i t will be found that they combine w1th explo
sive vio lence to fo rm an equal vo lume ofhydro'
chlo ric ac id .
In diffused daylight the combination takes place muchmore quie tly, and attempts have been made to util ise thi saction to measure the ‘ actin ism ofany l ight to which i tmaybe expo sed (see
‘_
actinometry The affini ty ofchlo rine forhydrogen i s so great that it causes a decompo s ition ofthe
water in an aqueous so lution, when expo sed to the l ight,though it has no power to do so ifkept in the dark.
The lates t discovery oflight causing a combinationbe tweena gas anda so l id is due to Mr. Franci s Jones , ofManchester.
He found that in Sunlight, ifsulphur was brought in con tact
with antimon iuretted hydrogen or s tibine , the o range sulphide
ofan timony was formed. The equation representing the re
action i s as fo llows
AntimoniurettedHydrogen SulphurAntimony Sulphide Hydrogen Sulphide
2 SbH36 S s ss 3 H,S.
With arseniuretted hydrogen (As H3) a l ike reactiontakes place .
1
After thi s briefresume ofthe sensitive compo unds thestudent will at once distinguish the advantage to be gainedby the employment ofthe simpler salts ofsilver for obtain ingimages in the camera . I t i s these alone which are suscept ibleofrapid deve lopment by exercising an attractive fo rce whenthe alteredmo lecules are few in number ; whils t all the o thercompounds require a
"
large number of part icles to be
changed in o rder that the image may be made visible at all.With the iron salts “
per se the developmen t by attraction may
be resorted to but i t will be found on experiment that theattractive fo rce i s so small that i t doe s no t nearly equal thatofthe silver compounds . Hence we may assert that, fo r producmgdeve lopable image s in the camera, the chiefpo rtion
of’the sensitive salt m'
ust consist ofone ofthese silver sal ts,
As beingsubstitutedfor Sb in the above equation.
On t/ze Support and Sabstratwn . 35
and that o ther metall ic salts can best be utilised for ob tainingimpressions by long expo sure, and are therefo re chieflyadapted for obtaining po sit ive proofs from negatives.
CHAPTER V.
ON THE SUPPORT AND SUBSTRATUM .
IN j udging of the kind of support on which to rece ivean image , whe ther i t be developed or fo rmed by the con
tinued action oflight, i t must be considered for what purpo se the image i s to be employed. If i t i s to be em
ployed as a screen, or a negative from which to fo rm a'
picture complementary to i t in photographic density and
po sition, then evidently the more transparent the supporti s,the be tter it will be for this particular
'
purpose. As a
rule,i t is only images impressed in a camera
'
which are
employed as ‘negatives,
’and as these may be said to be
invariably taken upon the sensi tive salts ofsilver, which are
easily acted upon chemically by extraneous matter, i t is
evident that a substance should be employed which is unaffec ted by them and by the agents which cause the developmen t ofthe image . In addition to this quali ty, a certain
amount ofrigidi ty in the support i s c onvenien t , though no t
essential , as the operations invo lved are ofsuch a character
as to cause this to be a desideratum. Evidently glass answersthe object mo s t tho roughly. Less fully does paper whenwaxed answer these requirements for with i t there is translucency and no t transparency, and none of the o therqual ities . A suppo rt ofco llodion and india-rubber com
bined, such as was at one time advo cated by Warnerke ,
answers to the first two requirements, and, since the operations invo lved in his method ofworking do not necessi taterigidity, 1t 1s a suitable one
36 On t/ze Snbstratznn.
To give a correct idea ofan image by reflected light, thatis,to look at it as a picture i s looked at, i t should appear
as highly co loured or as black as po ssible when contrastedwith the ground on which it rests . Fo rmed by development (after exposure in the camera), i t po sse sse s alwaysmo re or less dens ity, the densi ty approximately varyinginversely as the intensity ofthe reflected actin ic l ight whichhas acted on i t. If the developed image be of adark co lour, the proper effect of l ight and shade will bereversed. To give a correct representation, the depo si tmust be transparently zolzite, and the support dark-co loured
o r black . In the daguerreo type process the support i s ame tal plate, which is by contrast dark when compared with
the mercurially-developed image . In the co l lodion pro
cess, ifthe image can be made to appear whi te by reflectedlight
,the support may be ofglass , ifi t be backed with
some dark-co loured substance , such as black velvet or varn ish
, or it may be ofmetal darkened with some substancethat i s unaffected by the chemical agents employed (anexample of this we have in the ferro type plates). On
the o ther hand, ifthe picture be produced by a subse
quent operation from a negative, the image should be
as transparently dark as possible, and the gro und white .
In this case the support (unless o ther considerationsfo rbid i t) may be white paper, opal glass, or any o ther whitemedium. I t i s also wo rthy ofno tice that in o rder to produce a proper representation oflight and shade the groundshould always be in contact with the image. An exampleofa certain false gradation given in an image
,in which this
impo rtant rule i s neglected, i s to be found in the old co l lodionpositives on glass.Here tofo re no thing has been said regarding the vehicle
employed for ho lding the sens itive compounds in sitn on
the support, and thi s requires a detailed cons ideration. I tneed scarcely be said that some sort ofvehicle i s generallynecessary. The fact that mo st ofthe compounds employed
38 Tlie Daguerreotype.
l ight impacts upon surfaces : and as the surfaces ofs imilarparticles increase as the square ofthe diame ter, whilst their
masses increase as the cube, it i s evident the smaller theparticles are the larger will be the available area for the
same quanti ty. Co llodion i s a transparent, semi-viscousfluid
,made by disso lving pyroxyl ine (gun-co tton) m a
so lution ofe ther anda lcoho l, and is for o rdinary purpo sesto tal ly unacted upon by the sensitising so lut ion of silver
,
though , when specially prepared, it i s be l ieved that an o rganiccompound ofsilver is formed. Other advantages ofco llodionare to be found in the property it has ofsetting in a gelatinous
form, previous to its final desiccation, and also that the sol
vents used evaporise rapidly at o rdinary temperatures, leav
ing the salts, which are to fo rm the precipitate, ifno t inso lution, yet with the ir individual particles in such an ex
tremelym inute state ofdivision as to be undistinguishable
to the eye .
Aqueous solutions ofgelatine and albumen are equallygood so lvents ofthe salts alluded to. When, however, gelatine . i s employed, as in the gelatino-bromide process, the
direct combination between i t and the silver i s avo ided .
CHAPTER VI.
THE DAGUERREOTYPE.
UNDER the head of silver processes, the first that would
naturally occupy the attention is that due to Daguerre . I tis even at the present date adoptedfor some kinds ofwork ;for instance i t was employed by the French expeditions, sentto observe the transi t ofVenus in December 1 874.
The daguerreo type pro ces s consists , as already stated , ofthe formation ofa sensitive surface ofsilver iodide, o r s ilveriodide andbromide, on a si lvered plate, bymeans ofthe direct
M anipulations. 39
action ofiodine, or iodine and bromine . One ofthe mo stdifficult (and diffi cult only because the greatest cleanl iness inevery de tail i s required) parts ofthe who le process is the pre
paration ofthe silvered surface befo re i ts coming in con tactwith the halogen. The plates are usually copper, on which afi lm ofme tall ic s ilver i s depo sited by the e lectro -plating process, andwhen they leave the silvering so lution have, as a r ule ,a fro sted appearance . After be ing cut to the proper siz e , andthe corners clipped ofaboutgth ofan inch for conven ience’
sake, they are ready for po lishing. A plate in this statemay be placed on a flat table , andfour thin strips ofwoodnailed round it to prevent i t sl ipping. To the surfaceis then appl ied tripo l i powder in alcoho l with Cantonflanne l , and worked about to such an extent that it i s perfectlyfree from scratches, and i s fairly smoo th . The nextoperation consists in po l ishing it. This is effectedby means ofa buff. That which the writer has found effective i s madeby enclo smga wooden ball, ofthe siz e ofa small apple , in askin offe l t and then ofco tton woo l. Over this is stre tcheda piece ofthe finest Chamo i s leather. On to the surface of
the silver is then scattered a small quantity ofj ewe ller’srouge
,and the buff is caused to travel over the plate from
end to end and side to side alternately t ill i t become s ofthehighest po lish. This po l ishing should take place .almost im
mediate ly befo re the sensi tis ing operation i s commenced,o therwise there is a liabil ity ofthesurface
“
attract ing impurities fromthe atmo sphere. To sensitise theplate two sensitis ing boxe s are
required . A11 illustration ofthatemployed when dague rreo typewas commonly practised for po rtrai t work will give an ideaofthe sort of contrivance required . On the bo ttom of
the box c is placed iodine in powder A i s a piece ofcardboard, which fits into grooves -as shown. B B are the sup
Frc . 8.
40 Tlce Dagzcerrcotype.
po rts on which the silvered plate i s to'
rest . 1 The iodinewill vo latil ise at o rdinary temperatures, and condense on
the surface ofthe cardboard next to i t . When a platei s to be sens it ised the cardboard is reversed, and the iodinevo latil ises from the top surface on to the silver plate. The
plate gradual ly rece ives a thin coating ofiodide, passing
through various stages ofco lour. When a ruddy co lour isreached , i t i s placedin a similar box to that already described
(omitting the cardboard), at the bo ttom ofwhich is a mixture ofbromine and calcium hydrate . The bromine attacks
the surface, and with the iodide fo rms silver bromo - iodide.
When the surface assume s a steeLgray o r vio let co lour theplate i s remo ved, and once m ore placed in the iodine boxfor a third ofthe time o riginally necessary. In this statethe plate i s exceedingly sensitive, and i s ready for expo surein the camera. The expo sure may be made at once, or i tneed not take place for several hours ; C laudet, in fact, foundthat
,by keeping, the sensitiveness increased. The time
necessary to impress an invisible but developable image i svery short, afew seconds be ing all that i s necessary . Practicealone can tel l the exact time required, but it i s so on learnedapproximate ly after a few trials . The development i s ac
complished by exposing the impressed surface to the vapour
ofmercury . A cast-iron tray, with wooden sides and l id ,i s conven ien t : i t may fo rm a box similar to that shown
for the iodi sing operation. At the bo ttom i s placed a thin
layer ofmercury , the temperature ofwhich is raised to ’
about
1 50° Fahr. The plate i s placed in the box, face downwards ,
on the suppo rts, and the development is allowed to pro ceed ,the process be ing watched as it progresses by inspecting i tfrom time to t ime in a non-actinic light. Ifthe expo sure beright the image will be brillian t, ifunder-exposed i t will beweak ; whil s t ifover-expo sed it will be covered wi th -a ve i l
ofmercury.
1 When smaller siz es are to be used theymay be held in framessimilar to the inner frames ofa camera slide.
[nte/zsification ofclze Image. 4 I_
The development, it wi ll be remarked, i s due to the
attraction ofthe subiodide for the metalli c mercury vapour,and to no o ther cause. In order to fix the image the platei s immersed in a 10 per
'
cent. so lution ofsodium hyposulphite . After a few seconds the unalte red iodide Ag2l2and the AgI ofthe subiodide (Ag21) are di sso lved away, andthe image i s left as a white amalgam ofmercury and
s ilver on a darker co loured background. After a tho roughwashing in disti lled water the picture i s permanent, but itsappearance may be improved by ton ing i t i.e . intensifyingi t wi th go ld to darken the si lver, and render the amalgamstil l purer in co lour. This i s accomplished by pouring overi t,in such a quantity as j ust not to run over the edges
,
1 . Gold trichloride ‘
I gramme.
Distilledwater 50cc .
2. Sodium hyposulphite°
4 gramme.
Distilledwater 50cc.
The two solutions are well mixed together, and, afterflowing them on the plate, a spirit- lamp is moved aboutbeneath its bo ttom surface
,until the ton ing action com
mences. The mo re( rapid the depo sition ofthe go ld
, the
mo re satisfacto ry the image . When comple te, the platemust be we l l washed in a dish ofco ld water, and finallyrinsed with di stilled water. Drying is best accomplishedby gentle heat; applied first at one end, and gradually moveddown. Any large drops ofwater should be absorbed byblo tting-paper.
Daguerreo types may be reproduced by electro typy, ifthe plate be immersed almo s t immediate ly after toningin
the copper so lution. The o rdinary e lectro typing processanswers every purpo se : for the detail s
,reference must
be made to books treating specially ofthe subject. The
fact i s mentioned here , as i t shows that the image, afterall the se operations , i s in 7elief, though naturally to aVery limi ted extent
, yet stil l suflic lently to cause the
reflected light to give all the necessary gradations oflight
42 me Collodion Process.
“
and shade.i
sir W. Grove also introduced a method of
e tching daguerreo type plates by means ofthe battery. Heimmersed the plate in a so lut ion ofhydro chlo ric acid two
parts,and water one part, and oppo sed by a platinum
plate placed at °
z inches from i t. When the current wasgenerated by a couple ofGrove’s cell s, an oxy
-chlorideof silver was formed
,and after thirty seconds the plate
was found to be suffi ciently bitten. The oxy-chlo ride
was removed, and for fi ne wo rk was found ofSufficien t
depth to allow it to be printed from with prin ter ’s ink in theprinting-press. This process has not come much intovogue , as it is one which i s too fdelicate for o rdinary operations
,and the silvered copper i s expensive in comparison
with the o ther me tals employed for the purpo se . The mo st
recent development ofpho to -engravmgand the productionofrel iefs are described in a subsequent chapter.
CHAPTER VI I.
COLLOD ION.
PYROXYLINE i s prepared by acting upon co tton,paper, or
o ther kindred substances with a mixture ‘
ofn itric and
sulphuric acids. For an example ofthe process we may takeco tton ,
“
which has a defini te formula ofC Sulphuricacid has the property ofabso rbing water from any o rgan icsubstance with which it is in contact ; for instance a drop of
oil ofvitrio l on clo th or paper rapidly chars it,owing to the
destruction of the constituen t atoms, through i ts affin i tyfor water. Thus, ifwe take the co tton i tse lf, it
“
w ill be seenthat each mo lecule contains 6 equivalents ofcarbon,andj ust sufficient hydrogen and oxygen to fo rm 5 mo lecule sofwater ; the oil ofvitrio l is thus capable ofSpli tting upthe mo lecule ofco tton, appropriating the 5 mo lecules of
Tbe Cnefnistry of‘
Gnn Cotton. 43
water, and leaving the carbon behind.
1 Ano ther good ex
ample ofthe abstraction ofan equivalent ofwater froma mo lecule i s in that ofe thyl alcoho l , or spiri ts ofwine .
Ifthi s be distilled over in the presence of concentratedsulphuri c acid, we have e ther 2 as the product. When the
acid i s diluted with water, it s destructive power is limi ted,though as the water evapo rates from i t the power returns .Now the strongest ni tric acid which is usually obtainable
contains a large proportion ofwater. Thus nitri c acid,if
at 60° F. contains only 84 per cent . ofHNO3 , hencei t is that when this is mi xed with sulphuric acid, the water i sabstracted from i t, and the true ni tric acid (HNO3 ) i s left toact on any body with which it is brought in contact. Thi si s undiluted, and is c apable ofacting on co tton in a some
what peculiar way. It abstracts ei ther 2 or 3 atoms ofhydrogen
’
(according to the strength ofthe acids employed,and the temperature), and replaces them by 2 or 3 mo leculesofnitrogen tetroxide (N02) wi th the formation ofwater.
The fo rmula stands thusWater combinedwith the
Co tton Nrtn c acid Sulphuric ac1d nitric and sulphuric acidsHNo , H,so ,
Aq.
Water fo rmed by the decomposiPyro xyline + Sulphuric acid+ tion of the co tton, and that
combinedwith the acids
s O,2 H2O ‘1' Aq.
Co tton Nitric acid Sulphuric acid Water Gun Co ttonCGHWO5 + 3HNO3 HZSO, Aq
Sulphuric ac1d Water Water.H.SO, 3H20 Aq.
1 It 15 for this reason that ifthe most dilute sulphuric acid be spilton the clo thes, or passed through a filter-paper, and be allowed to
dry, a charring takes place . In the first case neutralisation With an
alkali, or in the second very thorough washing, will prevent thedisaster.
2 Hence the name sulphuric ether. The fo llowingequation exhibitsthe reaction .
Ethyl alcohol Ether Water abstractedby the sulphuric acid.CgII lzo z G
iHXOO H
zo
44 Collodion Processes.
The first be ing the gun-co tton ,as used for co llodion
,
and the second be ing the we ll-known explosweI
com
pound . I t will be no ticed that the sulphuric acid remainsunaltered
‘
in composmon, i ts so le function be ing to ab
so rb the water formed by the operation . The fact oftheexistence ofthe tetroxide ofn itrogen in the al tered co ttoncan be demonstrated in i ts combustion in an exhaustedglass vessel by the red fumes which tinge the gaseous products.
The same reaction as the above can be obtained by em
ployinggpotassium n itrate (KNO3 ) instead ofthe n itric acid
,
though in thi s case a portion ofthe sulphuric acid becomesconverted in to po tassiumsulphate. Thus
Potassium nitrate Sulphuric acid Potassium sulphate Nitric acid.
KNo , KH(SO,) HN03 .
The above equations represent the react1on that theoretically takes place when co tton i s treated with nitri c and
sulphuric acid in the above proportions,but there are o ther
po ints to be attended to in practice.
The proportion ofthe acids to each o ther materiallyaffects the properties . of the pyroxyline. Sulphuric acidparchmen ti ses paper when it i s immersed in i t or floated ini t, that is , renders it tough and ofclo se texture . The chem1
cal effect produced by the sulphuric acid is hardly known, but
ifpro longed, i t is known that the paper i s dissolved. Parchmentisedpaper treated with n itro -sulphuric acid has differen tqual i ties to that in which the parchmenti sing is omi tted. Wi ththe fo rmer a tough co llodion results, though it i s mo re powdery . An excess ofsulphuric acid beyond that necessary toproduce the reaction shown in the equations acts in a
'
similarway to treating the co tton first with the acid
,for it partially
parchmentises the co tton p revious to i ts conversion in topyro xyl ine , and as this is beneficial to a co llodion,
when no t
carried beyond proper l imits,an excess ofthis acid is always
employed.
The amount ofd ilution of the acids with water also
46 Collodion Processes.
The n i tric acid and water are first poured into a strongglaz ed po rcelain dish, and well mixed, the sulphuric acid i sadded last, the liquid be ing kept wel l stirred as it is pouredin. The temperature wi ll generally rise to 75
°or 85
°
(iftothe latter, i t may be suspected that the acids are to o dilute),and i t must then be allowed to coo l gradually to A
do z en ball s ofco tton woo l, 1 weighing about I grammes each ,having been prepared, should be immersed separately inthe fluid, and after thorough soaking (assisted by a glass orporce lain spatula, fig. be allowed to remain at the bo t
tom ofthe vessel. The immersion should take p lace rapidly,
o therwise decompo sition takes place , and this, when oncecommenced, will cause the temperature to rise rapidly, andthe who le ofthe co tton will be disso lved with the evo lutionofnitrousfumes. The balls must be left in the acid from ten
minute s to a quar ter ofan hour,and they are then presum
ably in a state ready for washing. The longer the immersion, the mo re l ikely are they to become inso luble in e therand alcoho l , approaching more nearly the state ofexplo sive
gun-co tton. They are next raised by the spatula, the excess 2
ofacid as far as po ss ible squeez ed out ofthem against theside ofthe vesse l , and thep they are dashed in to a vesselho lding a large quantity ofwater. All traces ofthe acids
are el iminated by washing in frequent changes ofwater, or,be tter still, in running water. To test when thi s i s comple te,a piece ofblue litmus-paper should be pre ssed again s t thewet co tton,
and ifafter two minutes it remains unaltered -it
may be assumed that the washing is comple te . The pyroxy
l ine should now tear easily, and no t be readily separableinto the o riginal balls, and should weigh about 30grammes.Ifthe o riginal fibre be easily di stingui shable , the tempe ratureprobably fe ll during the operation, or sufficient water ‘
was
The co tton should have previously been well steeped in soda andwater, and be then thoroughly washed and completely dried.
2 Ifthe precaution be no t taken ofsquee z ing out the acids, thereis a great probability ofa solution ofa portion ofthe cotton takingplace.
Manufacture ofPyroxyline. 47
not added. Ifthe we ight fall much'
below that indicated,thewater was probably a l ittle in exces s, or the temperaturewas too great. I t canno t be to o strongly impressed uponthe student that the strength ofacids i s al l-impo rtant
,and
ifthe amount ofwater present with them be above thatindicated, that
.
so much water must be deducted from thatgiven in the fo rmula. A specific gravity bo ttle i s a veryconvenient means ofascertain ing the strength ofthe so lution,
for,the specific gravity once known, the amount .oftrue
acid present can be foundfrom the tables given in the ap
pendix Other me thods for ascertain ing the specific gravitywill be found in mo st wo rks on Chemistry.
The next fo rmula for preparing pyroxyline ofthe samecharacter i s given without comment, as the above remarksapply to i t .
Sulphuric acid, I 8 42 170cc.
D ried potassium nitrate I 10grammesWater 283 cc .
Best dried cotton woo l 4 grammes.
Hardwich states that the chances offailure with thispro cess are very slight ifthe po tassium n i trate be not too
much contaminated'
with po tassiumchloride . In the aboveoperations a thermometer i s abso lu tely necessary. I t shouldno t be mounted in wood, but should be graduated on the
s tem i tself. I t may be suppo rted in a clamp , as shown inthe figure. For dry processes the fo rego ing fo rmulae givepyroxyl ine , which some consider as too tough and ho rny,and some ho ld, though the writer does not, that
.
this ise specially the case for pro cesse s where the sens itive sal tofsilver is fo rmed in the co llodion i tself(see chap. xvi ). A
modification in the propo rtions ofacid and Wa ter can be
made to suit those who prefer a mo re l impid co llodion. I t
has also been found by some wo rkers that in the latterproces s the presence ofa li ttle n i tro -gluco se i s a desideratun1
The po tassium nitrate sho uld be dried at a temperature ofabout1 20° Placing it in an air-bath is the most convenient method ofobtamvingthe temperature.
1
48 Manufacture ofPyronyline.
The fo llowing method secures its fo rmation thou gh, iftheresulting pyroxyline be -well washed, it is in a great measure
el iminated. I t would seem better to add the n itro -gluco se
to the co llodion, b ut as this has no t been e s tablished fromlong experience
,i t has been thought be tter to give the pro
cess as published by M . Leon Warnerke, in a commun ication to the Pho tographic Socie ty ofGreat Bri tain. Six
grammes ofthe finest co tton-woo l are put in to a porce lain j ar,and 2grammes ofgelatine disso lved in the smalles t quanti ty
F IG . 9 .
ofwater are added . The co tton i s impregnated with the
ge latine by pressing it with a wooden spatula, and when
this i s effected the co tton i s carefully dried by the aid of
heat . I t i s then ready for immers1on 1n acids which are of
the fo llowing strength
Nitric acid, p
WaterSulphuric acid,
Nitric acid, 1 4 2WaterSulphuric acid, 1
‘840
Soloents ofPyroxyline. 49
The acids andwater are mixed in the o rder named, andwhen a steady temperature of70
° i s obtained the gelatini sedco tton i s immersed in i t for twenty minutes . Wi th someco tton the
,
amoun t ofwater given above i s inadmissible,as
i t immediately disso lves . The propo rtions ofacids should
be kept, and the water diminished to such a degree that theso lvent action is reduced. After washing and drying
, the
resulting pyroxyline wil l be found to have lo st considerably
in we ight, and i t should be almo st powdery in appearance ,and readily dis integrable . I t will be found highly so luble ina mixture ofether and alcoho l , and as much as 2 per cent ,may be required to give a suffi cien t body to the co llodion.
H i therto co tton has alone been mentioned as capable offo rming pyroxyline but it may be stated that every analogous substance may be simi larly treated. Thus linen and
paper are amenable to the abo ve treatment, andfor somepurpo se s they give superio r results for ins tance, Whatman
’
s
drawing paper has been found by Wam erke to give betterresults than the gelatini sed co tton in the last process . Be ingalready siz ed with gelat ine , there is no need for the prel iminary treatment po inted out.
The action ofthe so lven ts employed in the co llodion on
the pyroxyline deserve s a passing remark,as many modifi
cations in the resulting film can be caused by judiciously
varying the ir propo rtions . The specific gravity of the
alcoho l employed should invariably be ascertained , as thecondition ofthe sensi t iveness ofthe plate depends muchupon i t s strength. With a. co llodion made at a low temperature, the presence ofa certain percentage ofwater is advisable, as i ts ho rny nature i s thereby modified, and a certaindegree ofporo sity obtained. A specific gravity of8 20 i sin this case admi ssible . W i th the pyroxyline such as thatobtained by the last formula, the water should be a min imum,
as it i s already po rous , and the presence ofwater i s aptto make i t ret iculated and ro tten. The specific gravi ty inthis case should rarely be over 8 1 2. An excess ofalcohol
E
50 Collodion Processes.
also tends to give poro sity, and therefore sensitiveness ; but if
the addition be carried to an extreme , the very po ro s ity diminishes sensitiveness, as the sens itive salts fo rmed in the filmcoagulate into too large particle s. Alcoho l also dimin ishes therapidity ofsetting. Ether, on the o ther hand, tends to clo sethe po res ofthe film, as is demonstrated by coating a platemade with an excess ofi t, when it will be found that a con
traction takes place,causing the film to leave the edges of
the plate . or to spli t on drying. The e ther employed should
be as pure as po ssible (thi s is no t ins isted on by manufac
turers ofco llodion) as o therwise it is apt to l iberate the
halogen from the disso lved salts , giving rise to an alkalinereaction which is one cause ofro ttenness in the film,
andan
apparent want ofbody in collodion.
l
The fo llowing are co llodionsfo r different processes.
For the wet process :
No . 1 . Pyro xyline , Hardwick’s formula 12 to 14 grammes
Alcohol, 8 20 450cc.
Ether, 725 550cc .
No . 2. Pyro xyline , Hardwick’s formula 12 to 14 grammes
Alcohol , 8 20 500cc .
Ether, 725 500cc .
No. 1 i s most sui table for cold, and No . 2 for warmweather.
For dry processes wi th the bath
No . 3. Pyro xyline , first formula 10to 12 grammesPyro xyline , last formula 4 grammesAlcohol, 8 13 o r '8 14 . 500cc .
Ether '
725 500 cc.
Water Q uant. suff.
The water i s shown in No . 3 to remind the student i t puts apower into his hand ofmodifying the co llo dion in stru ctureby its addition. I t frequently happens that No . 1 or 2
l The student would do well to try the e xperiment ofaddingasmall quantity ofcaustic potash to a phial ofcollodion, and no tingtheaction that takes place.
On Me Salts dissolved cu Collodion. 51
fo rmula may also be improved for dry processes by attendingto the amount ofwater present.
The next po int to be de termined i s the amoun t of
bromide and iodide to be disso lved in the co llodion, and todetermme the ir proport1ons 1t will be wel l to enter intodetail as to the ir behaviour when converted into the silvercompounds and expo sed to the l ight. Iodide ofsilver in afilm i s capable offorming a dense image with a sho rt ex
po sure, but the gradations in density are often wanting whenthe l ight is extremely bright ; added to which
,ifo rgan ic
matter be p resent with it even such as is to be found inmany col lodions , the picture i s apt to be veiled and wantingin vigour.
Bromide of silver, on the o ther hand, i s especiallyadapted for tho se co llodions which have an o rganic re
action. I t has usually been accepted that the iodide is themo re sens itive ofthe two salts
,but recent investigations
tend to show that the bromide has the advantage, bo thas regards sensitiveness and delicacy, when developedby method 3 (p. The failure ofthe bromide when
developed by method 2-
(p. which is wet-plate de
velopment, consists in its comparative insensitiveness to
very faint light as found in deep shadows . A bromo -iodide
ofsilver, however, combmes the advantages ofthe bromidewith that ofthe iodide for the wet process and certain of
the dry processes it possesses every essential quality for theproduction ofa good picture . The propo rtions ofbromine
and iodine in combination vary considerably, from 1 partofthe former to 10parts ofthe latter (which is j ust suffic ient to secure cleanl iness and freedom from veil with allo rdinary preparations of co l lodion and bath) to 25 partsto 1 . The latter proportion i s never employed except indry-plate processes. The iodide i s usually fixed at aboutfrom 6 to 10gramme s per l itre. The sensitiveness ofthesurface in all cases depends on the mode ofdeve lopmentemployed. Thus for a wet plate, Vogel has found that
E 2
52 Collodion .
the proportion of iodine to bromine should be about 4 to‘
r
to secure the greatest sensitiveness, whils t with the alkaline method it is dimini shed to the smaller proportion, ori t may be omitted altogether when a dry process is inquestion. At present we are considering the wet pro cess,and not the more modern dry processes where the conditionsare different. Themetal with which the iodine and bromine
are combined when introduced into the co l lodion serves toexercise a great influence on the sensit iveness ofthe surface . Not long ago Warnerke has stated that the metalscombine with the pyroxyline and fo rm compounds who secompo sition i s as yet undetermined,
-and thus the differencein structural effect and visco si ty exhibited be tween two :
iden tically similar co llodions when iodised with a cadmium
andan alkaline salt maybe accounted for in a great measure.
For experiment, i t will be advantageous ifthe student iodisetwo portions ofco llodion ; one with 4 grains ofcadmiumiodide, and the o ther with 4 grains ofpotass1um lodide , and
no te the difference in their behaviour when poured on aplate . With the latter he will find a free ly flowing fluid ;with the former one which is more glutinous, and difficultto manipulate.
I t also appears that the different me tal lic sal ts in so lution cause different degrees of sensitiveness in ‘ a film.
This has been investigated by Warnerke, who placesthem in the fo llowing o rder for imparting sensit ivenes s andintensity
Order ofsensitiveness Zn Cd Na F e NH,K U
Order ofintensity ofimage Zn U NH, Cd Na K F e
The alkal ine iodides are tho se which are,mo st prone to
decompose under the action ofether, particularly if i t bemethylated, hence, for a co llodion to keep long, it i s mecessary that the purest fo rm be employed . As before shown ,
when the iodide i s decompo sed, the alkal i decompo se s thepyroxyline
,rendering i t very fluid and defective in setting
54 Collodion Processes.
No . 2 i s suitable for dry-plate wo rk and for interiors,
but as a staple article No . 1 is recommended.
For a simple iodised co llodion the fo llowing formula
may be adopted :oNo . 4 . Ammonium iodide
Plain collodionOr,No . 5. Cadmium iodide 10grammes
Plain collodion 1 litre
No . 4 should be used immediately after making, whilst
No . 5 will keep almo s t indefin itely
The next fo rmula is for a simple bromi sed collodion
No . 6. Zinc bromide 16 grammesPlain co llodion 1 litre
For all the above iodides and bromide substitution maybe made with o thers
,and i t by no means fo llows that tho se
cho sen as examples will prove the mo st sensitive , thoughexperience has shown they give good results . I t i s customary in preparing plain co l lodion to omi t half of the
alcoho l , and to employ that halfas a so lvent for the halo idsal ts . Thi s is conven ient but not abso lute ly necessary. I t is agood plan to make a no te ofthe date ofthe manufacture ofthe co llodion,
as also ofits iodising ; useful info rmation i soften given by such memoranda.
TestingPlain Collodions.
Plain co llodion should be tested befo re lodising, and thefo l lowing tests may be applied, reco llecting that a film thatmay no t be suitable for the bath process may still be su itablefor an emulsion pro cess , and vice versa.
Coat a plate (in the manner described at p. and
ascertain ifwhen dry the film dry dead whi te , opalescen t , o rtransparent. Ifthe first
,i t is unsuitable for any proce s s ; if
the second, i t may be employed for emulsion wo rk ; whils tifthe third, i t may be suitable for any process .
Coat a plate, and, after the co llodion has set, mark ifi t
Cleaning tlze Glass Plate. 55
i s : powdery to the touch, or ifon applying the finger itcomes away in strips . Ifthe fo rmer, i t maybe good for dryplate wo rk
'
; if the latter, for bo th dry plates or the wet
process .Coat ano ther plate , and
, after setting, wash the film
under the tap ti ll all the so lvents are washed out,and no te
ifit take an even film ofwater or ifi t repel s i t at parts . If
the latter it is too ho rny to use in the bath processes ; alittle po tassium carbonate may improve it.No te ifthe co llodion flows freely, viscously, or lumpily.
Too limpid a co llodion will fail to give density ; too viscous-a co l lodion i s unsuitable for any but small plates , whilst alumpy co llodion will give i rregular images. The flowingqualities of a co llodion arising from the pyroxyline mayoften be co rrected by altering the proportions ofe ther andalcoho l .
Ifthe film be reticulated, havingmarks l ike a crape patternon i t, the so lvents may no t be sufficiently anhydrous
,or the
pyroxyl ine may be in fault, as befo re stated.
The co l lodion should also be tested after iodising; thedefects wil l be no ticed when treating of the defects innegatives produced by the various pro cesses.
CHAPTER VII I.
CLEAN ING THE GLASS PLATE.
THE plate, before be ing taken into use, should be mostcarefully cleansed from dirt ofany description. The success
ofa pho tographer may be said to depend in a great measure
on the effectual manner in which he completes this operation. The dirt that i s to be looked for on a glass plate i sthat due to the manufacture , that due to subsequent ex
posure to the atmo sphere and to the hands ofthe packers,
56 Collodion Processes.
and some times that due to the chemical compounds withwhich i t may have been in contact. Ordinary plates aresome times found to be gritty on what should be the po lishedsurface
,and the application ofacid may disso lve the grits
away. Hence it is a good plan to treat all new plateswith a so lution ofdilute n itric acid (10 parts ofwater to 1
ofacid). This will not rid them ofmechanical dirt,such
as dust o r grease. The presence ofdust i s readily ao
counted for, but the o rigin ofthe greasy matter i s far mo re
difficult to understand. Ifa plate that is tho roughly cleaned
be put away in a plate boxfor a few days, and be then exa
mined by breathing on i t, i t will be found that it shows signs
ofrepell ing the aqueous vapour from the breath in certainparts
,and that a subsequent cleaning ofthe plate i s neces
sary to render i t fi t for use. This phenomenon can be
accounted for on the suppo sition that organic matter ofa
fatty nature i s to be found in the atmo sphere, and when we
remember that the lungs expire not only carbon dioxide,but also various o rganic matters, we should expect that in
an inhab ited house this latter might condense on some
dry coo l surface . The danger ofusing plates on which
this deposit exists will be apparent by a simple experiment .
Rub a warm finger or hand over the plate, coat with co llo
dion,sens itise , but do no t expo se to light then apply the
developing so lution and watch the result. I t will be found
that where the contact has beenmade, a reduction Ofme tallic
silver will take place , and as developmen t pro ceeds a dark
stain will be produced. Imagine a similarly treated plate ,
prepared as before , expo sed in the camera and deve lopeda dark depo sit will take place bo th where the hand has
touched and also where the invisible image has been im
pressed. It may be said that all animal o rgan ic matter has
the property ofcausing a tendency for metall ic silver to be
reduced from the so lutions ofi ts sal ts. A similar remarkapplies to the mercury compounds which sometimes get in
visibly reduced in the surface ofthe glass . The composif
Detergents. 57
tion ofdust is ofa mo st varied nature , and no t unfrequen tlyconsists offerric oxide
,sodium chloride, and o ther earthy
constituen ts. The reduction ofsi lver ni trate in the presence
ofsome ofthese would be certain.
Alkal is have the property of converting greasy intosaponaceous matter, and spirits ofwine will di sso lve bo thsoap and grease ; hence bo th are employed as detergents.Mechan i cal dtrt requires friction to remove it, and this should
be j ust sufficient for the purpo se, yet not enough to inj ure
the surface ofthe glass. Such bodies we have in tripo l ipowder and rouge. The fo rmer i s recommended on account
of i ts be ing less gritty than the latter. The mo st common
cleamngso lution i s made as fo llows
Spirits ofwine 50cc.
Tripo li powder z— Q uantity sufficient to make a thincream
Ammonium hydrate. 1 cc .
Mr. Warren De la Rue for his astronomical pho tographyemployed a so lut ion ofpo tassium dichromate and sulphuricacid. This is doubtless a mo st effective detergent, but theuse ofsulphuric acid is open to objection on account ofthe
damage i t may do to the dres s or hands.The writer has heard ofa pro cess of clean ing recom
mended, in which i t was propo sed to employ po tassium
cyan ide, fo llowed by nitric acid. The student i s earnestlyrecommended no t to attempt this plan, as it i s po i sonousand highly dangerous (see p .
Bo i l ing the glass plate in caustic soda or po tash has alsobeen proposed. This is apt to inj ure the surface ofthe
plate, owing to‘
the slight so lubil ity ofvitreous matter in
so lutions ofthe caustic alkal is. Perhaps no mo re effectiveme thod for securing a clean plate can be adopted than
by firs t treating the plate with a co ld so lution ofcausticpo tash , rubbing it well in with a rag, and then immersing itin dilu te ni tric acid and washing under the tap . A finalthorough rinse in dist illed water, and a rapid drying in a
58 Collodion Processes.
water-oven, will leave the plate in as clean a state as can bedesired.
SENSITISING BATH.
The sens i tising so lution, that is , the so lu tion m which
the co llodion containing the so luble iodides or bromides, or
bo th, are immersed in o rder to fo rm the iodide bromide, or
bromo - iodide ofsilver, may be said to be invariably madeofsi lver n i trate disso lved in water. The purity ofb oth con
stituents i s of the highest importance,as any extraneous
matter may be fatal to obtaining good results in deve lopment. Distilled water i s naturally the purest fo rm ofwaterthat can be obtained, but even thi s i s sometime s contamimated with o rgan ic matter in so lution, which i s apt to
react the sensitive salt. The manner in which am
FIG . 10.
monia 18 carried over wi th the aqueous vapour i s wel l knownto any chemi s t, and in a similar way hydrogen sulphide can
be carried o ver. The latter contamination i s mo st hurtful tosensitiveness , and the fo rmer might cause fog. I t maybe useful to po in t out the best mode ofdistill ing Water in a smallway, in o rder to obtain abso lute purity.
Aglass re to rt i s always clean, and dirt can be more
readily seen than ifi t be ofmetal . The fo rm known as
Liebig’
s condenser i s therefo re recommended instead ofthe
o rdinary still. The water shouldbe placed to the leve l‘
ofthe
flask A shown in the diagram,and a little (say a gramme
to a litre ) caustic po tash should be disso lved in it.‘ This
will free the water of any ammoniacal compounds whenwarmed. The distillat ion take s place through the glasstube, b, round which is placed aglass jacket, c, containingwater. Co ld water is al lowed to enter the j acket by thetube , d, and the heated water is carried offby e an universalclamp, B, is u seful for ho lding the condensing apparatus inpo sit1on.
The first 50 cc. ofeach l itre distilled should be rejected,and the disti l lation should no t be continued beyond thatpo int where 100 cc . are left in the retort. The distillatemay then be considered to be pure enough for pho tographicpurpo ses . Ifan o rdinary wo rm still be employed, careshould be taken that the wo rm i s clean,
free from dust, andno t oflead. The water should be distilled over as before ,the first and las t portions be ing rej ected . Ifdis tilled watercanno t be obtained for making up the so lution, spring water,ifno t impregnated with sulphates, will generally answer.Fail ing these, river water, and lastly rain water, after twicefil tering through charcoal, must be reso rted to . At first i tmay seem strange to place rain water last on the l ist, buti t should be remembered that i t i s almost invariably col
lected from the roofs of houses, and i s con sequentlysure to be con taminated with o rganic matter
,and also
inorgan i c matter. Rain water,“
ifi t could be co llecteddirectly as i t falls , would save the necessity for usingdistilled water. A method ofpurifying o rdinary water forbath purpo se s i s as fo llows . Bo i l and fil ter i t, add a l ittlebarium n itrate to i t. and see ifi t turns milky. Ifsuch bethe case, add a small further quant ity, together with a fewcrystal s ofsilve r nitrate to each l i tre ofwater, and place inthe sunl ight. After a few hours’ expo sure, the organic
60‘ Collodion Processes .
matter and sulphates will be at the bo ttom of the con
taining vessel, and the supernatant water may be decanted,syphoned, o r fi l tered off. An excess ofbarium n itrate i snot hurtful to the solut1on. for, as will be seen at p. 62
,i ts
addition i s recommended .
Nitrate ofs ilver should be pure . The uncrystall isedwill be found suffi ciently free from n itric acid to be availablefor fo rming a bath so lution needing no do ctoring. I t i ssometimes adulterated ifany suspicion ofthis arise
,a cer
tain known quan tity of the crystal s should be di sso lvedup in water, and the amount ofsilver n i trate really present calculated
'
by any of the methods usually adopted .
Si lver n itrate is readily so luble in i ts own weight ofwater,but this strength would be quite unsuitable for a sensit is ingso lution for two reasons : first, silver iodide i s so luble to
a certain extent in silver nitrate so lution. The stronger thelatter, the greater the amount ofiodide disso lved. A varia
t1on ln temperature also affects the quantity capable of
be ing he ld in so lution. Now,even suppo sing that at the
temperature at which,the bath was fo rmed immersion ofan
iodised plate took place,the heat evo lved in the act of
combination be tween the so luble iodide and the si lvern itrate to fo rm the sensitive compound would be suffi
cient to cause the iodide in the film to be partially dis
so lved out. S econdly, the fo rmation ofthe iodide Wo uldbe so rapid that there would be a co arseness in the
particles unsuitable for rapidity. Sutton has demonstratedthat where any iodide i s in the so lution,
10per cen t . i s asgreat a strength as can wel l be managed, whilst a 5per cent .so lution IS the l imit in the o ther direction. When brom ide salone are employed
,the strength may be 1 5per cent , as the
silver bromide i s almo st inso luble in s ilver n itrate so lut ion.
In preparing a bath it is general ly saturated wi th s ilveriodide, to preven t the silver n itrate di sso lving away por tionsofthe sensitive surface . Some skilled pho tographe rs , how
ever,prefer the saturation to take place from the film
62 Collodion Processes.
there i s danger ofcompounds fo rming, which wil l combinewith i t, and finally cause decompo si tion be tween the new
compound and the silver salt .As the bath so lution gets wo rked, that is
,has many
plates immersed in i t, the original purity becomes im
paired by the accession of e ther, alcoho l, and variousn itrates from the co llodion, besides any extraneo us matterthat may accidentally be carried in. After a time the
vigour and cleanl iness of the deve loped image will befound to diminish , and the strength, & c .
,ofthe bath has to
be attended to . Gently warming it wil l get rid ofthe ether,
and evapo rating it to halfi ts bulk will get rid ofmo st ofthealcoho l . Ifo rgan ic matter be present, expo sure ofthe bath
(after neutralisation ofthe free acid with sodium carbonate)will cause metallic s ilver to be precipitated, and itse lfto beoxidised by the l iberated mo lecule ofnitric acid
,thus ren
dering it innocuous.Wi th certain co l lodions acetic acid will find i ts way into .
the bath,and the best me thod
'
of e l iminating the si lverace tates which will probably have been formed is to evapo
rate the bath to dryness and add some strong n i tric acid.
Thi s will l iberate the acetic acid,which maybe driven offby
a further application ofheat . None ofthese mode s oftreatment will eliminate all the impuritie s, for al l the fo re ignnitrate s (except ammonium) remain almost unchanged, evenby pro longed fusion no thing remains but to precipitate thesilver as chloride,or in the metall ic state. Ifa film
,after
withdrawal from the bath,presents an appearance as iffine
particles ofthe sensitive salt had been sprinkled over it,the
so lution i s over-iodized ’ that is,it i s super-saturated with
silver iodide. The disturbance made by the immers ion of
the plate probably causes the depo sit. Diluting to do ublei ts bulk , next filtering
,and then making up the so lu t ion to
proper strength , wil l be a cure, or, as some pho tographersaver
,the addi tion of2 per cent. ofbarium nitrate will an swer
the same end.
Development. 63
CHAPTER IX.
DEVELOPMENT OF THE PHOTOGRAPHIC IMAGE.
THE impo rtance ofa tho rough understanding of the ra
tronale ofdeveloping an image in the silver compounds is
no t to be over- rated, as a clo se study ofi t furnishes clues
to apparen tly mysterious results, which are so often metwithby every studen t in the art. The method ofdeveloping
the Daguerrean image has been already given, andwe pro
po se in this chapter to confine ourselves to that employed
in what is known as wet-plate pho tography and dry-plate
pho tography, and also that fo llowed in the calo type and
o ther kindred pro cesses.
I t wil l be reco llected that by method 1 the invis ibleimage was to be made visible by the attraction exercisedby the n ew compound formed after the impact oflight on
the o riginal one. As already announced in chap. iv. p . 24,
the change effected on a mo lecule ofsilver halo id i s its reduction to a lower type , i.e. one containing a lesser number
ofatoms. Thus Ag2l2 was reduced under certain circumstances to Aggl , the o ther atom ofiodine be ing absorbedby some body in contact with it. A similar change wasshown to be effected on the s ilver bromide and chloride .
We may, therefo re, take as a type any one ofthese . We
wil l choo se the iodide , and fo l low the development from
the earl ie st s tage , when used in the wet process .I t has already been shown at p. 19 that the building
up ofthe image is due to the we ll-recogni sed law that everyminute fre shly-fo rmed crystal attracts every o ther ofa similarnature, and that the fo rmation ofthe tree i s entirely due tothismo lecular attraction , and the slow reduction ofthemetal
fromits so lution. Ifthe metal were deposited rapidly the,
64 Collodion , Processes.
same law would stil l ho ld good, but the attraction ofone
reduced mo lecule on i ts immediate neighbour would begreater than that exercised by the metal adhering to the rod,as the probable distances in the one case would be far lessthan in the o ther. A particle ofsuch a siz e and weight
would therefore be built up before the metal on the rod
could draw it suffi ciently near to o vercome the fo rce of
gravity exercised on i t hence i t would sink to the bo ttomofthe containing vessel.
Ifwe take a so lution ofsilver n i trate and add to i t aso lution offerrous sulphate, we have an almost instantaneousreduction ofmetallic silver. Thus
Silver nitrate Ferrous sulphate Silver Ferric sulphate3 AgNO3 3 FeSO‘1 3 Ag Fe z (5005
Ferric nitrateFe (N03 )3
Any o ther oxygen-abso rbing medium which is incapable of
causing double decompos ition with the s ilver n itrate migh tbe substituted for the ferrous sulphate . By adding an acidto the latter the same action takes place , but much mo reslowly
,the time necessary to effect the to tal reduction
be ing dependent on the amount ofacid present. Suppo sing
by some means o r ano ther we are able to cause the first
crystals ofthe silver to depo sit themselves in certa in po sit ions, we may be certain from analogy that the remainingcrystals will adhere to these and build up a min iature silvertree. In the wet process, and also in the dry, we havemeans ofcausing these first particles ofsilver to depo sit onthe invisible image.
This invisible image i s formed of subiodide of silver
(Aggl). Only one ofthese atoms ofsilver is saturated ; theo ther is s till ready to combine with any o ther atom withwhich it has an affinity. Such an atom i t finds in freshlydepo si ted silver. The so lution ofs ilver n i trate is a lreadypresent in the wet process , and in the dry pro cesse s i t is
Crysfa/lz’
ne A ttracz‘z’
ozz. 65
added to the oxygen-abso rbing agent,which is employed in
bo th.
The firs t depo s ited crystals attract o thers, and thus animage i s built up . I t may, however, be asked how i t is thatdifferent density ofdepo sit is caused. The answer to this i sthat the invis ible image i s formed ofvariable quanti ties ofthesubiodide, approximate ly propo rtional in fact to the intensityofl ight acting on i t. At any spo t on the sensitive surface i ti s the integral ofthe attractions ofthe different atoms lyingclose to one ano ther that de termines the amount ofthe firstdepo sit, and the varying mass ofthis determines the distribution ofthe subsequent depo sitI t i s an axiom that the stronger the so lution of the
reducing agent the more rapid must be the depo s it, and i tmay be convenient here to discuss the bearing of this .Suppo se adjacent particles ofthe sensit ive surface po ssessseparate attractions of, say, I , 2, 3 , and 4 uni ts, caused by thedifferent intensities of l ight acting on tho se parts. The
probabilitie s are that the first metall ic silver atom depo sitedwi ll be drawn to the spo t po ssessing 4 units ofattraction.
Ifthe interval in time for the reduction ofthe next atoms
exceed that necessary for placing the first atoms in site ,
the attraction o riginal ly equal to 4 uni ts will become ap~
proximately 5, and the probabilitie s are that the larger proportion ofthe next reduced atoms will be attracted by the 5units than by the 3 ; and by the same action the 4 uni tsmay attract several atoms, whil st the 3 , 2, and I units mayhave attracted propo rtionally le ss. Ifthe reduction ofasuffi cient number of atoms to saturate the who le ofthe
atoms of Ag2I take place almo st“
simultaneously, the
probability i s that the difference in the increase ofattractivepower will be less marked. Thus 4 may become 5 ; 3 , 42 , 3 ; and 1 become 2. I t may therefo re be asserted thatthe po sition ofthe firs t depo sition will determine that subsequently taking place , provided the same rate ofthe reduction
be maintained . From the fo rego ing reason ing it will be
F
65 Collodion Processes.
"
apparent that the stronger the developing so lution the lessmarked will be the variation in density due to the differentin tensity oflight acting on the various portions ofthe sens itive salt.
The mo re viscid a l iquid and the smaller the mass oftheparticle , the slower will a particle trave l through the liquid.
An application ofthis law has been applied to deve lopment.A certain amount ofco llo idal substances, such as gelatine ,albumen, or these bodies acted upon by acids, i s added tothe l iquid in which the oxygen-abso rbing agent is dissolved.
Though the reduction ofthe silver n itrate to the me tallic state,
may take place as rapidly as in a so lution in which the co llo idalbody is omi tted, yet the time the metall ic atoms take to trave lthrough the viscous so lution is lengthened to such an extentthat an appreciable time is taken to form a visible particleofsilver. The time
,therefo re , taken to build up an image
i s longer than with a so lution in which the co l lo idal substancei s absen t ; i t is found that a small quanti ty ofthe '
colloid
will give sufficient viscidi ty to cause slow depo sition.
The examination under the micro scope ofan image de
veloped in the manner indicated above will perhaps throwmo re light on the subj ect than any verbal de scription thatcan be given . I t will be found that the who le ofthe imageis formed ofthese minute crystals, varying in siz e acco rdingto the length oftime which they took to depo sit . The ap
pearance ofthe film when the half- tones ofthe negative are
thus examined,will be as though i t had been sprinkled with
the metallic granules by means ofa pepper-box whilst theparts representing deep shadows will be represented by largepatches ofbare co llodion
,with here and there a crystal lying
embedded in the film. The studen t should take every Opportunity ofstudying the effect ofdifferen t kinds ofdeve lopment as regards the actual physical compo sition of the
image 5 and he may rest assured that the highest excel lencein any negative can never be attained when the depo sit i scoarse and highly crystalline . With a 2-inch object ive . i tshould appear as a stain on the film ofmore or less inten sity.
F ormulwfor Developers. 67
The fo l lowing are the formulae usually employedvelopment
No . I . Pyrogallic acidGlacial acetic acidA lcoho lWater
This developing so lut ion i s usually employed for simplyiodised co llodion, and i s useful when great densi ty in the
l ights i s required. The iron developers ofa weak and strongtype are as fo llows
No . 2 . Ferrous sulphateGlacial acetic acidA lcoho lWater
No . 3 . Ferrous sulphateGlacial acetic acidAlcoho lWa
'tc r
These fo rmulae give the l imiting propo rtions offerroussulphate to wate r admi ss ible, but any quanti ty between thetwo may be taken . For ordinary wo rk , about 40grammesi s usually taken, as giving the best results. The doublesulphate ofammonium and iron may also be substitutedfor the fe rrous sulphate , and i t has the advantage ofre
main ing in so lution unchanged for a long period.
The addition ofcopper sulphate to an extent equal tohalf the quanti ty of ferrous sulphate employed is alsorecommended by some operators, and i t has doubtless insome cases a beneficial effect.
The addition ofvario us co l lo idal substances to the de
velopers, as already stated, may some times be desirable,particularly where great density and fine depo si t are requisite.
Perhaps the best ofany i s that propo sed by Mr. Carey Leai t i s made as follows z— 3o gramme s ofFrench glue, o rgelatine , is softened in 50 cc. of water, to which 3—3 cc. of
sulphuric acid is addedfi The water is next ‘ boiled, and the
F 2
68 Col/oa’z'
orz Processes.
gelatine disso lves, and, after adding ano ther I O cc . ofwater,the bo il ing is continuedfor a couple ofhours. Five grammesofme tall ic z inc are next added, and the bo il ing continued
one hour and a halflonger. The so lution i s allowed to
settle, and the clear l iquid decanted off. To every 3 gramme sofferrous sulphate, r to 2 drops of this so lution suffices
to give suffi cient res traint, without the addition ofanyacet ic
or o ther acid.
Ferrous sulphate is a very unstable body, andwill abso rb
oxygen from the air, and speedily attain the ferric state and
as the latter salt i s incapable ofabso rbing mo re oxygen, It ISevident that the deve loping qualities are thus annihilated.
It has been in effect found that ferric sulphate i s a retarder,that is, a body which prevents the rapid depo sition ofthe
metalli c s ilver from the n itrate so lution. The lesson to be
learnt from this i s,that when the developer attains a red
colour i t must ofnecessity be slower in action than when ofthe o rdinary apple-green tint. A simple experiment with adeveloper containing ferric sulphate i s wo rthy of trial bythe student. Take
,say 3 grammes offerrous sulphate, and
having disso lved i t in 50 cc. ofwater, bo i l with strong n itricacid to such an extent that the addition ofa drop ofthe
so lution to one ofpo tassium ferricyanide produces no b lueprecipitate. Next precipitate the iron as ferric oxide byammon ia
,fi l ter, wash we l l, and disso lve up in the least pos
sible quantity ofsulphuric acid, taking care to leave a slightresidue undisso lved. Make up the quan tity ofl iquid to
10 cc .,and add 2 cc. to a so lution offerrous sulphate made
according to formula No . 3 , omitting the glacial acetic acid.
Deve lop a picture with i t, and no te the result.Attention should be paid in all cases to the crystal s of
ferrous sulphate employed. They are frequently mixed witha yellowish powder, due to the decompo sition ofthe sal t. In
common specimens thi s often bears a considerable proportionto the ferrous sal t i tself, andmust be allowed for in makingup the so lutions. The strength ofthe acetic acid is also
76> Co/loa'z
'
on Processes .
visual rays , 13 termed intensifying the image, and in bo th'
cases the result can be brought about by the same procedure.
The fo llowing are modes ofgiving intensity to the image .
I st. We may co ntinue the deve lopment ofthe image bymethod 1
,ifwe supply mo re free silver n i trate so lution to it
when exhausted and this will give us the necessary imten
sity. The theoretical considerat ions befo re no ted need no tbe again brought befo re the student, ne i ther i s any special
experiment necessary to impress them on his mind.
2nd. We may produce opacity to actinic rays by increasing the deposi t by o ther means. As an example of
what is meant, we may apply to the silver image a so lutionofmercuric chloride.
Mercuric chloride Silver Double chloride ofmercury and silverHgCl2 Ag AgHgClz .
This at “ first is grey (probably due to the fo rmation atfirst ofsilver subchlo ride), but it finally becomes a purewhite. I t will be no ticed that each atom ofsilver attractsone atom ofHgClz . As regards opaci ty without regard to
co lour,the image must evidently be mo re Opaque . It is,
however, as regards actinic rays much less o paque than
when the image was ofthe grey due to the silver.An application of ammonium hydrate to i t, however ,
converts it into a jet black or deep brown.
Here we have a still further depo sition on the silver atom,
which is therefo re denser, and, be ing black, i s very
'
opaque
to actin ic rays.
As ano ther example ofth is mode ofintens ification we
may instance the effect ofcopper bromide on me tallic silver,and the subsequent treatment ofthe depo sit thus formedwith silver ni trate 1
Silver forming+Copper Silver Copper
the image+bromide
_
bromide sub-bromideAg CuBr2 AgBr CuBr
For a detailed account see Pfiotograpfi z'
c7ou rnal, April 1877.
F ormat/2 for [ntezzs zfi era 71
‘When treatedwith-
Silver nitrate we haveCopper
Silver Copper Silver Silver Silver nitrate inbromide sub-bromide ifnitrate bromide sub-bromide so lutionAgBr CuBr
2 + 2AgNoa= AgBr An r
I t wil l be seen how immense ly the depo sit on the image i sincreased by this method .
Las tly,intensity in an image may be secured bv sub
stitutin‘
gsome o ther metal for the silver by chemical means.For example
,we may apply a so lution ofplatinum tetra
chloride ; the silver will be converted into chlo ride, and the
platinum will be depo sited in i ts place . The silver chloridemaybe subsequently disso lved away by sodium hypo sulphiteo r ammon ia, or by many ofi ts we ll-known so lventsFrom a study ofthese methods it will be apparent that
methods 2 and 3 must each be carried out on an imagefrom which everything else i s removed but the me tall icsilver ; metho d 1 may be employed without such removal .
The formulae for the first method are as fo llows
No . I . Pyrogallic“acid 4 grammes
Citric acid 4 to 8 grammesWater 1 litre
No . 2 . Ferrous sulphateCitric acidWater
Withthe latter intensifying so lut ion de tail in the shadows isoften brought out, though absent in the deve lopment, butthe fo rmer i s the mo st efficacious fo r rapidly giving opacityto the image. With each ofthe above
'
a few drops ofaso lution of
Silver nitrate 20grammesWater, 500cc .
must be added immediately befo re application to the film.
These inten sifying so lutions may be applied”
to the image
either befo re o'
r'
after fi xing ;'
tho se which‘
fo llow, however,
72 Collodion Processes.
require the unaltered iodides and bromides to be previouslydisso lved away.
Iodine ‘
I gramme
Po tassium iodide '
2 gramme
Water 50cc .
The iodine (which is he ld in so lution by the he lp ofthe
po tassium iodide) converts a po rtion ofthe reduced me
tallic silver into iodide,and when continued but for a
short time the image has a bluish-green tint, which is mo re
non-actinic than if i t were left in the metall ic state . If
this be no t sufficient a so lution of
Potassium permanganate grammeWater 50cc .
may be flooded over i t. The permanganate i s decompo sedin coming in contact with the silver iodide, and inso lublemanganic o xide i s precipitated on the image .
Ano ther form ofintens ifier is made by
Mercuric chlorideWater
Po tassium iodideWater
The latter i s added to the fo rmer till the red precipitate of
mercuric iodide‘
i s on the po int ofbecoming permanent.
This so lution applied to the image converts the si lver intodouble iodide ofmercury and s ilver, which i s very non-ac
tinic in character ; o ther similar methods may be adopted ,all depending on the fo rmation of double metallic com
pounds . By converting the silver image into iodide by theapplicat ion of the iodine so lution, and then flo oding withsodium sulph-antimoniate (Na2S,Sbs5) commonly knownas Schlippe
’
s salts,
a . .scarlet deposit i s produced of
silver sulph-antimon iate in which 2 atoms ofsilver replacethe 2 atoms ofsodium, the iodine combin ing with the
sodium. This method ofintensification i s due to Carey
Sc/z/cppe’
s Set/ls. 73
Lea, who described it in a paper which appeared in Feb.
1865, in the American ‘ Journal ofPho tography .
’Schlippe
’
s
salts are prepared by taking
Antimonybisulphide (finelypowdered) 18 partsDried sodium carbonate 12 partsCaustic soda 13 partsSulphur 35parts
These are ground up into a fine paste with a l ittle water,and transferred to a we ll-clo sed stopped bo ttle
,complete ly
filled with water. After digestion and agitation for twentyfour hours, the clear liquid i s filtered off, and allowed to
evaporate spon tan eously in a closed'
vessel over sulphuricacid
,till lemon-co loured crystals ofa regular te trahedral
shape are obtained. These are disso lved in water immediately befo re use, as the so lution depo sits an antimonycompound when kept. The mo ther l iquor maybe employed
for intensifying, but does not answer so wel l as the saltitself. The quali tyofthe co lour is dependent on the amountofsilver converted into iodide or chloride.
When great densi ty is required without gradation of
shade, the fo llowing fo rmula i s efficient when preceded bya saturated so lution ofmercuric chloride.
The effect ofthis compound, as already po inted out, is
to fo rm a double chloride ofsilver andmercury,grey at firs t
,
but which subsequently becomes converted into a pure whitedepo sit. When in this state if
Ammonium sulphide
Water
i s applied, a double sulphide i s fo rmed ofan intense black.
Dilute ammonium hydrate may also be employed, as alreadystated, in place ofthe sulphide .
As regards the treatmen t ofan image with copper bromide ,this salt may be formed by disso lving I gramme ofcoppersulphate in 10 cc . ofwater andadding an equivalent ofpotas
sium bromide to i t. Thi s so lution i s flowed over the plate ,and after a whitening action on the film and thorough
washing,a 20per cent. so lution ofsilver ni trate i s applied.
F ixingtbc Image.
As regards fixing the image,no thing need be said ex
c eptingthat the so lvent used must be incapable ofreadilyattacking the metallic image, and such are the so lut ions ofsodium hypo sulphite and po tass ium cyanide . I t will beuseful here to po int out the mode by which this so lution i s
effected. Suppo sing, for instance , the image be deve loped
on the iodide ofs ilver ; we have on addition ofsodium
hypo sulphite e i ther
Silver Sodium . Double hyposulphiteiodide . hyposulphite ofsilver and sodiumAgI Na
,s,o ,AgNa8 203
2 AgI 3 Na28203 AgZNa4 36 203) 2 NaI
The first silver hypo sulphite i s very so luble in waterwhils t the last is very inso luble we have, therefo re , in u singsodium hypo sulphite, a danger of the fo rmation of the
inso luble compound— a danger not to be under-estimated inthe matter ofsi lver prints, when the el iminat ion ofthe lessso luble compound is a matter ofgreat difficulty.
Wi th po tassium cyanide the danger doe s no t exis t, forthough silver cyanide i s fo rmed, yet i t i s readily so luble in a
small excess ofthe po tassium compound.
Silver Potassium Double cyanide ofsilver and Potassiumiodide . cyanide . potassium iodide .
AgI 2 KCN AgCN‘KCN K I
Instead ofAgI in the above equations, we may substitute nearly every silver compound— thus AgNOs, AgCl,AgBr, AgOSiO Po tassium cyan ide , however, has the
drawback that it is excessively po i sonous , and tha t thepresence ofacid causes i t to evo lve hydro cyan ic ac id
,a
gas the deadly effects ofwhich it is unnecessary to commenton . Ano ther drawback to i ts use i s the danger that e xis tsof i ts di sso lving up the fine ly depo sited me tallic silve r, ofwhich the half- tones of the image i s compo sed . Ifused
Vczrfiisé iiz'
g. 7
in a sufficiently weak so lution, however, the solvent actionneed not be feared . All traces ofthe hypo sulphite and;
cyanides should be removed by tho rough washing, o therwise
the transparent parts ofthe image might disco lour, or a disintegration ofthe film might take p lace through crystallisation .
The fo llowing so lutions are tho se generally employed
Sodium hyposulphite 100grammesWater 500cc .
Po tassium cyanide 30grammesWater 500cc .
Vam is/ziflgl/ze Film.
The co llodion film be ing excessively del icate and easily
torn or scratched,pho tographers have adopted the plan of
covering it with a transparent film ofhard resin. This i s
effected by disso lving the re sin in spirits,such as alcoho l
,
and flowing it over the surface. In practice i t will be found
that, in o rder with safety to cover the film without dissolving or disintegrating it, the specific gravity ofthe methylated alcoho l , with which for economy it is made
,should be
greaterthan that employed in the manufacture ofthe co llodion . I t may at first sight seem strange that alcoho l should
be capable ofattacking the pyroxyline, but it must be re
membered that undiluted methyl compounds are so lvents ofit, and, unless sufficien t water be present in the varn i sh tocheck the tendency , a disintegration at least will take place .
I t must also be remembered that the rate at which theso lvent evaporates will cause a difference in the transparencyofthe coating. Ifit be allowed to evapo rate spontaneously,the alcoho l evapo rates first, and leaves the water behind, and,as anyone wil l find ifhe drop a l ittle varnish into water, theresin at once separate s in minute particles, which , when so
un ited together, give a translucent depo sit, caused by the reflections ofthe various surfaces . On the o ther hand, ifheat
76 Collodion Processes.
be applied,ar_ d the water be caused to disappear as rapidly ornearly as rapidly as the alcoho l, the re sin will dry transparent.the heat be ing sufficient to cause the par ticles to be bound oneto ano ther, thus e liminating all chance ofparticular reflection .
The resin should be as co lourle ss as po ssible, as eventhe thin coating given to a negative picture i s often sufficientto cut offmuch ofthe actinic l ight ifi t be ofa red or ye llowt int. As an experiment, it is only nece ssary to disso lve red
Australian gum in Spirit or water, and apply it to a po rtionofa glass plate, when i t will be found that sensitive chloridepaper darkens much less rapidly where covered with i t thanwhere it i s bare .
The constituents ofmo st varni shes usually compriseamongst them lac and sandarac, but it is a matter ofthegreatest n ice ty to proportion them in such a manner that thefilm shall no t split after expo sure to any great variation intemperature. The cause ofthe contraction that takes placei s not accounted for ; i t seems that some resins have a
property ofattracting mo isture, and almo st becoming hydrates. This might cause an expansion ofthe film, whilst a
rise oftemperature might cause con traction. The wholeblame, however, must no t be laid upon the varnish , as theco llodion film
,when not free to expand and contract as it
l ikes, may often produce'
the same effect. The fo llowingvarni shes have been found satisfacto ry :
U nbleached lacSandaracCanada balsamOil ofthyme or lavenderAlcoho l, °83o
Seed lac 120grammesMethylated spirit I litre
The lac is allowed to remain in contact with the so lutionfor two or three days, with o ccasional shaking ; after whichthe supernatant l iquid is decanted off
,and thinned down
to‘
proper fluidity.
78; Collodion Processes Wet Plate.
Coating t/ze Plate witfi Collodion .
The plate may be held in i ts centre by a pneumaticho lder, such as that in figure 1 1
,or at the corner by the
fingers, ifcare be taken that no po rtion except theF IG . 1 1 . edges are touched. From a half-filled 6-ounce
bo ttle, or from what is known as a co llodionpourer (fig. the co llodion should be carefully poured upon the plate
, so as to form a cir »
cular poo l at the end farthe st away from the
man ipulato r, and gradually be allowed to coverthe entire surface, the wave flowing from the
right-hand to the left-hand top corner, fromthence to the left-hand and right-hand bo ttomco rners, and finally into a sto ck bo ttle, whence ,after decan tation, and (ifnecessary) dilution
with 2 parts ether to r ofalcoho l, i t can be again employed .
When the co llodion is thus poured off,the plate will be
in nearly a vertical po sition ,
FIG 1 2 . anda gen tle rockingmo tion
shouldbe given to i t to prevent the co llodion se ttingin ridges but the precaution should be taken no t togrind the edges agains t thebo ttle, o therwise particles
of glass may appear on
subsequen t plates. In ho t
weather the co llodion does
no t take so long to set as in co ld. The state ofthe film
can be always ascertained by cautiously touching the left
hand bottom co rner with the finger. When no lo ngertacky
, the plate i s ready fo r immersion in the bath . The
co llodion should be fi l tered ifnecessary , o r i t may be
decanted from a stock bo ttle by one ofthe ordinary syphon
arrangemen ts .
Sensitising t/ze Platesl 79
The film ofco l lodionhaving set, the plate i s immersed in
the sens i ti sing so lution contained in a vertical or horiz ontalbath
,the fo rmer being recom
mendedfor small plates , thoughthe latter i s essen tial for largesiz es. A
”
‘ travell ing bath ’ i sperhaps the best fo rm ofbathho lder, as i t i s usefulfor indoo randalso for outdoo r wo rk. It isofthe fo rmgiven in the figure.
The top ofthe glass so lutionho lder B , which i s he ld in acase A, i s clo sed by a watertight india—rubber top D
,
screwed_
down by the screws c as shown. The .
‘ dipperemployed for carrying the p late into the So lution during
Pro . 13 .
F IG . 14. FIG . 15.
the operation sensiti sing may conven ien tly madeof PURE silve r wire , of the accompanying shape . I t i susually,
'
however, made ofebonite orglass.~
When the platei s covered with the so lution by a steady downward mo tion
ofthe dipper into the bath, i t i s moved slightly up anddown“
80 Collodion Processes Wet Plate.
in the fluid to wash offthe e ther from the surface ofthe
film, and, when all greasy appearance has vanished, i t may
be left quietly at rest for from one to five or six minutes,according to the temperature and amount of bromide 1
presen t. The mo tion ofthe plate in the bath at first is impo rtan t as, ifneglected, streaky negatives are apt to result,especially in summer weather.I t need scarcely be said that the silver n i trate so lution
should be free from all sediment befo re a plate is immersedin i t, and it should be kept in o rder as shown at p . 62.
After be ing very slowly withdrawn from the bath,capillary
attraction will be exercised by the so lution in the bath on thatleft on the film, and there will be but a slight quant ity left on
the plate, quite insuffi cient to cause the necessity oflong draining. On the o ther hand quick withdrawal necessitates longdrain ing on a pad ofblo tting-paper the edge that o ccupied
the lowest po sition in the dark sl ide should be pressedagainst i t. When the surface appears free from excess of
mo isture, the plate i s placed in the dark slide , taking care
that the edge that is to o ccupy the top place in the camerais kept in the same relative po sition. The sl ide i s clo sed
after the back ofthe plate has been dried with a piece ofrag
or blo tting-paper. I t is here presumed that the camera
i s in po sition, and that the view has been focussed, fo llowingout the rules given in Chap . XXXIII. , p . 240, and that
the expo sure is given also in acco rdance with the remarks
to be found on p . 258 .
D evelopment.
Having decided which developer Is to be employed,
making the decision after a careful study ofthe picture , and
no ting its peculiari ties, the plate 15removedfrom the dark slide ,
the same precaution ofkeeping uppermo st the edge which
The greater the amount ofsoluble bromide in the collodion the
longer it takes to sensitise fully.
82 Collodion Processe Wet Plate.
whil st,on the o ther hand, one no t suffi ciently opaque will
yie ld a grey print, which is unsatisfacto ry. The opacity
must be j udged ofby the colour ofthe depo sit as we ll as by
the density,though the fo rmer need no t be taken into
accoun t when the iron developer has been used, as thesilver depo sit caused by it i s ofa blackish grey. Ifa pyrogallic acid developer be employed the co lo ur is ofa de
cidedly reddish tint, and propo rtionally non -actin ic, hence
great j udgment i s necessary to ensure a really good resu l t.When intensity is procured by using the pyrogallic so lution
the same remarks ho ld good, though the co lo ur is never somarked as when aris ing from deve lopment. Whatever coursebe decided upon
,i t should be bo rne in mind that the general
character ofthe finished negative will always bear an exactrelation to that given by the primary development . Thusa flat-looking developed image will yie ld a flat-looking pic
ture, whilst one full ofgradation will yield one similarlygraduated.
Should intensification be necessary,the operato r must
determine whether i t would be mo re advantageous to conductit befo re fixing the image, or afterwards. Should ove r-exposure have been given the latter will be advisable , whils t , ifumdue exposure, i t should certainly take place befo re fixing . The
intensifier should be poured over the plate,and, whilst
so remaining, a few drops ofthe silver n i trate solu t ion (p .
71 ) should be dropped in to the cup , and then the in ten sifierpoured back . The so lution i s again swept o ver the plate;and the required densi ty is obtained by removal ofthe si lver.I t. has been a po in t causing some discussion as to
whether a deve loped picture may see l ight before be ingintensified. The answer to this seems simple . W i th aniodised film, whi ch has been wel l washed after de velopmen t, it may be expo sed to to lerably bright light witho ut any danger ofproducing a ve il by the action ofthe
intensifier, since silver iodide i s almo st insensitive to l ight,except in the presence ofan iodine absorben t. With a
F iring t/ze Negative. 83
bromo-iodised film mo re caution i s required, though the
writer has never found that a sho rt expo sure in a moderatelystrong light i s hurtful . Wi th a bromised film the less ex
posure given between the two operations the better.When in tensifying after fixing
,i t i s customary to flow a
l ittle iodine (see p . 72 ) over the film,then to expo se i t to
l ight,and afterwards to use the pyrogall ic so lution. This is
nearly useless unle ss a li ttle free silver n itrate be present, o rall excess ofiodine be washed out, any trace ofwhich wouldrender the expo sure inoperative. The writer recommends a
l ittle bromine water instead ofthe iodine, for reasons whichwill be apparent on reading the chapter on emulsions .
In intensifying after fixing, there i s a danger ofstain ing
the shadows with a reddish stain . This seems to be mo re
due to a pyrogall ic stain than to depo sited silver, and can
usually be go t rid ofby a l ittle acetic acid diluted with an
equal bulk ofwater .Fo r landscape or po rtrait negatives i t i s seldom wise to
reso rt to any method of intensification, except that wi th
silver, as there i s great risk ofmaking the half- tones too
opaque . The iodide ofmercury fo rmula (p. 72) is perhapsthe best, ifanything more be necessary.
This operation call s for l ittle remark . The plate may be
immersed in a vertical or horiz ontal bath ifthe sodium hypo
sulphi te so lution be employed, or i t may be applied by flow
ingi t o ver the plate this should always be done with the cyan ide so lution . Atten tion should be paid to see that all the
iodide , bromide , or bo th be disso lved away. Thi s can beascertained by reversing the plate and no ting ifthe yellowishgr
'
een co lour due to them be absen t. Finally the platesshould be we l l washed and drained. A neat con trivanceforho lding the plates when drained i s shown in fig. 1 6. As
i t fo lds up it i s suitable for fie ld wo rk,though a draining
G 2
84 Collodion Processes I/Vet Plate.
box. i s usually carried, made as in the accompanying sketch17.
F IG. 16, F IG . 17.
Varm'
sfiiflgtae [Vegative.
The plate may be allowed to dry spontaneously o r by
the aid ofheat ; the latter method gives a slightly denser
image,and therefo re a negative should never be heated when
parts ofi t are dried by o rdinary evaporation . Befo re applying the varn ish the plate must be warmed (see p. 7 to
cause the varnish to flow,and also to preven t it drying rmatt .
The varni sh is applied like co llodion,the same pro cedure
be ing fo llowed exactly. When all that will has run backinto the bo ttle
,any excess that may have co llected at the
co rner endmay be remo ved by pressing the glass -on a padofblo tting-paper. The plate must again be warmed.
The sOurces ofheat are various . In India, or ino the r hotclimates it will be found that expo sure to the sun ’s; raysimparts sufficient warmth to the glass . In temperate cliimatesthe neatest way ofattain ing the proper temperature i s byplacing the plate in a ho t-air bath
,as used in chemical
o perations (see fig. 26) fail ing which,a clear fire, a Btunsen
ro se burner, or a paraffi n lamp maybe broughtinto requ is i tion.
A naked spirit-lamp is dangerous without great care, and
the so lven ts ofthe varn i sh,be ing highly inflammable, r eadily
catch fire from any _naked flame.
86 Colloa’iofi Processes W
'
et Plate.
kept for a long time befo re development should be sen sitised
(or finally dipped) in a weak bath, and only immersed in i tsuffi ciently long to cause all repuls ion be tween the surface ofthe plate and the so lution to be overcome . A co l lodioncon tain ing a larger than usual propo rtion ofbromide i s alsorecommended to secure freedom from stains.
The usual explanations given as to the cause ofmarkingsl ike watered silk, i s that the co llodion contains too much
“
iodide, i s too alcoho lic, or that the pyro xylin i s to o strong. The
remedies have already been indicated. Black stains at thecorners ofthe plate are often caused by the bath so lutionflowing back over the sensitised surface, after having been incontact with the wood ofthe dark slide.
Transparent markings are much more common in co ldthan in hot weather. They generally arise from unequalsensitising ofthe film in the bath, andfrom the deve lo per
refusing to flow.
A want ofsharpness in a picture may be due ,to in
accurate fo cussing, to a want ofachromatism in the len s,or
to the camera be ing shaken accidentally by the wind, o r bvthe sinking ofthe camera legs during the expo sure . Ifthe
lens be in faul t there i s no help for it but by ascertaining howmuch further backwards or forwards the ground glass ofthe
camera ought to be shifted in o rder to get the sharpest
result po ssible . Thi s can easily be found by actual trial ,and when no ted the ground glass may be permanen tlyplaced in such a po sition relatively to the glass plate in the
dark slide, that when the picture i s visually in focus the
po sit ion ofthe sensitive plate shall be chemically infocu s.
A blurring ofthe image can easily be accounted for ;though, perhaps , there has been mo re contro versy on the
subj ect than on any o ther pho tographic phenomenon. I t i susually ascribed to geometrical reflections of the inc identrays coming through the lens from the back surface ofthe
glass,and no doubt, in some cases , this is abso lutely true,
though in o thers i t requires a mo re complete explanation.
I rradiation. 87
I t must be borne in mind that the rays oflight do not strike
the surface ofthe plate perpendicularly except at its centre .
The accompanying diagram shows a glass plate , A A, ofexag
gerated section, with rays oflight passing through the opticalcentre, C, ofthe lens, B , coming from a bright l ine, a o. The
ray e c D i s perpendicular,and the ray (3 c E makes an
angle with the pe rpendicular. Thi s last ray,after passing
through the co llo dion film (which for the time we may con
sider transparent) would be bent inwards to F,and a
F IG. 18.
po rtion would be reflected from the back surface ofthe plate,and strike the thin co llodion film again at G . From G, apo rtion might be reflected again
,and so on . Evidently , in
this case, a blurring might take place , but always outwardsfrom the centre ofthe plate . If
,however
, the ray ofl ighto c E pro ceeded from the extremity
,o, ofthe do tted l ine, oa
’
,
which may be suppo sed to represent a bright line oflight,then no blurring would be apparent, as the blur from i twou ld be covered by the image E M
, of the bright l ine.
Now in practice blurring is usually mo st intense when adark obj ect, such as a tree, i s opposed to a bright obj ect,such as the sky in which case we may suppo se o a
’
to be a
section ofthe sky,and a c ofthe tree, which we may suppo se
to be a dark l ine in section. Here the blurring is evidently
88 Collodion Processes I/Vet Plate.
no t due to reflections ofthe incident rays from the glass. To"
account for it, we must look to ano ther feature ofthe sensitive surface . Ifa sens itised film be examined under the
‘
micro scope it will be found to consis t ofminute grains ofsilver bromide, iodide, or bromo - iodide
,and each ofthe se
grains individually must reflect mo re or less l ight from i tssurface . A beam oflight
,therefo re,must be dispersed in every
direction,and, as has been shown ,
1 the light striking at anypo int ofthe film is scattered and reaches the back surface
ofthe plate as a disc, with intensity gradually diminishing
from the centre. The reflection from that surface becomesmo st no ticeable when the critical angle of the glass i sreached. The direction that the rays take in striking the
particles i s no t ofany great moment, as the difference inintensity ofthe reflections in any one direction i s very slightwhen the angle do e s no t differ very largely from a right angle .
Hence i t is seen that blurring really takes place from thiscause in all parts ofa picture taken on a glass plate , butthat i t IS naturally mo st apparent when a bright light i soppo sed to a deep shade. There i s still another po in t in
this particular scattering ofthe rays to take into account ,and that is
, the lateral scattering. Suppo sing the inten sityofthe l ight in the lateral direction to be only ofthatin the perpendicular
,the penetration into the film would stil l
be cons iderable, and a blurring would result on thi s acco unt.In photographing fine l ines close together thi s kind ofblurringi s often mo st apparent , a black line be ing often filled up ,o r rendered too fine . I t has been argued that blurring is al sodue to the lens
,but a serious consideration ofthe matter
will show that such an effect i s hardly po ssible ifi t be to lerably achromatic.
The blurring caused by the reflection ofthe scat te redrays from the plate can be destroyed by using an opaquebody
,on which the co llodion shall rest
,or i t can be partially
el iminated by placing a backing of some black or non
1 See London, Ectz'
nourg/z, and Mag. January 1875.
90 Colloa’ion Processes Wet Plate.
Ifa pyrogall ic acid developer be employed,that given
at p . 71< is perhaps as good as any. An iron deve loper
may be advan tageously made with a large propo rtion of
ferrous ni trate , in o rder to secure the White depo sit . The
fol lowing i s a formula usually adopted.
Ferrous nitrateFerrous sulphateNitric acid,A lcoholWater
Should the depo sit fo rmed by this deve loper be too
granular,a l ittle mo re ferrous sulphate must be added.
The pyro xyline for the co llodion should be prepared withweak acids , about equal parts ofthe sulphuric and n i tricacids be ing employed, with as much water as they willbear without di sso lving the co tton woo l when the tempera"
ture i s lowered 5° below that given for preparing negative
pyroxyline.
The co llodion should contain more ether than alcoho lwhen such pyroxyline is employed.
~
The writer has foundthat the fo l lowing gives satisfacto ry results
Ether, 725 .
A lcohol , ‘8 12
Pyro xyline
The fo llowing may be added to this quanti ty of plainco llodion
Ammonium iodide
Cadmium bromide
“
As in iodising negative co llodion, i t may be found ad
visable to om i t 1 50 cc . ofalcoho l from the co llodion and
to disso lve the iodide and bromide in i t, and subsequently tomake the addi tion when the co llodion i s required for u se .
A l ittle tincture of iodine, enough to give a sherry colo u r
to the co l lodion i s usually necessary to secure sufficiently
D ry-Plate or Alkaline Development. 9 1
dense pictures. The deve lopment should no t be carried tosuch an extent as in the negative pro cess . A picture on aglass suppo rt, when viewed by transmi tted light, should, infact, lo ok under-expo sed. Ifthe supportfor the co llodion bea ferro type plate, the image may be developed very readilyso as to give the best effect, or the black background can
be used to ass ist the j udgment. I t need scarcely be said thatwhen glass i s emplo yed the back ofthe plate should have"
black velve t or black varnish in contact with i t. Bates’
s
black varnish is recommended for backing the plate. The
fixing so lution i s the cyan ide so lution given at p. 75.
CHAPTER XIV.
DRY-PLATE OR ALKALINE DEVELOPMENT.
As we proceed to the practical part ofdry-plate processes,i t will be found that the sensitive salt which is principallyemployed is the bromide ofsilver. Sometime s the
‘
silverbromide has a large propo rtion of iodide in combinationwith it and sometimes chlo ride, but i t may be t aken as anaxiom that the really effective salt ofsilver is the bromide ,though the conditions ofsensitiveness may be altered by
the pre sence ofe i ther or bo th ofthe o ther two salts . In
the development ofdry plates by what is known as thealkal ine method, instead ofthe image be ing bu il t up froma silver salt external to the film,
i t is buil t up from the so l id
silver salt in the film i tself. The iodide is very feeblyamenable to th is treatment, but the bromide and th
chlo ride are readily acted upon by the deve loping agents .These agents are in reality compounds which have a strongaffini ty for oxygen. Pyrogallic acid and its congeners , incombination with an alkali, are tho se most usually employed ,though in some quarters, mo re particularly on the Continent,the o rganic ferrous salts are in equal favour. The behaviour
9 2 Eaperiments in Aleoline Development.
ofthe alkaline pyrogall ic acid is almo st precisely similar tothat ofthe latter salts ; and any argument or experimentthat is applicable to the one i s also appl icable to the o thers.
Thefo llowing experiments shouldbe made by the student,
that he may become acquainted with all the phenomenaconnected with this class ofdeve lopment .
1 . Precipitate pure bromide ofsilver,say 4 grammes ,
and wash thoroughly then place i t at the bottom ofa testtube, and cover it with a so lution ofpyrogall ic acid (about
F IG. 19 .
‘
3 gramme to the 100 to within a sho rt distance ofthe top. Having drawn out from 5inchtubing a fine funnel
,let him place the end drawn
out just above the bromide, and then pour intothe funnel 3 or 4 drops ofstrong ammonia. I twill be seen almost immediately that a blacklayer fo rms above the bromide, that the silver isreduced, and that the action continues fo r acertain time
,and then stops . The blackenin
’
g
of the l iquid will be found to be due to the
al teration in the pyrogall ic acid,consequent
on the abso rption by the alkali ofthe brom ineabstracted from the bromide.
2. Repeat the experiment, replacing the pyrogallic acid by dilute ammon ia (say, 8 80sp. gr.diluted with 10times its bulk ofwater), and dropinto the funnel a small quantity ofstrong pyrogall ic so lution. The phenomena presented willbe sl ightly different. A cloud will instantly fo rm
in the ammon ia, and if the surface of the bromide hasbeen pro tected by a small diaphragm ofpaper, the who le
ofthe so lution may be poured off, and the surface ofthe
bromide will be found almo st unchanged. The differenceis caused by the solnoility ofsilver bromide in ammonium
hydrate ; and the portion held in so lution i s consequen tlymo re readily reduced than that remain ing in the so lid s tate.
3 and 4. Repeat these two experiments, substitut ing
94 Dry-Plate or Alkaline Development.
Suppo se we have a mixture ofbromide and sub-bromideofsilver, i t is very easy to theo retically determine whichcompound would be firs t reduced to the metallic state by
the action ofthe alkaline reagent. I t is merely a qu estion
as to which requires the least wo rk to be done on it as towhich will be first reduced. In the case ofthe bromide of
silver two atoms ofbromine have to be removed befo re theme tall ic s i lver results, and in the o ther only one atom. I tis evident, then, that in such a mixture as we have assumed,the sub-bromide would be the first to be reduced, and no t
til l that had been effected would the bromide be attacked .
In the case ofa pho tographic image on a plate befo redevelopment, the amount ofsub-bromide i s infinitesimallysmall ; but, infin i tesimal as it is, that must be reduced to
the metallic state befo re the neighbouring molecule s of
bromide can be attacked. I t might be thought, as th ismust be so rapidly done, that ' the bromide also would be atonce reduced. But here ano ther phenomenon comes in toplay. I t appears that the bromide ofsilver and freshlyreduced metallic silver canno t exist separately in contact,but that immediately new sub-bromide ofsilver i s fo rmed.
Thus
Freshly reducedMetallic Silver
Ag2 Ag.
,Br2 2 An r.
land Silver Bromide become Silver Sub-Bromide .
At any rate , an action ofsomewhat this description take splace in the film. An interesting experiment i s confirmato ryofthis. Take an o rdinary dry plate, such as an albumenbeer plate, and expo se it in the camera. Coat halfofitwith a bromide emulsion, and develop i t by
”
the alkal inemethod . That part coated with the bromised co llodio nwill be found to acquire density. When dry, remove the
film from offthe glass plate with gelatini sed paper, and alsocause the adhesion ofa similarly prepared ge latine pape rto the surface primarily next the plate . When nearly dry,the expo sed film can be split offfrom the bromi sed film
Gallic A cid in Development. 9 5
and on examination i t will be found that there i s an image
on bo th films. ' Ifthe sensitive salt in the co llodion film
expo sed in the camera be iodide,an image may be de
veloped, tho ugh it will be weak.
The fact remains, then, that this action takes place, even
though the fi lms be separated by a very thin layer ofalbumen.
I t will also be apparent that the imagewill be stronger when
deve loped with ammonia than with po tash , for with the
fo rmer the silver can be deposited from the so lu tion. The
writer has been able to intensify images from alkaline or
neutral so lutions of sodium hypo sulphite and po tassium
cyanide,in which have been dissolved si lver chloride, by
this action ofpyrogallic acid .
The restraining action ofso luble bromide (po tassium,&c .)i s mo st probably due to the fact that these salts fo rm doublebromides with the silver, and no t with the sub -bromide,as might be expected from chemical considerations. To
reduce the double bromide to metall ic silver evidentlyentail s more wo rk than the reduction ofthe silver bromideby itself, and the freshly precipitated silver has also to
loo sen the bond be tween the double bromide befo re i t cancombine with the silver bromide to form the fresh subbrom ide, hence the reduction i s slower. In practice this isthe case . A developer in which a full dose ofbromide i spre sent develops mo re slowly than one in which it is presentin small quantities only.
I t should be no ted that the same -treatment ofthe bromide i s effective when gallic acid is employed instead
'
of
pyrogallic, the power ofreduction ofthe fo rmer be ingsmallerthan that ofthe
‘
latter. This fact proves that with a weaklyfo rmed invisible image a strong reducing agent should beused
In the silver bromid‘
e emu ls ion plates,for which this
deve lopment i s particularly adapted,it will be no ticed in
subsequent page s that in o rder to obtain pictures which arefree from ve il or fogone ofthree conditions is necessary— u
9 6 D rn late or Alkaline Development.
eithei' there'
must be al ittle so luble bromide in the fi lm, or
e lse ifthe re be an excess ofsilver n itrate over that necessary to combine with the bromide there must be some freemineral acid or the excess must be converted into silverchlo ride by the decompo sition ofsome metallic chlo ridepresent.These condi tions are apparently conducive to the for
mation ofbright images . I t will be profitable, however, toconsider the probable cause ofthis . Bromide ofsilver isu sually fo rmed by the double decompo s ition ofsilve r nitrate
and so luble bromide, such as a proportion ofpo tassium withthat ofcadmium. Cadmium ando ther dyad metals mayfo rm
two bromide s, the o rdinary bromide and a sub-brom ide. In
o rdinary circumstances the latter compound is found invery small quantities, but when it come s in contact withsilver nitrate , a sub-bromide ofsilver, or, o therwise, bromideofsilver
, with'
unattached atoms ofmetallic silver, is formed ;When the so luble bromide i s in excess, these mo lecule ssupposingthem to be halfmo lecules ofsilver bromide togetherwith the attached atoms ofsilver — readily attract the exces s ofso luble bromide and, when these atoms ofexcluded si lverlo se the ir nascent power ofattraction,
they become '
in~
capable ofcausing a reduction ofthe neighbouring bromidewhen a deve loping so lution i s applied. If
,however, the se
sub-bromide mo lecules remain as such,the actio n ofthe
reducing agent s i s to attack these first,and the reduced
silver exerts i ts power in determining the reduction ofthe
ne ighbouring mo lecules ; in o ther wo rds , causes reduct io nwhere light has no t acted.
Some times the metallic or o ther so luble bromides arecontaminated with minute trace s ofoxides ; when this i sthe case, n i tri c acid converts these into nitrates . This same
acid also wil l act upon the sub-bromide , though no t on the
sub-chlo ride . Thus AgQBr is decompo sed into AgBr and
AgN03 or ifnitric acid be applied to the soluble bromide“
,
such as ofcadmium, before its contact with the silver nitrate,
98 Dry-plate or Alkaline Development.
befo re the bromide itselfis attacked. Time i s in reality'
the factor we have to pay every attention to . Ifwe can
extend the t ime ofreduction, we can make certain that thesalt acted upon by l ight is alone reduced in the firstinstance, and allows the secondary action ofthe fo rmation
of‘
new sub -bromide to take place befo re the bromide 15
attacked. Whe ther the restraint be physical o r chemical,ofone thing we may be certain, that such restraint implie sa less energy capable ofbeing brought into play in a certain time and ifthe t ime be sufficiently long, the energyat any instant will be insufficient to reduce the bromide
,
though it may suffice to reduce the sub-bromide . Ifwe
compare the above method of deve lopmen t with thatgiven in Chapter IX. ,
we shall see that the latter i s theonly one which 18 feasible for the wet proce ss
,and the
pre sent one for dry processes. In the wet pro cess there i s aso lution ofsi lver n itrate on and in the film the
’
unbonded
atom ofs ilver in the subsalt is in a state possessing the
greatest attractive activity, and development must takeplace sho rtly after expo sure. Ifthe second method wereadopted, all the si lver nitrate would have to be eliminatedfrom the film by pro longed washing : in practice it is
found that the resulting image i s liable to be ofunequaldens ity. Again, the proportion ofbromide to iodide in
the co llodion i s small, and as the iodide i s only affe c tedby intense ly strong alkaline developers, the chance of
ve1l ing the image through the reduction of the bromideunacted upon by light is increased, ifsufficiently st ro ngso lutions are employed to cause the reduction ofthe sub
iodide .
With dry processes the advantages rest with the alkalinemethod. The development takes place ho urs, or e venmonths, after expo sure ; consequently, if the method in
Chapter IX. be adopted, it is qui te po ssible that the freeatom ofsilve r in the sub-iodide or sub-bromide has partiallylost i ts nascent energy, and that when the free silver nitrate,
Alkaline Development. 99
together with the developing so lution, is applied to the
surface ofthe film, the intensity ofmo lecular attraction i s
diminished. The same amount of attractive power maybe obtained by increasing the number ofmo lecules actedupon by light hence what in the wet process would be asufficient exposure , for this development, in the dry may beto tal ly inadequate. Besides this, in the mo st recent process,in which gelatine fo rms the vehicle in which the sensitivesalt is embedded, the addition ofs ilver n itrate i s impracticable, since i t would only induce stains over the who le
surface and obli terate the image, even ifthe image could bereadi ly developed.
Coming to the second method, however, the result differs.Even ifthe s ilver atom ofthe sub-bromide be inert, theagents employed will still naturally first attack the re
mainder ofthe molecule, by taking up the bromine and
l iberating the o ther atom ofsilver. Now thi s liberation takesplace in clo se contact with ano ther atom, andas the attractionvaries inverse ly as the square, or more, ofthe distance , amuch less attractive fo rce i s necessary in o rder to draw the
l iberated atom to i ts partially saturated ne ighbour. The
atom once in situ attracts the o ther depo siting atoms , andan image i s rapidly bu i lt up. As a matter offact, in tho sepro ce sses in Which bo th methods of development are
possible i t is found that alkal ine deve lopment causes a gainoffrom 4 to 50 times in the matter ofsho rtening expo sure,mo re particularly when a large quantity ofsilver bromide i spresent from which the atmo sphere i s excluded by its be ingembedded in a comparatively thick film,
as in the gelatinobromide proce ss .
The o rganic sal ts ofiron (ferrous) have been alluded toas acting in the same way as the pyro gallic acid and alkali.I t is mo re easy to give the chemical reactions which takeplace with the fo rmer than with the latter. The typical saltmay be taken to be ferrous oxalate disso lved in a so lution of
neutral potass ium oxalate. The decomposition ofthe sub-s
H 2
100 D ry-plate Processes witk tkeBat/z.
bromide entails an alteration in the ferrous salt emp lo yed as
fo llowsFerrous Oxalate and Silver Sub-Bromide give Ferric Oxalate and
3 2 An r Fe.
Ferrous Bromide and Silver.Fe .Br2 4 Ag.
In some processes the citrate and tartrate ofiron are em
ployed. The same fo rm ofequation appl ies also to them.
CHAPTER XV.
DRY -PLATE PROCESSES WITH THE BATH .
IT i s no t proposed to enter into details ofmany dry-platepro cesses with the silver bath, as they can be asce rtainedby the consultation ofvarious manuals . At the same time itis thought advisable to enter mo re fully than usual in to the
theory ofthe subject. The course usually adopted for theseprocesse s i s as fol lows
The plate is coated with a preliminary substratum ofge latine, albumen, Or india-rubber, or e lse is given an edging with
one ofthem. The co l lodion is then applied, and sensitisingtakes place in the usual mannerf The silver n i trate so lutioni s next thoroughly washed offin distilled or rain water, andwhat is known as a preservative i s flowed over the surfaceofthe plate. The preservative may be partial ly washed off,or i t may be allowed to dry on it in undimini shed strength .
The plate i s now in a state ready for expo sure .
The preliminary coating or edging ofalbumen i s given to
the plate in o rder to secure the adhesion ofthe co llo dionfilm. I t i s found in practice, ifthis be omit ted, that thefilm
, on being wetted, becomes non adherent,and floats
Off. The substratum, as it is technically called, must alwasbe ofsuch a nature as no t to inj ure the bath so lution, and, toguard against all risk, i t is advisable that every portion of
102 D ry-plate Processes wit/c t/ze Bat/z .
impermeable to all so lutions so much so that a horny'
col
lodion will often refuse to deve lop . I t has been the practicewith many practical pho tographe rs to keep the iodi sed col
lodion till it is tho roughly aged, and has a ruby tint from
the el imination ofiodine from the iodides ofthe alkali s,and the consequent combination between the alkali and thepyroxyline . This effect is precise ly similar to that obtainedby a modification ofthe ratio ofthe acids to the water in
the manufacture ofthe pyroxyline. Co llodion may also berendered porous by adding water to a po rtion ofi t to sucha degree that it gives a reticulated film,
and by then addingthe remainder unwatered. A sl ight opalescence ofthe film
is no t Objectionable,and i t may even dry almo st matt, so
long as the necessary dis integration of the pyroxyl ine issecured, since it is found that v
'
arni shing takes away thedead appearance to a great degree .
The sensitising bath should be such as to give a gooddense film after the plate has been immersed some t ime.
The so lution employed for the wet negative process i s of
proper strength,unless the co llodion be highly bromised , in
which case the amount ofthe Silver nitrate maybe increasedto halfas much again
,or even to twice the quantity.
After sensitising, i t is usual to wash the plates to such an
extent as to free them from all s ilver n itrate so lution . The
first washings are usually made in di st il led or fi lteredwater.Rain water is often recommended, but the operator shouldbeware ofi t, unless it be very clean and at least be twicefiltered. Perhaps mo re bad dry plates are produced by theuse ofimpure washing water than by anything e lse. Whenthe del icacy ofthe effect that is produced by the ethe realwaves ofl ight“ on the pure products is taken into consideration , it will be apparent that every extraneo us force whichwill overthrow the equilibrium ofthe particles should beavo ided . Thus we might expect that hydrogen sulphidemight cause that overthrow
,and it inevitably produces
fog. A moderate proportion Ofiron in the water would
APP/fl ingtlze Preservative. 103
also produce like results . When the first excess of the
silver so lution i s washed away the danger ofthe use of
impure water diminishe s rapidly, and almo st any ordinarilypure water may be brought into requisition, but in any casea final rinse ofdistilled water i s to be strongly recommended.
Care Should be taken that the surface ofthe plate after with
drawalfrom the bath is covered by the water without stoppage . The fi rst washin
'
gs may be we ll perfo rmed in a di sh,and an even covering of'the surface can be attained after afew attempts . Washing in dist illed water should continuet ill all repellent action due to the alcoho l and ether containedin the bath so lution disappears.
Applyingthe Preservative.
The'
prese rvative i s usually appl ied by floating it on
the surface ofthe film for about a minute. Care should betaken that its strength is no t dimini shed by too much water
be ing left on the plate . In some cases it may be appliedby immersing the plate in a flat dish o r dipping-bath con
taining it. There are some objections to this mode of
application however.I t will be convenient here to discuss the ends to be
obtained by the use ofa pre servative . 1 . I t must be an
iodine o r bromine abso rbent, for without this quality thefilm manifestly might be insensitive . 2 . I t must be capable
offi l ling up the minute po res ofthe co llodion, so that onre -wetting after drying it may give access to the developingso lution. 3 . I t must act as a pro tective varn ish against theatmo spheric influences . Regarding the firs t po int there i sno t much difficulty, as nearly every o rganic an imal
.
o r
vegetable compound is capable ofcombin ing with iodine .
Under the head ofabso rbents we may rank tann in, pyro
gallOl, gallicfl
acid,gums , ge lat ine, albumen, caffeine, theine ,
and o ther like bodies. The second requirement maybe me t
by the employment ofsome ofthe above, or by the addition
to them ofsugar in various fo rms. The last requirement is
104 D ry-plate Processeswit/z t/ze Ballt.
more difficult to meet, and i s very Often neglected, as it en-f
tails that the body should not be hygro scopic. The drawback to anyprocesses, for instance, in which the preservativescontain gum arabic is that mo isture i s attracted, and the sen
sitiveness ofvarious parts ofthe plate i s affected. No bettervarnish i s known than albumen,
though this has its disad:
vantages as regards rapidi ty,unless the greate r propo rtion
ofi t be removed previous to desiccation, or unless it i tself
becomes a vehicle for ho lding the sensitive sal ts, as in the
co llodio -albumen process. In the writer’s Opin ion an un
exceptionable preservative has yet to be found . I t appearsdubious whether it will not become advantageous to dispense with i t al together, when the balance between the
pyroxyline and sensitive salts is properly adjusted, as in thecase ofemulsion plates. I t must
,then , be bo rne in m ind
that the word ‘ preservative ’ i s only employed for want ofa better.
Dryingt/te Plate.
Ordinarily speaking,the film is allowed to dry Spon tane
ously, for which purpo se a cupboard or box should be fi ttedup in the manner described in the various handbo oks .Ano ther plan that may be adopted by the student, iftheplate be no t too large , is the use ofthe ho t-air bath, employedin chemical labo ratories . The author has found that up to85x65 inches this method is useful . I t i s found conven ien tto allow the doo rs to be left Open til l the surface moisturehas disappeared
,after which they may be clo sed and the
plates be allowed to dry at the higher temperature . Halfa do z en plates may be dried by thi s means in halfan ho ur .
GUM -GALLIC PROCESS.
Of dry-plate processes, only two will be described inde tai l ; descriptions ofo thers can be foundin various pract icalwo rks on the subject. The first that will be describe d isthe gum-gallic process
,as introduced by Mr. R. Manners
106 D ry-plate Processes wit/z t/ze Bat/t.
panying ske tch . The pump, or o ther exhausting apparaJ
tus,&c . i s attached to india-rubber tubing. It is preferable
F IG . 21 .
to fi lter the so lution whilst warm when co ld the pores of
the filter-paper are rapidly fi lled up, and the so lution re
fuses to pass. It may be necessary to fix into the funne l aplatinum fo i l cone, made by cutting a piece ofplatinum fo il
in a circle , and cutting offa secto r,as indicated in the annexed figure. In
any case the filter-paper shouldbe thin
andfree from iron.
1
The preservative so lution is floated
o ver the plate, and, after remaining on
about a m inute , is allowed to dry. If
the surface appear dull, i t should be
dried by art ificial heat previous to expo sure in the came ra.
The expo sure varies acco rding to the developer em
ployed. In developing a plate by anymethod, i t is usual
to apply a narrow edging ofindia-rubber solution round the
F IG. 22.
This may be detected by moisteningit with hydrochloric ac id,and lettingfall on it a drop ofpo tassium ferro -cyanide.
A cidDevelopment; 07
plate by means ofa piece ofstick, or by a piece ofblo ttingpaper held in the fingers and run round: A prettier piece of
apparatus which is sometimes employed to give the
edging is the fo llowing -A came l’s -hair brush,B,is he ld in po sition by a couple ofwire loops,
c C,inserted in a
_
stick,A, and so arranged that the
brush may be lowered into the so lution withoutwetting A . The bo ttom edge
r
ofthe stick is brought
against the edge of the plate , so that the brushrests on the fi lm,
and having drawn round the plate,a neat
.
edging is given . The strength ofthe indiarubber so lution may be five t imes that given on
p . 101 . Ifthe first deve loper be used the expo sure
may be from four to twenty times that required to give
an o rdinary we t plate , whilst ifthe alkaline developer be
employed it wil l develop we ll with the expo sure ofa wet
plate.
F IG . 23.
Acid I ron Developer .
I . Ferrous sulphate 2 grammesWater 30c c .
2. Ge latine 4 grammesGlac ial acetic acid 60cc
Water 400cc .
The ge lat ine may be disso lved in the water, and the
glacial acetic acid added to i t. This rs~
qu1te as effective ,disso lving the ge latine fi rst in the acetic acid
,and the so lution
i s much mo re quickly made . Three parts, by measure, of
No . 1 shouldbe m i xed with 1 part,by measure
,ofNO 2
, and
after fil tering the developer i s ready for use . I t is better tomix them only a sho rt time befo re they are required, asaslight precipitation takes place ifthey be kept long toge ther.To every 4 cc. ofthe developer 1 drop ofa 60 per cent.
so lution ofsilver n itrate should be added, and the application be immediate ly made to the plate .
The following is the alkaline developer which may be
employed
108 D ry plate Processes wit/c t/ze Bat/z.
I . Ammonium carbonateWater
Ammonium hydrate , '880
Water2. Potassium bromideWater
3. Pyrogallic acidAlcoho l
With this the fo llowing proportions
No . 1 solution 4 parts2 2 parts (1 part in co ld weather)3 1 part.
These are well m ixed immediately before use, and after
the plate has been mo istened by water are flowed o verit. Ifthe expo sure has been ofright duration the image
Should immediately appear, in which case the so lut ion Should
be flowed back into the developing cup, and the detai l beallowed to
‘ come up ’ by the small quantity remaining in
the film. When this i s secured ano ther part ofN0. 3 maybe added, and density will gradually be attained. The
writer prefers not to give the full density by'
the alkalineso lution, but rather to gain i t by the application ofthe
pyrogallic acid intensifier with the silver nitrate so lu tion
(see p.
'
Ifthis pro cedure be adopted, the developmentshould be stopped immediately all the detai l is visible byreflected l ight, and the surface should be floode d with a1 per cent. so lution ofacetic acid in water. The intensifica
tion next pro ceeds as in the o rdinarybath dry-plate processes .With an under-expo sed picture , ifthe detail does no t
appear with the above propo rtions ofthe alkaline developingso lution,
a newmixture i s made, nearly all ofNo . 2 so lu tionbeing omitted . Unless the expo sure be very much undertimed
,this is usually efficacious. Wi th an ove r-expo sed
picture the image will flash out ; the develope r mus-t atonce be washed off, and a double amount ofNO. 2 added,
or reso rt may at once be made to the acid intensification.
1 10 D ry-plate Processes wit/c t/ce Bat/z.
The negatives taken by the gum-gallic proces s, under
favourable circumstances, are everything that can be wished
for, be ing de licate, full ofdetail, having the we ll-knownbloom, and be ing fairly sensitive even with the iron de
veloper. In the hands ofManners Go rdon i t has proved
the mo st trustworthy ofany bath dry-plate process (except
one) with which that eminent pho tographer has wo rked .
ALBUMEN BEER PROCESS.
This process has been fully described in ano ther wo rk by
the wri ter, l and i s given as there in described. I t was in troduced by him for so lar pho tography, and was employed by,the English Transi t ofVenus expedition. I t i s
,however,
equally adapted for landscape wo rk, and i s very certa in inits results . The co l lodion employed can be that described atp . 53, though for mo re rapid
~work the fo llowing is better
Alcoho l, °825 450to 350cc .
Ether 350to 450cc .
Pyro xyline 12 grammesAmmonium iodide grammesCadmium bromide grammes.
The re lat ive proportions ofether and alcoho l are ad
j usted acco rding to the temperature in which the plates haveto be prepared.
Wi th the ordinary samples ofco llodion the usual silvern itrate bath (p. 6 1 ) can be used, but with the co llodion madeas abo ve i t is advisable to use a bath containing 16 per
cent. of'
silver ni trate . In bo th cases rapidity is increasedby the addition of2 per cent. ofuranium nitrate. It has
been found advantageous to'
dip the plates at first in the
weaker bath , allowing them to remain in i t for a couple of
minutes, and then to transfer them to the stronger for tenminute s mo re . This mode ofprocedure give s very sen s it ive and Opaque fi lms
, the greater part ofthe'
actinic raysbeing thus utilised. The sensitiveness, however, greatlydepends upon the poro sity ofthe film, and every effo rt
Instruction in Pkotograp/cy . Piper and Carter.
Preparation ofAlbumen. I I I
should be made to attain the maximum ofthis quali ty withoutinj uring the texture ofthe film. The addit ion ofthe larges tpracticable amount ofwater to the co llodion tends to
"
givethis des ired po ro sity. After sensi tis ing the plate is sl ightlywashed, and the first preservative applied
,which is
A lbumen 100cc.‘
Water 100cc .
Ammonium hydrate 12 cc .
This is beaten up into fro th (or i s mixed by pounding itin a mo rtar wi th silica) , and, when settled, the clear liqu id isdecanted Off. The so lution 15 mixed, immediate ly 2 beforeuse, with an equal quanti ty ofo rdinary beer or stout, and
floated over the plate. When bo ttled beer is used , i t i s advi sable to drive offall the carbonic acid by a gentle heat.
The excess i s drained off, and the film tho roughly washedunder the tap for a couple ofminutes, and i s final ly rinsedwith a so lution ofplain beer, to which 1 per cent. ofpyro ~
gallic acid has been added.
The plate i s dried in the o rdinary manner.The expo sure with wel l-prepared dense plates is Often
as Sho rt as that necessary for wet plates, but great latitude i sadmi ssible. With 20times the min imum expo sure nece ssaryto secure a good negative there need be no dange r ofve i l
The development need no t be'
effected for at leas t amonth after expo sure . The fo l lowing so lutions are tho seemployed
1 . Pyrogallic acidWater
2. Ammonium hydrate , 8 80Water
3 . Citric acidAcetic acid (glacial )Water
4. Silver nitrateWater
Dried albumen 5grammes to 100cc. ofwater— may be substi
tutedfor the 100cc . ofalbumen.
“
2 This precaution is necessary, otherwise the tannin ofthe beer isprecipitated by the albumen.
1 12 Dry-plate Processes wit/i t/ce Bat/z.
The description ofthe development i s taken from the workalready referred to .
‘The washing water before development should be ofatemperature not less than 15
°C. When the plate i s washed
,
the fo l lowing deve loper is employed — To each 50 cc.
ofNo . 1 are added 10 drops ofNo . 2,and after we l l
m ixing with a stirring rod the solution i s floated over theplate.
‘Almost immediately the image begins to appear, andaftera few seconds ’ interval the detail can be Seen by reflectedlight to gradually deve lop. Ano ther 7
’ drops ofNo . 2 are
again added to the so lution, which is once mo re floated overthe plate. Twenty drops ofNo . 3 are next poured into thedeve loping cup, and the so lution from the plate poured intoi t . Again the plate is rinsed, this time by the acidified pyrogallic so lution, and intensification given by the use ofit witha few drops ofNo . 4 . It is no t advisable to al low too muchdetail to come out with the alkaline so lution, but to allow apo rtion ofi t to be brought out by the subsequent treatmentwith pyrogallic acid and silver. The alkaline developer re
duces the bromide salt, and leaves the iodide to be attackedby the si lver so lution. I t will be remarked that no restrainersuch as bromide isemployed ; the albumen disso lved by theammonium hydrate plays the part ofa re tarde r, but no t as adestroyer ofthe latent image When the image appearssufficiently dense, it is fixed by either sodium
’
hyposulphite
or by po tassium cyan ide
One po int in the preparation ofthese plates canno t betoo strictly attended to viz . to keep the fingers
‘
awayfrom al lcontact with the film during preparation . A to uch, however
Slight, will cause a stain, and unsightly markings extendingacro ss the plate have been traced to the same cause.
I 14'
Collodionfenntlsion Processes.
coat plates with these two specimens re spectively, expo singbefo re the co llodion has become dried. No te
i
their be
haviour with the alkaline develope r (see p. I t will befound that with the plate in which there is excess ofbromidethere will be no deve lopable image, whilst with that preparedwith excess ofsilver n itrate there will be a fog over theimage. Next take plates prepared with the same co llodionsand wash tho roughly under the tap . Bo th now will givego od deve lopable pictures, but that having an exces s of
brom ide will reqmre a longer expo sure to give a go od negative. Next, take similarly p repared plates, and, after washing, flow over them a so lution of tannin, and the image swill be found to be more readily developable . Again, prepare an emulsion as befo re, using a so luble metal l ic chlo ridein stead of the hydro chlo ric acid, and
,having divided it
into -two portions , add an excess and defect ofSilve r tothem respectively. Prepare plates as above, and n otice thebehaviour. I t will be found that with the sl ightest exc essofSi lver there will be inevitable fog, ,
whils t with the defectthe behaviour will be the same as that given above . Per
haps the mo st sensitive emulsion may be prepared byhaving a Slight excess of silver n i trate and n itric acid
,
omitting the chlo ride altogether.The use of silver chlo ride in the emulsion secu res
density it does no t ofnecessity secure freedom from fog,but, being more so luble in ammonia than the bromide, the .
ammonium pyrogallate readily diso lves it, and immediate lyprecipitates it on the parts acted upon by light. It i sbe l ieved that this simple explanation is capable Ofrenderingclear the use ofit, as recommended by vario us writers .The fo llowing rules may be laid down
rst. That no thing but silver bromide is necessary to give a
goo d .image , ifthe so luble bromide be in excess.
z ud. That ifthere be an excess ofSilver ni trate, theemulsion must be acidified with ni tric or o ther m ineral acid,o r be neutralised by certain metallic chlorides, to sec urefreedom from fog.
Experiments.
‘
I 15
With regard to the first part ofthis second rule, i t will beremarked that the same necessity arises in the bath processeswhere much bromide i s present in the co l lodion . I t mustalso be bo rne in m ind that ifthe first rule be fo l lowed; the
density ofthe developed image wil l be strong ; whilst ifthelatter (unle ss chloride be present), i t may be weak, unlesssome density -giving body, such
‘as silver n i tri te, gluco se,&c .
,be added to the emulsion, in which case go od density
can be obtained.
Having made an emulsion as described, i t will be well to
make ano ther simple experiment . Coat a plate with it, andallow it to dry. On drying, it will be found that the so lublesalts have crystallised on the surface ofthe plate, renderingthe development ofan image almo st impo ssible . Here
,we
have evidence that i t i s necessary to remove these salts.There are two ways ofaccomplish ing this , e ither by washing
the plate after be ing coated with the co llodion,“
or bywashing the who le bulk ofthe emulsion after allowing itto ge latini se by evapo ration ofthe so lvents. In the lastmethod the washed emulsified co l lodion i s dried, and
“
the
resulting pellicle i s again disso lved in ether and alcoho l.It has certain advantages about i t which canno t be
'
gain .
said ; thus, the so le manipulation in getting ready a do z endry plates is to coat them with the emulsion,
‘
and thenallow them to dry. I t also has drawbacks ; one ofthe
:
principal ofwhich is the l iabili ty to spo ts on the negative, apo int which is difficult to understand, Since they probablywill be ent ire ly absent on plates prepared with the sameemulsion unwashed.
I t is no t propo sed to enter into de tails ofall the differentvariet ies of the emulsion processes two distinct variations will be given, one ofwhich will be typical ofan
emulsion where the coated plate alone is washed, and the
o ther ofa washed emulsion. Bo th these wil l be Of the
Simplest characte r, and have succeeded in the hands ofthe
writer.
I 16 U nae/asked Emulsions.
U NWA SHED EM U L SI O N S.
Canon B eeefiey’s Process.
Thefollowing are Canon Beechey’s directions, which if
followed wil l give to lerably certain results. Take
Cadmium bromide (dried) 90grammesAlcoho l, ‘805 I litre,
and allow the mixture to stand, and then decant from it
any quantity that may be required. To each 100cc . ofit
add 1 6 cc. ofstrong hydrochloric acid.
Ofthe above so lution take 50cc.
Ether 720) 1 10cc.
Pyro xyline . 2 to 2 5grammes.
The pyroxyline shouldbe that prepared at high temperature, and may contain n itro gluco se if thought advisable
(see p. I t will be found necessary that it should standat least a day before being used fi ltering through tow onlypartially free s i t from small particles ofundisso lved co t ton.
Ifmuch of the emulsion is l ikely to be required, one
ofthe tall graduatedglasses, as in fig. 24 , will be found con
venient gany quantity can then be syphoned or decanted off.
When the co llodion is clear it is ready for sensi tis ing :that partwhich i s to be emulsified should be poured in toa glass beaker. For every 100 cc. ofthe above
,take 56
grammes of silver ni trate and powder it carefully in an
agate mortar, or by means ofa glass stopper on a th icksheet ofglass . Place it in a test-tube
,with j ust sufl
‘icient
water to disso lve it, and add to i t 30 cc . ofalcoho l,This alcoho l i c so lution ofsilver nitrate should be addedto the co llodion (fig. 25) drop by drop, and the emulsionshould be stirred cont inually whilst the additions are madefinally, the test-tube should be rinsed out with ano ther
30 cc. ofalcoho l, and added to the co llodion.
After the final addition the emulsion should be ve ry '
smoo th and rather thick, though when poured upon a strip
18 Collodion-emulsion Processes.
geneous, o therwise the film will appear granular.’
This is
effected by shaking it in the bo ttle halfan hour befo re i tis appl ied to the plate. When the co llodion i s set, i t is im
mersed in a dish ofdistilled water or fi l tered rain water tillall the repellent action between the so lvents and the wateri s e l iminated, and till the great excess ofs ilver n itrate and
the o ther so luble salts i s washed out. I t may then be passedthrough ano ther dish ofwater iffound necessary, and finallyal lowed to rest in a dish containing beer, to every li tre of
which 2 grammes ofpyrogallic acid has been added. The
best kind ofbeer is that known as sweet ale, the saccharineandguinmy matter being more abundant in it than in thatknown as bitter ale. Any trace ofsilver which
‘
may remainin the film combines with the o rganicmatter, and the dangerofve il is thus reduced. The drying is conducted in the
usual manner,care being taken not to disturb the plates till
they are thoroughly desiccated.
Ifthe calculation as to the amount ofs ilver ni trate necessary to combine with bromide and hydrochloric ac id be
made, i t will be found that there i s a considerable exce ss ofsi lver n itrate in the above emulsion. The o rganic matter ofthe preservative i s present to give intensity during deve lopment .
The development will be carried out by the alkal ineme thod given at page 108, or the ferrous oxalate at page 109 ,the who le ofthe descriptions applying to the process underconsi deration.
CHAPTER XVII.
WASHED COLLOD ION EMULSIONS.
WE now come to the class ofwashed emulsions . Thereare almo st endless varieties ofpreparation
,but experience
seems to show that the simpler the formulae are kept themo re certain are the results. The fo llowing is a mo de
Was/zed Colloa’z'
orcEmdlsz'
ons. I I 9
of preparation which has almost invariably given rapidand exce llent results , and the writer strongly recom
mends it.The plain co llodion i s prepared as fo llows
Ether, °
730 50cc.
Alc ohol , °820to ‘830 25 cc.
Z inc Bromide 5grammesPyro xyline to grammes.
The variation in the amount ofpyroxyline rs g1ven, aso n i ts qual ity largely depends the amount which is essential.Wi th o rdinary pyroxyline the smaller amount will suffice
,
whereas, ifi t be ofa sho rt pulverulent class,the larger
quanti ty wil l be necessary. The writer rec ommends the
o rdinary tough pyroxyl ine,prepared from o rdinary co tton
previously bo iled in strong alkal i,and in the strength of
acids given at p . 45. The z inc bromide may be disso lvedin
i
the alcoho l,with a small amount ofwater in addition
,if
found necessary. To the above quantity ofz inc bromideshould be added about 30 drops ofnitric acid, or 1 smalldrop ofbromine . The reason for e ither ofthese additionshas already been given. Ifthe bromine be employed, careshould be taken to est imate the quant ity ofsilver nitratewith which it will combine. Thi s may be convenientlyexecuted by dropping, say, 3 drops into 50cc. ofwater, andprecipitating with a standard so lution ofs ilver nitrate, or
by taking care to have an excess ofsilver filtering, washingthe precipitate , and gently igniting it
,in o rder to convert
the silver bromate into si lver bromide, and then we ighing it.When the bromised co llodion i s perfectly clear from all
floating particles, which can be secured by allowing themto sett le, o r by fil tering them through co tton which hasbeen previously we ll washed and rinsed with alcoho l, i t i sready fo r the addition ofthe silver n itrate. I t is wel l toallow an excess at least of{3 per cent. ofthe silver salt ifgreat sensitiveness is required , o therwise the bromide maybe allowed to be sl ightly in excess . To convert the above
120 Washed Collodion Emulsions.
amoun t ofz inc bromide into silver bromide would theoretically require 756 grammes, but in practice i t is found thatthis amoun t canno t be depended upon . When nitric acidis used with the z inc bromide i t will be found that 85grammes suffice . When the bromine is used the amountrequired must be subject to experiment. The student mayfind i t convenient to add the silver n itrate so lutio n little byl ittle till he hits the po int where an excess commences, andthen to add5per cent. more ofsilver ni trate . To ascertainwhen the excess o ccurs , a drop ofthe emulsion, from t imeto time be tween the additions ofthe silver n itrate solution,should be poured on to a glass plate, and a l ittle potassiumchromate dropped on to i t a red co loration due to silverchromate shows the slightest excess .
The si lver ni trate i s disso lved up as in the last pro cess,and poured in as already described . It may be as wel l tono te that finer-grained emulsion i s some times made bykeeping out half the co llodion
,adding the who le of the
silver little by little,and then stirring in the o ther halfof
the co llodion.
In o rder to obtain a maximum sens itiveness, the emulsion should be left fo r from 24 hours to 60hours, the t imedepending much on the kind ofpyroxyline employed . If
a large batch ofemulsion i s to be made up, it may beadvisable to prepare 50 cc . first
,and at the expiration of24
hours, 48 hours, 60 hours , to wash it as directed be low,and test its qualities and a no te should be made ofthe
period at which it seems mo st sensit1ve, and at the samet ime free from fog. I t is no teworthy that the washed emulsion usually appears to possess the same qualities as theunwashed. If therefo re
,this process of testing be c on
sidered too tedious,the emulsion may be tested at intervals
in the unwashed state, or, to Speak more co rrectly, afte r ithas been washed after coating the plate . When the emulsion i s in a proper state , it should be poured out into a flatdish, and be allowed to set. A gentle agitation with a glass
22 Wasnea’ Collodion Emulsions.
water i s not hurtful to the emulsion,and if
,after hardpres
sure with a spatula on a square ofthe pel licle, the re is no tsufficient mo isture to damp blo tting-paper, it may at oncebe transferred to a bo ttle to re-disso lve . The bo ttle em~
ployed should be capable ofho lding twice the amount ofso lvents that will be used, as space is requiredfor shaking:
F IG. 26.
The so lvents em
ployed are equal partsofpure ether and ab
so lute alcoho l, 100 cc.being employed for
every gramme of
pe Xyline employed.
Wi th some pyroxyl ine the result ing images
are deficient in vigour.
To co rrect this, to the
first wash water a strongsolution
'
oftannin, o r
salicine,&c .
,may be
added.
A modification of
the -above emulsionmaybe prepared by emulsifyingwith an exc ess
(say 5 grammes) of
silver nitrate, after 15
to 20 drops ofstrong
nitric acid added to
each 100cc. ofthe col
lodion. After the addition of the excess of
silver ni trate a sufficien t quant ity ofsome metallic chlor ide,such as ofcobalt, may be
“added, in o rder completely to
neutralise that exce s s ofsilver, and to leave a slight exrcess
ofthe so luble chloride; This method is due to Mr.New ton,
and in his hands appears to work satisfactorily. The presence
Dryingtne Plate. 123
ofa free chloride i s not so destructive ofsens itiveness as the
free bromide, hence the preference that i s given to . the
fo rmer over the latter for neutrali sing any excess ofsilver
nitrate. I t i s often useful to keep the pores ofthe col
lodion open by a li ttle resinous matter, such as gum am
moniacum. This gum i s very inso luble, and, ifemployed,a saturated so lu tion ofi t in alcoho l should be prepared,and the resulting varnish should replace the alcoho l em
ployed for re-disso lving the emulsion pellicle. With allthe washed emulsion processes the plate is coated as witho rdinary co l lodion and allowed to dry, no preservative be ingnecessary. The dark heat which is radiated from a slab of
iron placed o ver a spirit lamp or Bunsen burner is recom
mended byMr.Woodbury,to cause the rapid evaporation of
the so lvents from the coated plate. The plate must not beexpo sed to the naked flame from these sources, as the blueco lour is sufficiently intense to cauSe a veil to spread,
overi ts surface on deve lopment. Washed emuls ion plates willkeep indefinitely bo th befo re and after exposure , as will theemulsion ifall excess ofsilver nitrate be washed away. The
exposure necessary 1s large ly dependent on the presence of
so luble bromide or chlo ride and on the ir quantity. As a rule
the plates require halfas much expo sure again as a wet plate.
The development i s conducted as laid down for the prev1ous process , and calls for no especial remark.
CHAPTER XVIII.
THE GELATINO-BROM IDE PROCESS.
IT would not be advisable to enter into all the variousmodifications which could be described regarding this process . I t . i s thought sufficient to describe but two methodsof preparation of the plates
,bo th ofwhich give results
which cannot readily be beaten, e i ther for rapidity or for
brightness in'
the resulting negatives . This process i s themost modern one
,and it s advent opened out a new era in
I 24 The Gelatino é oromia’e Process .
pho tography,since sens itive surfaces can now be prepared
which are 60 times mo re rapid than the o lder we t processwhich has been described. I t must
,howeve r, be said that
tnis gain in speed does no t at the same time s ign ify gain
in beauty ofresult. For giving beaut iful prints we believethat the co llodion pro cesses surpass this newer one, asthe picture is so much more under contro l in the matterofgivingintensity.
The gelatino -bromide process, as its name s ign ifies, i s
one in which gelatine acts as the vehicle in which the sensit ive sal ts are embedded, and i t very much dependsquality ofthe gelatine as to the rapidity and freedo
disfigurements which the plates prepared with i t
The ge latine employed should be of the pure st, and itshould no t set at too low a temperature when i t i s l iquefied.
It may be stated, as a rule, the good gelat ine will absorb
enough co ld water to disso lve i t, ifthe temperature i s raised
above 33°
C . Ifonly one quality ofgelat ine i s to be used,He inrich’s medium hard wil l be found very su itable , but we
prefer to mix this with Nel son’s No . 1 pho tographic ge latine , as we shall presently describe . One fact must always
be bo rne in m ind, that frequent reheating ofgelatine spo ils
i ts setting qual ities hence care must be taken as to this
by keeping back a certain quantity ofit during the preparation ofan emulsion which requires much heating. The
sens itive salt or salts are emulsified in the gelatine verysimilarly in manner to the ir emulsification in the co l lo dion
ulsion processes described in the last two chapters.
The general o utline ofthe process i s as fo llows
Disso lvingthe soluble haloids. Addinggelatine to the emulsion._
Dissolving the silver nitrate in Settingofthe emulsion.
a so lution ofgelatine. Washingthe emulsion.
Emulsification of the silver Drainingthe emulsion.
haloids. Cleaningthe glass plates.Increasingthe sensitiveness by the Coatingthe plates.application ofheat or o ther Dryingthe plates.wise. Packingthe plates.
126 Tne GelatinoLoromia’eProcess.
but after bo iling great density ofimage i s secured togetherwith rapidity .
'
I he bo il ing ls analogous to the ripen ing inthe co llodion emulsion pr .ocesses I t i s conven ient toempty the emulsion out ofthe bo ttle into a flask capableofho lding at least 200 cc. ofl iqu id, and this is placed in
a saucepan ofwater, heated over a Bunsen burne r. The
flask should be placed in a jacke t ' to prevent the access oflight to the sensitive material i t contains . A tin canisteranswersvery we ll , if
'
a clean co rk be loose ly placed in neck
ofthe flask. The water‘
should be brought up to bo il ingpo int, and kept in a state ofebulli tion for about 40m inutes.There seems to be a time when maximum sensit iveness isarrived at, after which there i s agradual decrease in sens i
tiveness, and the emulsion i s apt to become granular;
The bo iling makes the particles coarser than when the’
emulsion was first mixed, and on examining a drop ofi t ona strip ofglass the transmitted co lour wil l be found to be”
changed. Instead ofthe ruddy tint,i t wil l appear to ' be
bluish,with perhaps a tinge ofo range in i t. Whi lst the
boiling is taking place No . 5 should be swe lled in 65 cc. ofwater and disso lved by the aid ofheat. I t and the emu lsion;
are then coo led down by allowingcold water to run o ver
the flasks contain ing them till a temperature ofabout 23° C 3
i s reached. The emulsion and the gelatine are then m ixed,and wel l stirred up together, so that the mass is approxif
’
mately homogeneous. A white jam-
po t i s an exce llentreceptacle for the mixture, since it will then set in a so lidlump and be ready for the next operation.
As far as this point the two methods are dissimilar, buthencefo rward the manipulations for the two are the same.
We therefore shall proceed to give the alternative me thodofpreparing the emulsion.
The same quantities of the different materials are
we ighed out as befo re, and the numbers we refer to are the
same as tho se previously employed.
NO. I is disso lved in 3 cc. ofwater, No . 2 in 40 c c. of
Boilingt/ze Emulsion. 127
water. No . 3 i s soaked, swo llen, and dissolved in the same
water in which No . 2 i s disso lved . NO. 4 i s disso lved in ,
30 cc. ofco ld water, and ammonia diluted to halfstrength, i s added drop by drop til l the o xide ofsilver firstformed is redisso lved. No . 5 i s disso lved in about 60cc. ofwater. The so lution ofgelatin e -
and bromide (No s. 3 and
2 ) is allowed to coo l to about 20°
C .,and then the ammonio
(
nitrate of silver so lut ion is added drop by drop, w1th
constant shaking'
or stirring. The iodide so lution (No . 1 )lis finally dropped in. Ifthe so lution ofNo . 5 be at o nce
“
gadded a fairly rapid emulsion w il l be fo rmed
,but ifthe
eemulsion be allowed to standfor from 18 to 24 ho urs beforethe addition i s made, a much mo re rapid plate can be pre
pared from i t. The emulsion will then be grey by transmitted light. The emulsification takes place in a coolso lution, and therefo re there i s less liabil ity for the gelatineto be acted on inj uriously by the ammonia than ifheat beapplied . In warm
l
weather as much as halfofNo . 5 maybe at once added to the emulsion to prevent i ts becominggranular. The emulsion i s then, as in the first method
,
transferred to a jam -
po t and allowed to set.
In ho t weather it is as well to let the jam-po t and its
contents stand in iced water to enable the emu l s ion to set
rapidly and fi rmly. A certain amount offirmness is adesideratum,
as in the subsequent washing ifonly slightlyset there will be too much adherent water.
The Washing has next to be effected,and th is can only
be carried out by'
causing the set emulsion to be dividedinto very fine shreds . Squeez ing the emuls ion throughcoarse canvas, such as is used for Berl in woo l wo rk, or
,
‘
be tter stil l, through mo squito ne tting,breaks the emulsion
up to a proper state ofsubdivision. The contents ofthe .
jam-
po t are placed in a square piece ofsuch netting, twistedup in i t
,and squeez ed through into a jar ofwater, the canvas
being held beneath the surface to prevent the shreds rej oin
ing. When all i s squeez ed through, the particles ofgelatine
128 The Gelatino-oromia’e Process.
may be transferred to the canvas once more, and be freely’
doused with water from a water-tap or by hand, and thenleft to soak in water for halfan hour. The squeez ing
operation i s again performed, and after ano ther half-hour’s
washing the emulsion may be drained. T his may be carried
o ut by placing it on a hair sieve, or o ver the mo squito
ne tting. A couple ofhours’
rest should render i t sufficiently
free from al l adherent water.The emulsion may now be retransferred to the jam-
pot,
which should be carefully cleaned, and then surrounded with
hot water ofabout 30° C. till i t is well disso lved . The
addition of°03 gramme ofchrome alum i s a safe ty to theemulsion. I t may be disso lved in a small bulk ofwater
and then dropped in. Next 14 cc. ofabso lute alcoho l ares tirred into the viscid mass. If extreme rapidity be
required, to the amount ofemulsion here suppo sed to be
made 10 drops ofammonia diluted with 100 ofwater maybe stirred in, the temperature ofthe liquid be ing kept upto 33
°C. for a couple ofhours .
After fi l tering, the emulsion i s now ready for coatingthe plates. This may be carried out by allowing i t to ,
perco late through a double thickness ofswan’s-down calico .
The plates are cleaned with n itric acid and water, andthen wel l washed in clean water. They are then dried witha clean diaper duster, and placed in a heap in the dark roomfor the next operation, after removing all adherent dust orfluffby a soft brush.
The emulsion, which should now be in a flask, must be
kept in water of“about 44°C., and, suppo sing a who le plate
(85in. x65in.) i s to be coated, about 70 cc. ofit put in to aglass measure which has a lip, and a poo l ofthe warmemuls ion i s poured in the centre . In fact
, the plate is'
coated as with co llodion (see p. except that it is. no t .
drained so c ompletely,a fair amount being left on the p late.
The plate i s next placed carefully on a perfectly level shelfofslate or thick glass, and kept there till the gelatine is set,
‘
130 T/ie Gelatino-oroinicle Process.
led, terminated with a tap, which allows any air to bego t ridof, which would o therwise st0p the flow into c c . At H i s atap, which allows the who le apparatus to be emptied atpleasure. K 15 a hot-air shaft
,be ing some four fee t above
the box. I t i s terminated by a bend in two directions, andcan be fi tted with a cap, ifrequired, in which are pierced o rifices. Beneath are a couple ofventilating inlet pipes , likewisebent in two directions . L L L i s a false bo ttom,
pierced withho les, on which the drying racks are placed. F i s a gas jet,which heats the water. (The cupboard is shown with only
one door.) Each door is made light- tight bymeans offi lle ts,which need not be described. The hinges are pianofo rtehinges. The piping ismade ofcompo s ition gas-pipe
,though
perhaps iron would be better ; still, as they are, they answerperfectly .
The interio r ofthe cupboard must be maintained at aconstant temperature ofabout 25
°C.
,and the gas must be
regulated accordingly .
A gas regulator is an excellent piece ofapparatus tohave to keep the cupboard up to this temperature. I tmust be reco llected that the great desideratum in drying theplates is a constan t circulation ofair rather than any greatheat. The plates should be dry in at least 24 hours, but iti s well to keep them as if drying in the cupboard for
48 hours, to ensure perfect desiccation.
The plates have next to be stowed away, and there is nobetter plan than to place them face to face in pairs, and
then to make up bundles ofthese pairs, six plates in each,and tie them firmly together. Perfectly clean and dry pinkblo tting-paper may also
“
be placed between each pair of
plates, and the bundles made up as befo re.
Exposure ofGelatino-oromicle Plates.
CHAPTER XIX.
EXPOSURE AND DEVELOPMENT OF GELATINO-BROM IDE
PLATES.
THERE are one or two contrivances in the marke t whichserve to indicate the speed— i.e. the relative sensitivenessofa plate . Such a contrivance is Warnerke
’s sensitome ter,
which is a mo st useful piece ofapparatus . A very fairjudgment ofthe rapidity ofa plate may, however, be arrivedat by expo sing different po rtions for 5, 10, 15, &c . secondsto the light ofa candle placed 1 2 feet offthe plate . I t willrequire a very rapid plate to show any expo sure havingbeen given to the po rtion which has really been expo sed
5 seconds. The plate after preparation should never seeany light which is inj urious to i t, and great care should betaken that in the preparation the light which must perforcefal l on i t is ofthe least hurtful co lour and ofa very feeblecharacter. The same remark applies to placing the platesin the sl ides, and in the ir
“ subsequent development.As to expo sure some thing must be said. The plates pre
pared as described in the previous chapter are exquis itelysensitive to white l ight . Wi th a rapid rectilinear lens byDallmeyer, us ing the smallest stop (No . an open landscape ou a bright spring day i s impressed in halfa second,always suppo sing there are no very near deep shadows, whichwill require a slightly longer expo sure to
.
bring out detailin them.
Wi th these particular plates,'
when they are coveredfairly thickly with emulsion, the exposure maybe pro longedto 3 seconds under the same circumstances without anyde triment to the resulting negative. This 15 what is technically called latitude ofexpo sure . A negative with such a
pro longed expo sure requires care in developmen t, but allK 2
132 T/ze Gelatino-bromide Process .
details in every part ofthe picture can be brought out withoutdiffi culty ifproper precautions are used. Wi th a large r stop
,
say No . 3 , which is usually small enough to place the who le ofa picture in good fo cus , only ofthe above least expo sureneed be given
,which is about g
1th ofa second. For suchexposures uncapping the lens ’ is impracticable, and reso rtmust be had to some sort ofmechanical shutter, a description ofsome ofwhich will be found in a subsequentchapter, as will be the choo sing ofthe picture .
When the sl ides containing the plate s are carried to thecamera they should be shielded from l ight
,and the front
board of the sl ide should only be withdrawn when the
fo cussing clo th is thrown o ver the camera, also to pro tectit from light .
Light usually enters a lens through the slo ts which are
cut in the brass mounts for the diaphragms o r stops . Thiso rifice should be covered over with an india-rubber band .
I t also frequently happens that the l ight coming throughthe lens is reflected on to the s ides ofthe camera. Thismay be overcome by placing an oblong diaphragm behindthe lens just large enough to cut offall light which doesno t fall on the plate . The inside ofthe camera should beofa dead black, and, in fine , too much care canno t be takento ensure the absence ofall reflections.
For developing these plates there are almo st innumerableformulae
,but the writer gives two for alkaline development
which he has found wo rk really well with him, and alsoone for ferrous
"
oxalate.
The first fo rmula stands thus
Pyrogallic acid
2 . Po tassium bromideWater
Ammonia I partWater 9 parts .
Sodium sulphite A saturated so lution in water.
I 34 l e Gelatino-oromia’e Process.
The rationale of this is that the developer soon exhaustsitse lfin the parts ofgreatest densi ty, whilst i t i s almo stunaltered in the parts where no de tail has put in an appearance . When the detail has been brought out in thi smanner
,the image should be intensified by a fresh appli
cation ofthe deve loper.In the case ofso -called instantaneous pictures
,when
expo sure has been excessive ly sho rt, i t is a good plan to
soak the plate in 3 cc. ofNo . 3 and60cc . ofwater, and afte ra couple ofminutes to add to the developing cup the propo rtions above indicated ofNos. 1
,2,and 4 . This wil l
cause what would o therwise be an under-expo sed pictureto develop as ifit had rece ived a longer expo sure .
Ano ther developer which the w riter can recommend
from personal experience 1s a fo llows
I . Potassium carbonate, pure 3 partsWater 8 parts
2. Potassium bromide 1 partWater 25 parts.
3 . Pyrogallic acid dry.
4 . A saturated so lution ofsulphite ofsoda in water.
cc . ofNo . 1,
‘
5 cc. ofNo . 2,
°
2 gramme ofNo . 3 . and4 cc .
ofNo . 4 are mixed, andmade up with water to 70“cc. and
applied. Thi s is a no rmal developer. The writer prefers tocommence with 2 cc. ofNo. 1
, with the same amounts ofNos. 2, 3 , and 4, and to add mo re ofNo . 1
,ifrequired For
instantaneous pictures the plate may first be soaked in 33 cc .
ofNo . 1 and 60 cc. ofwater, and the bromide, sulphi te,and pyrogalli c acid subsequently added. Negatives developed with this developer are del icate
,but the density
may be increased by a final addit ion of2 drops ofammonia,8 80. The ferrous oxalate developer may also be used as
prepared at p. 109 . The writer recommends that it shouldbe used ofhalf-strength (i.e. diluted with water) till all thedetail appears, after which a fresh quantity ofthe strongdeveloper is applied, to which 1 cc. ofa 25 per cent . solution
[Vets/Zingt/te Negative. 135
ofpo tassium bromide i s added. This will bring up the
negative to the necessary density.
In the case ofinstantaneous pictures the developer should,as before , be used half-strength , but an addition to every
50cc. of20drops ofa 2 5 per cen t so lution ofhypo sulphiteofsoda in water should be made. This addit ion gives marvellous developing properties to the ferrous oxalate, increasingits power three o r four-fold.
The plate after development must be well washed, andshould be fixed in a bath ofhyposulphi te ofsoda, as given atp . 75. Films containing much iodide take somewhat longto fix
,especially ifthe emulsion has been prepared by the
ammonia process . The plate should be thoroughly washedfo r a couple ofhours in constant changes ofwater. Whenthe film i s tender, and has a tendency to leave the plate ,between the developing and fixing operations it should beimmersed for five minutes in a saturated solution ofalum, the
film be ing washed bo th befo re and after the immersion .
When finally drying the negative care should be taken thatno drops ofwater are left on the film, as they are apt to
cause marks . Our advice i s to sponge the surface ofthe
negative with a very soft sponge befo re i t i s dried. Overexposed pictures, and pictures taken on plates on which thelayer ofemulsion i s small (as in some cheap commercialplates), will be found to lack density, and they must be in ;
tensified. Unfo rtunate ly the surface ofge latine almo stpreclude s the intensification be ing carried out as for wetplates
,owing to the gelatine staining. Reso rt is therefo re
had to mercury intensification, and i t is believed that thetwo fo llowing are permanent .
The fo llowing so lutions are prepared
1. Mercuric chloridePotassium bromideWater
‘
2. Silver nitrateWater
36 Tne ' Gelatine-bromide Process.
To' No . 2 i s added a 20 per cent. so lution ofpo tass ium
cyanide in water, till the precipitate ofcyanide ofsilver firstthrown down is very nearly redisso lved . The so lutions arefi ltered.
To intensify an image,the plate when dry is immersed
in water for five minutes, and placed in a di sh contain ingNO. 1 . Here it is allowed'
to bleach tho roughly. It i s thenwashed in running water (or constant changes ofwater) fora quarter ofan hour, when i t is placed in a dish contain ingNo . 2. The image immediately. takes a black co lour andbecomes dense. The film must be next tho roughly washed.
I t may happen that the density is too great, in which case itcan be reduced by immersion in a 4 per cent. so lution of
sodium hypo sulphite.
Instead ofNo . 2, a saturated solution ofsulphite ofsodamay be employed, and the same mode ofreduction pursuedifthe intensity is too great.Befo re the negative is taken into regular use for printing
it should be varnished. The varnishes given at p. 76 wil lanswer, and the manipulations given at p. 84 should befo llowed.
D efects in Gelatine Negatives.
The worst defect ofall is the'
tendency ofthe film to
leave the plate. This is usually cured by immersion in the
alum bath between fixing and developing, as 1t generallyhappens that this ‘frilling ’
ofthe plate takes place in thehypo sulphite of soda . so lution. A badly cleaned plate isalso conducive offrill ing
,for which there i s no cure.
Blisters are usually fo llowed by frilling, and what applies
to i t applies to them. Green fog is seen when alkaline de
velopment i s employed the emulsion from which the plates
are prepared is then at fault. ’
General fogi s usually due to the emulsion, but when i tis no t very bad
'
the plates may be used by giving slightlypro longed expo sure and adding treble . the amount of
138 Paper Negatives.
The paper employed should be as tough and grainlessas po ssible, capable, however, ofho lding sufficient ofthe
sensitive compound to give abody to the image. Good
English paper ofthe consistency ofmedium Saxe answersevery purpo se . The great drawback to all papers ofthe
present day seems to be the chance oftransparent spo tsappearing during development, and
'
a consequent damageto the image. What is the chemical nature ofthese spo tsis not known, but they can generally be got rid ofby brushing a dilute so lution ofhydrochlo ric acid over the surfaceofthe paper, and then tho roughly washing offall excess ofacid. When dry the paper is ready for impregnatlngwiths ilver iodide. This last is fo rmed by taking
No . 1 . Silver nitrate 3 grammesD istilledwater 20cc. 1
2. Po tassium iodide 3 grammesDistilled wate r 20cc
No . 2 i s poured into the so lution ofNo . 1 with constants tirring
,and a precipitate ofsilver iodide ISformed. The
po tassium iodide be ing in slight exces s , acertain quantityofthe silver iodide 15 held in so lution. The precipitate 1s
allowed to settle at the bo ttom ofthe glass measure (inwhich we will suppo se the two so lutions to have beenmixed)and the supernatant l iquid is
_poured offwater is again added,
and after stirring i t_
i s again poured off. This operation of
washing is continued some three or four times , o r until theso luble po tassium ni trate i s nearly e liminated.
The silver iodide i s next disso lved in a so lution ofpotas
s1um iodide.
Po tassmm iodide
Water
This is poured on the silver iodide and wel l stirred.
As this quantity will not effect complete so lution, crystalsofthe po tassium salt must be added till after «much stirringthe so lution i s semi- transparent or milky.
Preparation ofSensitive Paper 1 39
The paper i s next cut to a convenient s iz e, and i spinned on a flat board. The so lution is applied by a brush
ofco tton-wo o l, a good form adapted for the purpose be ing given in the figure . A IS a glass tubeofabout 20 centimetres long, and above 1 centime tre diameter. A loop ofstring, B, passes through
the tube , acro ss which is placed a thin tuft ofco ttonwo o l , c . The loop is then pulled up into the tube ,a sufficiency ofco tton-wool being allowedto remainexternally to fo rm the brush . I t is advisable first towash the woo l in a weak solution ofalkal i in water,taking care , however, that none ofthe alkal i remains in the fibre, and that it is tho roughly dried beforebe ing used as a brush.
The so lution i s brushed up and down and across thepaper, till the who le surface has rece ived a unifo rm coatingofthe disso lved iodide . When partially dry the paper isimmersed in a dish ofdistilled water, all air-bubbles be ingcarefully removed from the surface. After soaking for acouple ofminutes it i s removed to a second dish , and subsequently to a third dish .
”
The water removes the po tuss ium iodide , and leaves a primro se -co loured silver iodide on
the surface ofthe paper. After the washing has been con
tinued two or three hours, the paper is hung up and dried.
In thi s state i t is nearly insens itive to light (though not quiteso
,as the iodide i s in the presence ofo rgan ic matter), and
can be sto red away between the clean unprinted leaves ofabook . When required fo r use, the paper is pinned on to
the board as befo re , and a mixture ofthe fo llowing so lu tionsis brushed o ve r it
F IG. 29 .
No . 1. Silver nitrate 5grammesGlacial acetic acid 8 cc.
Water 50cc.
No. 2. Saturated so lution ofgallic acid in distilledwater.
To every=c c . ofNO. 1 add 60 cc. ofdistilled water,next 1 cc. ofNo . 2, and finally 30 cc. ofdistilled water.
140 Paper Negatiz'es.
Ifthe temperature be high, the water must be inc reased
to such an extent that immediate reduction ofthe silver
n i trate may no t take place . After well mixing, the so lu
tion 15 applied lightly, but plentifully, to the iodisedpaper with
' the co tton-woo l brush alreadydescribed, and allexcess blo tted offon filtering-paper ofthe purest descript ion. Two sheets are then placed back to back with blo tt ing -paper between them.
The paper i s most sensitive in i ts moist state, but i ti s also capable ofgiving pictures when dry
,or un ti l the
surface ofthe paper becomes disco loured by a reduction
ofthe gallate of silver. For expo sure in the camera ashee t may be placed between two pieces ofglass
,or the
corners may be gummed on to a shee t ofglass, the papertaking the po sition ofthe co llodion film ofthe o rdinary processe s. The expo sure varies considerably according to thepreparation ofthe paper and i t should always be sufficientlypro longed to give a trace ofthe sky-l ine on the undevelopedpaper. To develop the picture, the paper must be pinnedon the board as befo re, and equal parts ofNo . 1 and No . 2
applied,with similar quantities ofwater as already indicated.
Thi s i s applied with the brush , and i s continued t ill thedeveloping action begins to flag. When this is the case, thegallic acid so lution, No . 2
,i s applied
“ very lightly,unti l
the deep shadows begin to dim by transmitted light. The
development must now immediately be arrested, o therwisethe picture will be ve iled.
For an under-expo sed picture more ofNo . 1 should beused than that given, and ifover-exposure be feared (indicated by the picture be ing fairly visible), No . 2 sho uld bein excess. A l ittle consideration ofthe se po ints wil l showhow development may be equalised in dark parts . An
artist in the production ofthese pictures will be able toproduce a picture by a little attention to the above detail s,whilst a mere manipulato r would probably produce no thingbut an image wanting in del icacy and gradation.
142 Paper Negatives
There are various processes for the production ofpaper
negatives extant, amongst which maybe mentioned tho se of
Le Gray, Blanquart-Evrard, and Prichard . That "due to
Le Gray wasa t one t ime a great favourite, i ts distinguishing
feature being that the paper is waxed befo re be ing sensit ised .
The waxed paper is immersed in a so lution ofpo tassiumiodide and bromide , together with Sugar ofmilk, and after
drying is treated wi th a so lution ofsilver nitrate, acidified
wi th glacial acetic acid. The deve lopment is carried on
much in the same way as that indicated in the abovepro cess, the paper being submerged in the fluid . This lastpro cess, perhaps , i s be tter adapted to care less manipulation
than that described above , as all danger ofsta1ning the back
ofthe picture i s avo ided.
There are several negative and po sitive papers in the
market at the presen t time, some ofwhich are very exce llent. Eastman’s negative paper i s prepared with a gelatine
emulsion, such as was described in Chapter XVIII ., and
can be deve loped by any of the developers given in
Chapter XIX. The method ofho lding these papers in
situ in the camera will be given in the chapter devo ted to
apparatus . After usingthe ferrous oxalate developer befo rewashing
,we usually pass the paper through a bath ofwater
which has been rendered Slightly acid by hydrochlo ric o r
sulphuric acids. This prevents the fo rmation ofany oxideofiron in the water, and removes the o xalates . The negative after well washing is fixed and again washed. The
paper i s rendered translucent by waxing, or by applyingcrude paraffin to the back. The Eas tman Company alsoi ssue what they call stripping films. These , after fixing,are transferred on to a collodionised plate, placed underpressure for a quarter ofan hour, and then treated l ike acarbon print (see page be ing immersed in ho t water.The paper support strips offand leaves the film bearing thenegative image on the glass plate . A thin sheet ofgelatineis then soaked in water and applied to the film. The two
Silver Printing. I43
are dried toge ther, and when thoroughly desiccated the
negative is s tripped offthe glass. This thin film can be
printed from either side .
Mr. Warnerke has introduced a negative tissue which is
remarkable fo r the fact that it consists ofa paper, suppo rt,rendered almo st transparent by some preparation ofvarnish,and coated on bo th sides with a gelatine emulsion. Suchtissue is deve loped by the alkaline o r ferro us oxalate de
veloper, and is printable at once wi thout further~
operationsbeing gone through.
Mr. Vegara has taken up a paten t ofthe late Mr. W.
Wo odbury fo r the production ofa tissue ofvery much the
same character. The fault in all paper negative s IS the
presence ofa certain amount ofgrain, which is very objec
tionable for some purpo ses, mo re particularly in smal l
pictures.
CHAPTER XXI .
S I L V E R “ P R I N T I NG .
IN the fourth chapter the results ofthe action oflight onsilve r chlo ride and o rganic compo unds ofsilver were shown .
In this part i t is propo sed to treat the subject rather morefully, as Silver printing entire ly depends upon it
The student woulddo we ll to make the fo llowing experi
men ts for himse lf, as , by so do ing the rationale o f'
the variations in the pro cesseswill become familiar to him,
andmanyfailu res will be avo ided bya study ofthe theo ry.
Take any o rdinary paper which contains Siz e ofsome
description, and immerse it in a so lution ofsodium chlo ride .
Sodium chloride 1 grammeWater . 50cc .
Hang it up and allow it to dry, and in non-actinic l ight
(adopting the manipulations which Will be presently"de
144 Silver Printing.
scribed) float several pieces ofconven ient dimensions‘
on a
so lution ofsilver ni trate for three minutes.
Silver nitrateWater
When dry to the touch,place one ofthese piece s under
a negative in a printing frame, and expo se i t to the actionofthe sunl ight ; after a few seconds o pen the frame in
a subdued light, and no te the result. The parts actedupon by light will have a vio le t tint, and if ammoniabe applied to a po rtion ofthe darkened paper it will befound that the image almo st entirely disappears . For
reasons already given this will indicate t hat the s ilverchlo ride i s disso lved. Allow further play of sunlight,say fo r a couple ofminutes, and again no te the re sult.It will be found that the image i s much redder in
colour, and that ammonia fails to remove all the co lo ration. From this we infer that the o rgan ic compoundfo rmed by the s iz e ofthe paper and the silver nitrate i sacted upon . Next take ano ther sheet ofthe same paperand wash out al l excess ofsilver n itrate , and expo se undera negative, and examine the print at the same intervals as
befo re . I t will be found that the sho rt expo sure produceshardly any perceptible darkening, whils t with the longer iti s much less than in the previous experiment. From the
results ofexperiments already detailed in the fourth chapter,i t will be seen that the absence ofs ilver nitrate prevents thedarkening ofthe silver chloride , and that the o rganic c ompound is the more impressionable . A minute examinationofthe image will also show that there is a spo tted irregularappearance in the darkest parts. There i s
'
an easy
theo re tical explanation of this .“
The chlo rine l iberated
from the darkening silver chlo ride i s taken up by the
o rgan ic compound, bleaching it to a certain extent, formingwhite chlo ride ofsilver,
-which in i ts turn is'
capable ofbe ingacted upon by l ight. Experiments with similarly washed
146 Silver Printing.
certainty. Albumen coagulates in the presence ofni tric’
acid, and also at 65
°C. It is precipitated
,but not coagu
lated by alcoho l . It combines with the metals,prominent
amongst which is the compound it fo rms with silver. Silveralbuminate i s whi te, turning a dark-red brick co lour in
the presence ofwhite or o ther actin ic l ights . The change
is speedily effected, and, l ike o ther o rgani c compounds of
silver, is not disso lved by ammonia after darkening, though
the addition ofan alkali speedily disso lves the white albuminate . This alone prevents the adoption ofan alkaline so lution
ofsi lver, such asthe ammonio -nitrate ofsilver, for sensitisingpaper coated with this and some so luble chloride, as the
effect would be s imply to disso lve it. Gelatine, which is thesiz e used in some papers, combines with silver, and forms a
red tint on expo sure to light owing to its co lour, and the
greater difficulty oftoning, i t is not usually employed in printingoperations. Starch (C9H1005) fo rms a compound withsilver, which on exposure to light darkens to a more vio le tco lour than e i ther ofthe preceding. I t is largely used insizmgpaper, and i t is consequently necessary to no te thisco lour.From the forego ing remarks i t will be seen that all the
bodie s which are employed in siz ing o rdinary paper willcombine with silver, but there are o ther reasons why albumeni s that which i s usually employed. I ts adoption i s due to
the fact that i t remains on the surface ofpaper, forming asmoo th and thin layer, which is capable ofho lding in situ
the different chlo rides , and that on the application ofsilvernitrate this de licate film i s converted into an o rganic saltofs ilver together with the silver chloride. In all printingoperations one po int is a desideratum, viz . that the imageshould be on the surface ofthe paper, and not sunk into i t.The impo rtance ofthi s may be tested by sensitising albumenised paper on the reverse s ide, and endeavouring toohain a print on the albumen surface in the o rdinarymanner. I t will be found that the image will appear feeble
Tneory ofToning. 147
by reflected ligh t, though by transmitted light it wil l appear
we l l-defined and dense When albumen i s usedfresh,andin
a slightly alkaline condi tion, the resulting print po ssessesgreater stabili ty than any ofthe fo rego ing substances as re
gards delicacy ofimage i t canno t be surpassed. U nfortun
ate ly albumen is mo st easily applied to the surface ofpaperwhen slightly acid, the acidity be ing due to decompo sition,and the resul ting compounds fo rmed are mo re liable to
change .
It'
may be asked, why no t print in pure silver chloridealone , held in situ by some vehicle, such as co llodion ? Thisis no t impo ss ible though impracticable, as the reduction of
the si lver chlo ride image by the fixing so lutions i s so great thatthe print would be wanting in vigour. With the addition of
some o rganic compound, however, i t becomes quite feasiblebut then,be i t remembered, the depth obtained is due to thato rgan ic substance . Thanks to the discovery ofMr.WhartonSimpson, that si lver chloride and some o rganic compounds ofsilver (amongst which we may name citrate ofsilver) willemulsify in co llodion, prints are readily obtained by theco llodio -chlo ride, and po ssess a beauty which canno t be surpassed. In the next chapter this process will be describedmere ly mentioning/en passant that in this process, as inany o ther in which printing on silver chlo ride takes place,an excess ofsilver nitrate is beneficial .The co lour ofprints obtained is always objectionable if
fixed directly after taking out ofthe print ing frame ; andreso rt is had to an operation called toning to render it morepleasing. This toning may consist of gilding the silverimage
,platinising it
,or substituting some o ther metal for it.
The co lour ofthe silver print when appearing through thiso the r metal may give a pleasing tint, or it may fail to do so
,
acco rding to the extent to which the operation i s carried. It
wil l be seen, in the pract ical instructions in printing, that thepictu re is mo re or less washed in water befo re toning. Byimmersion in water the vio le t-co loured image becomes of
'
a
1. 2
148 Silver Printing.
red co lour to what this change i s due i s rather uncertain .
I t has been held that it i s due to the water disso lving a certain part ofthe .
s i lver oxide . I t may be due to a differ entcompound being fo rmed by the combination ofwater withthe altered compo und, but this i s doubtful. In o rder to tonethe picture
,certa in so lutions ofgo ld, platinum,
o r o therme tals are made
,and the print immersed in them the first
ofthese me tals, in the shape ofgo ld trichlo ride,i s that
usually employed. I t is, therefo re, propo sed chiefly to con
fine the remarks on toning to that process in which thatmetal is principally employed . The go ld trichloride has theformula Auc l3, which i s a fairly stable compound. Ifits
temperature be raised to 170°
C . it becomes decompo sed,a pale yellow and inso luble powder, go ld chlo ride, AuCl, resulting, chlo rine be ing evo lved. When the fo rmer salt ofgo ldismixed with a so lution ofsilver n itrate, the chlorine leavesthe go ld to form s ilver chloride, and the latter salt ofgo ld i sfo rmed. Acetates
,as also the carbonates ofthe alkalis, are
capable ofprecipitating go ld from a neutral so lution in the
presence ofany disturbing cause— such as o rganic matter.I t will be seen from the formulae given for toning so lu
tions, p. 158, that one contains chloride oflime, and as anexample ofone kind ofton ing this one will be considered.
Ifa print so tho roughly washed that all excess of silvernitrate i s eliminated be immersed in this so lution,
it will befound that the go ld depo sits very slowly, and that the imagebecomes feeble and spo tted in appearance , whereas with aprint in which the excess of s ilve r ni trate has been bu t
partially removed, the toning or gilding action takes placemuch more readily.
Chloride‘
oflime i s a mixture ofcalcium chlo ridewith calcium:hypo chlo rite (CaCl 2), be ing made by passingchlorine over. calcium hydroxide, or common slaked lime .
Calcium hypochlorite
2 Cal~120 2 C12 2 H20 CaCl2 CaCl,O2
Calcium hydrate Chlorine Water Calcium chloride
150 Silver Printing.
immersed in a so lution oflead nitrate, and again slightlywashed. On applying the chlo ride ofl ime toning bath , theprint quickly changed to a rich brown co lour
,and, after
fixing,had all the qualities ofa properly toned pr-int . In
this case the lead combined with the chlo rine,and acted
like the s i lver nitrate . Ano ther toning bath, consisting of
lime wate r and go ld trichlo ride, was prepared . Two wellwashed prints were immersed, and left respectively forthree minutes and fifteen minutes on the latter a sl ightdeposit ofgo ld was visible , and also a diminution in the
depth ofthe print after fixing. With the former the printwas less affected. Prints in which silver nitrate and lead
n itrate were present bo th toned admirably, but rather toorapidly fo r safe ty. On examination a trace ofhypochlo rousacid was found in the toning so lution. A slight addition
ofchloride ofl ime was next made , and prints in which silver
and lead n itrate were present were immersed in the so lutionthey toned
.
gradually and regularly. This last experiment,which was confirmed by o thers, showed that the calcium
hypo chlo rite contained in the chloride oflime acted as aretarder to the toning Operation, as the chlorine containedin hypo chlo ro us acid combined with the silver nitrateequally with that evo lved from the precipitating go ld. Thismanifestly would check the depo sition ofthe go ld.
Ano ther toning so lution used is one made with sodiumacetate and go ld. In practice i t i s found that toning takesplace mo st regularly when the print has been previously wel lwashed. On adding a so lution ofsodium acetate to silver
nitrate , a sparingly so luble silver acetate and sodiumn itrate are fo rmed by double decompo sition. If
,then,
the s ilver n itrate be present in the print, the greatest
po rtion of the adjacent sodium acetate i s decompo sed,
and sodium n itrate left in its place. The sub-chlo rideand oxide ofsilver bo th seem to be as readily attackedby chlo rine as the si lver
‘
acetate . Hence the chlorine,having no thing at hand to absorb i t (sodium nitrate not be ing
Tneory ofToning. 51
able to do so ), attacks the silver of“
the print and produces
the bleaching action already referred to . When al l the freesi lver nitrate , however, i s washed away, the conditions arechanged the sodium acetate will abso rb chlo rine , andforma chlorace tate and hydrochlo ric acid, as indicated in the
fo l lowing equation
Go ld Sodium Sodium trichlor Hydrochlorictrichloride acetate GOId
acetate acidz AuCi, 2 Au Nac ,01,o , 3 HCI.
Eventually an evidence ofthis reaction may be tracedin the fact that the so lution becomes acid, and refuses totone .
The forego ing experiments exemplify the followinglaws
That a neutral so lution ofthe go ld ton ing bath isnecessary.
(2 ) That some active so luble chlo rine abso rbent must bepresent, e ither in the print or in the so lution.
That when the aflinity ofthe absorbent for chlorinei s vio lent i ts action must be retarded.
In considering any toning so lution, these three qualifications must be taken into account, and ifone ofthem be
vio lated a perfect print must no t be expected.
The theo ry offixing prints is the same,as already in
dicated at page 74, and need scarcely be touched upon.
The reason why po tassium cyan ide canno t be usefully employed as a fixing agent has been already shown to be due tothe fact that the organi c oxide ofsilver is so luble in its so lut ion . I t must be strongly impressed upon the student thattwofo rms ofdouble hypo sulphites ofsilver and sodium are
fo rmed, one ofwhich is so luble and the o ther inso luble (seep . The so luble fo rm undergoes a change in l ight whichrenders i t inso luble hence fixing the print in daylight
shou ld be avo ided.
When pr1nts are immersed in a so lution ofsodium hypo
sulphite, a certain po rtion ofthis salt is combined with the
I 52 Silver Printing.
double salt ofsilver formed. Every print immersed therefo re leaves a smal ler quanti ty ofthe uncombined sodiumhypo sulphite in so lution and s ince the double salt ofsi lver
and sodium i s so luble in the uncombined hypo sulphite, it
fo llows that care must be taken not to fix too largean area ofprint in the same so lution, o therwise the in
so luble salt will be fo rmed in the pictures. The effect ofthis i s seen in the fading ofprints . No amount ofwashingwill e l iminate this inso luble fo rm. The acid vapours to befound in the air will decompo se i t, and cause a liberation
ofsulphur compounds, which gradually bleach the blackportions of the image, and give the whites a j aundicedappearance .
Even where the soluble double hypo sulphite has beenformed, washing the prints in a thorough manner isessential for permanency , for any trace ofit will decompo se in a similar manner, as will al so the sodium hyposulphi te i tself. In the writer’s opinion the prints should be
'
immersed in two separate so lutions ofthe sodium hyposulphite the first will fo rm the necessary so luble salt , andthe latter will cause i t almost entire ly to disappear, all tracesbe ing subsequently e liminated by the washing water. SomeAmerican writers have propo sed to shorten the washing of
the print by a final immersion in a so lut ion of iodine,tetrathionate being fo rmed ; the reaction would be asfo llows
Sodium hyposulphite Iodine Sodium tetrathionate Sodium iodide
2 Na,S,O3
I2 NaS,O6 2 NaI .
Bo th these salts are so luble in water, but less so than thesodium hypo sulphite, and the tetrathionate appears to be
more readily decomposed. If a silver compound be present with the hypo sulphite, the same reaction apparentlytake s place , though an exact analysis of
‘
i t has not yet been
undertaken. The quality ofthe washing water i s impo rtan t- it should ifpossible be rain water, o therwise pure spring
154 M anipulations in Silver Printing.
lowing fo rmula may be used for the albumen so lu tion, withwhich to coat the pape r
Ammonium chloride 10grammesSpirits ofwine 15 cc .
Water 135 cc .
Albumen 450cc .
The first three are tho roughly mixed, and the albumen,derived from the whi tes ofeggs
,i s gradually added to the
so lution . Perhaps the simplest way of effecting so lutionand perfect mixture i s to halffi ll a bo ttle with the albumen,and then to add a fair supply ofroughly powdered glass.Shaking the bo t tle will cause the flocculent matter to be
F IG . 30.
broken up, and leave it in a state ready for fil tering through
sponge or wel l -washed tow. The paper,having been cut
into sheets ofconvenient siz e, is floated on the fluid,con
fained in ' a dish, the hands grasping its two oppo siteco rners . The convex surface ofthe paper thus fo rmed isfi rst brought in contact with the so lution. As the hands aredrawn apart, the paper pushes out all air-bubbles befo re i t,and at length lie s in perfect contact with the so lution . I t is awise precaution to take, however, to raise the pape r from one
co rner, to make certain ofthe absence ofall air-bubbles, andthen to allow it to remain at rest on the so lution for a minute.
I t is next removed, and hung on a line to dry, being heldby a couple -of,American clips, or thrown over a stretchedco rd. Thi s last plan i s apt to cause markings
,tho ugh it i s
probably necessary when large sheets ofpaper are manipu
SensitisingBat/z . 155
lated, owing to their tendency to tear ifonly suspended by
two corners . When dry the paper will no t be flat, and shouldtherefore be ro l led and put away be tween flat boards.When a print having a dull surface i s required, the fo llowing
fo rmula is sometimes used
Ammonium chloride 6 grammesGelatine ‘6 grammeWater 300cc .
The gelat ine i s first dis so lved in ho t water, and then theremaining sal t added. The paper i s floated for three minuteson this so lution.
Ano ther mode ofproducing a dull surface, and whichis very effective, i s to use resinised pape r. The annexedfo rmula is wo rkable, and is due to Mr. H. Cooper, jun.
Frankincense 1 gramme
Mastic '8 gramme
Calcium chloride from '
5 to I gramme
Alcoho l 45. cc .
Good Rive paper i s immersed in this so lution for halfaminute, after which it is ready for floating on a moderatelystrongsensiti s ing bath .
Tlie SensitisingBat/t .
When a paper is weakly salted, say, having halftheamount ofchloride ‘
given in the formula for albumeni s ingpaper, a weak sensitis ing bath is usually employed, whereaswith paper strongly salted, o r for the resinised paper, one
somewhat stronger i s necessary. The fo l lowing fo rmulaewill show what the extreme strengths of so lution shouldbe
6 grammes100cc.
I 5grammes100cc.
The paper is floated on e i ther ofthese so lutions in the
manner given for albumenising paper, the time ofcontact
156 M’anipulations in Silver Printing.
varying fromthree minutes in hot to five minute s in co ldweather. It should be removed slowly from the sensiti singbath to prevent waste ofso lution, and when hung up to dryby an American clip in the dark room, the drainings should beco llected by attaching a slip ofblo tting-p aper to the bo ttomco rner. I t is always advisable to have one co rner lowerthan the o thers
,as the sensitising so lution thus drains more
equally away.
In o rder to preserve sensit ised paper from co lo rationdue to the decompo sition ofthe o rganic salt ofsilver
,i t may
be placed between sheets of blo tting-paper impregnatedwith sodium carbonic. Ready sensiti sed paper is sold in themarket , the disco lo ration be ing prevented, as a rule, by theaddit ion ofnitric acid in some fo rm.
Printing tlze Picture.
Printing operations are rarely carried on in the same tem
perature and state of' atmo spheric mo i sture as those in which
the paper is dried hence i t is advisable to allow the paperto assume the conditions ofthe fo rmer before rigidly confin ing in the frame. The fact that paper expands in mo is tair at once shows that the dimensions ofa pho tograph can
never be re lied upon as be ing accurate. Measurements haveshown that a drawing
,for instance, will vary as much as 1 per
cent. in certain conditions ofthe atmo sphere. The sameremark applies to plans printed in the o rdinary lithographicpress ; the scale i s never co rrect except under fixed con
ditions.
The negative should be placed in the printing framewith the varni shed side next the paper. A convenient fo rm
offrame, and one which is usually employed by pho to
graphers, i s shown in the diagram. B i s a shee t ofthickplate-glass, which rests in a frame, A.
‘ The negative is
placed ou
'
the glass, the sheet of paper over it , then asmoo th felt pad
, and over this a back, 0, hinged in the
158 M anipulations in'
Silver Printing.
The fo llowing toning baths may be considered asstandards c
Go ld trichlo ride 2 5grammeChloride oflime '
25grammeChalk (precipitated) I teaspo onfulWater 1 litre .
The water should be bo iling ifthe so lution be required
to be used at once , o therwise it should stand in an unco rked
bo ttle for twenty-four hours
Gold trichloride '
25gramme
Sodium acetate 7grammesWater 1 litre
Thi s should be mixed a day befo re be ing used. A very
excellent toning bath for ready sensitised paper is as fo llows
1 . Borax 24 grammesWater 1 litre.
2 . Gold trichloride °
25grammeWater 1 litre .
The borax should be disso lved in the water with the aid
ofheat. Nos. 1 and 2 should be mixed in equal parts im
mediately before use tofo rm the toning bath. Befo re ton ing,the so lutions should be fi ltered in a clean dish, slightlywarmed ifthe weather be co ld. The prints are placed in
water ofabout 15°
C.,and the washing continued as indi
cated in the last chapter,acco rding to the toning bath
employed . They are then immersed in the toning so lu tion
three or four'
at a time,and the dish is kept in constant
mo tion,so as to allow an equal toning action throughout .
I t is likewise essential that no two prints should sticktogether, for the same reason. Acco rding to the co lour of
the print desired, so must the cont inuationofthe toningaction be regulated. Ifa rich chestnut brown be required,
Toningana’F iringt/ze Print. 159
but very l ittle apparent change in the co lour ofthe prin t is
necessary , whereas ifan engraving black tone i s sought, theaction must be continued till the image i s decidedly blue.
I t is no t to be inferred that these rule s are abso lute in
every case ; so much depends on the siz ing ofthe paper
and on the amount ofchlo ride present that they are no t
applicable in all cases, but with Saxe paper, prepared asgiven in the fo rego ing fo rmulae, they will ho ld good.
The fixing so lution i s made up as fo llows
Sodium hyposulphite 200grammesWater 1 litre .
Between ton ing and fixing it is essential that the printsshould be well washed . The necessi ty ofthi s maybe understood by referring to p . 151 . I t has been sometimes recom
mended to acidify the washing water, but the proposer ofthisplan can have had no thought ofthe danger to the per
manency ofthe prints which he therebyintroduced an acid
at once begins the decompo sition ofthe hypo sulphite. The
writer strongly urges the necessity of a s trongly alkalinecondition ofthis bath , and in practice he adds 50 cc .
.of
strong ammonium hydrate to it when fixing prints . Mr.
J. Spiller was the first to point out the use ofammon ium
carbonate in the so lution ; he showed that it disso lved out acertain compound left in the whites ofthe picture , whicho therwise was inso luble, and which readily decompo sedunder atmo spheric action. The pictures should be immersed in the so lu tion for ten or fifteen minutes, the time
varying according to the thickness ofthe paper they shouldthen be washed (unless they be placed in a second so lutionofhypo sulphite, as already suggested), rapidly at first andafterwards mo re slowly. Perhaps the best way ofeliminating the greater part ofthe hyposulphite i s to place the
prints in a large tub ofwater, which is kept in mo tion, andafter five minutes’ washing to place them in a smaller
160 Illanifinlations in Silver Printing.
quantity ofwater. After this they may be removed to awashing trough, where the water will be changed seve ral
t imes an hour. The accompanying idea ofawashingtrough
may prove useful. It is one which was designed and is
employed by Mr. England, and has answered we l l thepurpo se for which it is intended. A is a trough, at the side
ofwhich is a syphon , S, the inside legreaching to within 2
o r 3mill ime tres ofthe bo ttom,the bend
'
ofwhich i s a little
below the top ofthe trough. B i s a cradle , pivo ted on, a
rod, E, which passes through the sides ofA, as shown. c is
a water-whee l ,attached to the wall, on to which a gentle
stream ofwater the tap, F , plays . G is a small arm at
F IG . 32.
tached to the axle ofthe whee l, having a rodsuspendedfromit,which 1s attached to the cradle
,B . As the whee l slowly
turns the rod is raised, and the . prints are caused to moveabout in the water. The water runs into a trough throughthe pipe H, and when i t reaches the top ofsyphon pipe thetrough gradually empties itse lf
,leaving the prints o n the
gutta -percha strips which fo rm the bo ttom ofthe cradle .
I t will be seen that the supply of. water must always berather less than that which the syphon is capable ofcarryingaway. A useful addition to the trough i s a ho riz ontalpipe attached to the we ll ofthe wheel, moving from side to
162 Col/odio and Ge/m‘ino-e/zloria’e Processes.
material ofwhich they are made should be , ifpo ss ible, glass
or po rcelain, and never tin or z inc.
The defects in prints due to defective manipulation, andnot to want ofartistic skill, are but few in number. Red
marks that repel the toningso lution can usually be tracedto contact with hot and mo i st fingers . A red tone afterfixing is due to an insufficient depo sit ofgo ld, and a bluetone to an excessive depo s it. The whites may appearyellow through the fixing so lution be ing of insufficientstrength , or through paper being used when the sensitivesurface shows signs ofdisco louring through too long keeping. The general cause ofthe fading ofprints has alreadybeen detai led.
CHAPTER XXII I .
COLLODIO AND GELATINO-CITRO-CHLORIDE PROCESSES.
TH IS pro cess i s intended to be employed for printing onglass or paper, and for permanent s ilver prints no thingbetter can be desired. The fo llowing is a formula which istakenfrom the published pro cess ofMr.Wharton Simpson
No . I . Silver nitrate 4 grammesWater 4 cc .
No . 2. Strontium chloride 4 grammesAlcoho l 60cc .
No . 3. Soda grammeAlcoho l 30cc .
NO. 4. Citric acid 3 grammesAlcoho l 30cc.
To every 50 cc. ofplain co llodion 1 cc . ofNo . I i sadded, be ing previously mixed with 2 cc. ofalco ho l, ino rder to prevent precipitation ofthe pyroxyl ine. Next 2 cc.ofNo . 2 are added with constant shaking, and '
5 cc .
ofNos. 3 and 4. In a quarter ofan hour it is fit for use.
MakingEmulsion. 163
I t will be no ted that there i s a large excess ofsilver nitratepresent. The amount necessary to combine with the
strontium chlo ride is only 2 9 cc . and with the sodiumcitrate 18 cc. ofsilver ni trate
,there i s, therefore, present
mo re than double the amount ofsilver n itrate necessary
to combine with them. As already shown,thi s exce ss is
necessary. In practice, particularly when printing on glass , .
i t has been found very difficult to prevent the salts crystallisingin the film whilst drying and in o rder to overcomethis so urce ofannoyance
,a me thod analogous to that of
the washed bromide'
emulsion pro cess may be employed .
The above propo rtions ofstrontium chlo ride and sodiumcitrate may be kept, but the silve r ni trate should be reducedto one -half. The plain co llodion i s made up with halfthe so lvents usually employed to disso lve the pyroxyline,and consequently only halfthe above quanti ty is used inm ixing the co llodio -chlo ride . After the emulsion i s fo rmedit is po ured into a dish, allowed to set
,well washed, dried,
and then disso lved up in the proper proportions ofso lvents ,in the alco ho l ofwhich T
1; cc. ofthe si lver nitrate so lution
has previously been added. In this state the co llodiochlo ride contains the same necessary excess of silver
nitrate , but the strontium and sodium n i trates are absent.This diminishes the risk ofcrystallisation taking place inthe film,
and with a certain class ofpyroxyline this i s
en ti re ly avo ided. The silver citrate supplies the necessaryo rgani c matter by which a vigo rous image is obtained.
If a glass plate has to be coated with the emulsion,the same directions as tho se given for coating emulsionplate s should be fo llowed, with the addition that it is well todry the film befo re a fire, and to print whil st it is still warm.
When a paper has to be coated more difficulty is found,
The paper must be strongly siz ed ; o rdinary paper allowsthe co l lodion to penetrate through its pores, and a mealyappearance i s some time s the result. 'Arrowroo t paper, supplied by mo st dealers in pho tographic materials, _i s perhaps
M 2
164 Col/oo’io and Gelatino-o/z/oride Processes.
the best kind . Obernetter, ofMunich, uses an enamelpaper as a support. A similar paper is prepared bycoating o rdinary paper with a strong so lution ofge latine,in which barium sulphate
,known as ‘Mo untain snow
,
’is
mixed. When dry, this gives an impervious skin to the
surface ofthe paper. The paper is pinned on to a board,the edges be ing turned up 2 or 3 m illimetres, and at oneco rner a spout is fo rmed,from which the collodion is pouredoff. The emulsion i s now applied as ifto a glass plate .
Some operato rs find that by fuming the film with the vapour
of ammonia, after thorough drying,"
increased vigo ur isimparted to the print. In any case this end may be
attained by applying a so lution ofgallic acid and ace tate of
lead,together with a few drops ofa so lution ofsilver n itrate .
The print may be toned in any ofthe o rdinary toning baths .Ammonium sulpho -cyanate and go ld have been recom
mended, but the tones thus obtained vary greatly in richness.
For printing on glass a special printing frame has beendesigned
,but thi s is no t required ifthe precaution be taken
to gum a strip ofpaper along.the corresponding edges of
the sensitive plate and ofthe negative. They may then beseparated one from the o ther with the certainty that theywill fall into the ir o riginal po sition . The prints are fixed insodium hyposulphite, made as under
Sodium hyposulphiteWater
An' immersion ofe ight minutes in this so lution i s sufficient .A ge latino -citro -chloride e mulsion, which was intro
duced by the autho r, may be made as fo llows
1 . Sodium chloridePotassium citrateWaterSilver nitrateWater
3. GelatineWater
166 Printingwit/i I ron and Uranium Compounds.
&c .,by contact . An exception,
however, i s in the platinotype process, which i s now very generally employed, and to
which a separate chapter is allo tted.
PrintingProcesses zoitnSalts ofI ron.
Sir John Herschel invest igated the relative sensitiveness ofthe differen t salts ofiron, and came to the con
e lusion that the double citrate ofiron and ammonia wasmo re readi ly acted upon by light than any o ther, whils tafter it came the double oxalate of iron and po tassium.
To produce the former salts, take a weighed quantity of
ferrous sulphate, dissolve'
in water,and boil with
’
n i tricacid till i t i s tho roughly oxidised and in the ferric statenext precipitate with ammoni um hydrate, and wash the
ferric oxide in warm water to get rid ofall the so luble salts .Transfer the washed oxide into a glass beaker and graduallyadd a so lut ion ofcitric acid
,and warm. When a small
trace offerric oxide remains undisso lved, the addi t ion of
the citric acid should be stopped. Take the same amount
ofcitric acid already added to the ferric oxide, and carefully neutralise it with ammonium hydrate, testing the
operation with l itmus -paper. Then mix the two so lutionstogether and evaporate to dryness over the water-bath , andwhen sufficiently concentrated allow the crystals of the
double citrate ofiron and ammonium to separate out. Aftercarefully drying between blo tting-paper they are ready foruse . The do uble oxalate ofiron and po tassium may be
prepared in a similar manner. When required to renderpaper sensitive the fo l lowing proportions shouldbe taken
Double citrate ofiron and ammonium 10grammesWater (distilled) 100cc.
This is applied to the paper with a brush, or e lse the paper
may be floated on i t. When dry it is expo sed beneath anegative from a minute in bright sunshine to a quarter of
an hour in diffused light, when it is ready for development,
He rsclzel’
s Processes. 167
though the imagewi l l be barely visible. Ifa blue picture berequired, all that is necessary is that the print should be
immersed in a so lution ofpo tass ium ferri -cyanide . After afew seconds the image will be found perfectly developed.
A copious washing in water (in which a little citric acid hasfi rst been disso lved for the first washing) will d is solve out
all the so luble salts , and leave the blue image unchanged.
The theo ry ofth is reaction has already been explained in
Chapter IV.,and need not again be discussed . When pic
tures were developed by this method the process was calledcyano type by Sir J . Herschellf
Instead ofdeveloping with'
the po tassium ferri-cyan ide,the expo sed paper may be immersed in a dilute and neutral
so lution ofgo ld trichlo ride. The go ld gradually depo sit son the exposed po rtions and gives a purple image . Thisme thod of producing pictures on an iron salt has been
called the chrysotype . The reduction ofthe go l d fo llowsfrom the fact that theferrous salts are capable ofreducingsalts ofgo ld to the metallic state when coming in contactwith them in so lution. In the case ofpictures taken bymeans ofthe double oxalate ofiron and ammon ium,
it is
we l l to add tq the go ld so lution a little neutral ammonium
oxalate. The development in this case takes place veryrapidly. To fix the pictures they should be immersedin water sl ightly acidified with hydrochloric acid, and thenbe tho roughly washed.
An expo sed paper prepared with any double sal t of
i ron and ammonium may be developed by floating it on
a so lution ofsilver n i trate to which a trace ofgallic acidand ace ti c acid have been added ; the ferrous salt reduces the
silver n i trate , and causes the metallic s ilver to deposit where
the ferrous salt existed. The gallio acid subsequently cause safurther reduction ofthe silver
“
nitrate,and the fi rs t depo si t
o fsilver attracts the fo llowing. An image i s thus built up.
Founded on the same reaction pictures may be obtained
by means of“platinum t e trachlo ride, mercuric .
chlo ride, and
168 Printingwit/z I ron and Uranium Compounds.
po tassium dichromate, &c . ,though greater expo sure with
these is necessary.
Ano ther modification ofthe -iron process described i sthe production ofa po sitive from a po sitive. I t i s founded
on the fact that po tassium ferrocyanide fo rms an inso lublecompound with a ferric salt and not with a ferrous salt. If
,
then ,a ferric salt be acted upon by light i t gets reduced to
the ferrous stale, and ifthe paper be floated on po tassium
ferro cyanide,on the part unacted upon by light a blue pre
cipitate i s formed,and on the part acted upon a slight stain
ofl ighter blue. To prepare a paper which shall give clean
prints i t i s usual to mixthe iron salt with a so lution ofgum,
and to develo p with amixture offerro andferri -cyanides of
po tassium . Bo th the expo sed and non -exposed parts ofthepaper are tin ted blue, but the gum disso lves offthe portion
ofthe paper expo sed to light, carrying that po rtion ofthe
blue co louring matter with it which was caused by the ironwhich was reduced to the ferrous state and which hadcombined with the po tassium ferricyanide thus leaving
the part unacted upon by light behind as blue on a whiteground . This process i s useful for copying tracings, but
the paper in this case must be placed in contact with the
lack ofthe tracings . In engineers’ drawing-offices paper ofthis de scription has come into very general employment,as i t can now be purchased of
‘
excellent qual ity.
About 1857Salmon and Garnier brought out a processdependent on the fact that the ferrous salt resulting fromferric citrate ismo re hygro scopic than the ferric citrate itself.Paper coated with the ferric citrate i s expo sed, and -thencovered over by an impalpable powder, such as plumbago .
The surface i s then gently breathed upon, andmo re or less of
the powder adheres, approximate ly in the inverse ratio ofthe
amount ofactinic light that has been allowed to fall on i t .When sufficient intens ity is secured the non-adherent powderis removed by a soft brush . The unaltered citrate 1s easilywashed out ofthe film
,leaving the powder image on the
170 Printingwit/i I ron and Uranium Compounds.
presence ofo rganic matter. The fo llowing may be taken
as a good strength ofso lution
U ranic nitrate 40grammesDistilledwater 250cc .
The paper should be floated about e ight m inutes,as for
sensitising paper in the silver bath . When dry it is readyfor expo sure
,which is somewhat long. To produce a brown
picture, float the expo sed surface on the fo llowing
Potassium ferricyanide 1 grammeNitric acid 2 dropsWater 250cc .
In about five minutes the who le ofthe detail will be visible.
After thoroughly washing in slightly acidulated water theimage will be fixed.
To produce a grey picture, the expo sed paper should befloated on
Silver nitrate 2 grammesWater 40cc.
Acetic ac id 3 or 4 drops.
The image appears very rapidly, and attains full intensityifthe expo sure have been sufficiently long. Ifit be weak, afew drops ofa saturated so lution ofgall ic acid added to theabove will produce the desired effect. Washing in waterwil l fix the picture
,though care should be taken that no
chlorides or carbonates are present in i t. Ifany doubt existas to the ir presence
,sodium hyposulphite must be reso rted
to . The picture maybe toned with go ld, platinum, o r o thersalts
,as may be desired.
Uran ium will also reduce the so luble salts ofgo ld to themetallic state hence a picture maybe deve loped with these .
A pleasing varie ty in these prints can be made bym ixing with the uranic so lution some ferric salt
,and de
veloping with the po tassium ferricyanide. The resultingtone i s richer and quite as permanent. Various o thermodifications have from t ime to time been made for theproduction ofdifferent shades ofco lour in the print.
Tlie Platinotype Process. 171
CHAPTER XXV.
THE PLATINOTYPE PROCESS.
THE beautiful platino type pro cess , which is rapidly gainingfavour, i s dependent on the reduction ofa ferric salt to theferrous state. Mr.Willis , to whom the discovery ofthis processi s due
,found that when a platinous salt . the chlo ro -platinite
ofpo tassium,was mixed with a ferric oxalate and then floated
on a hot so lution ofneutral po tassium oxalate,that where
light had acted, there the platinum salt was reduced to the
metallic state . The fo l lowing seems to be the reaction :
Ferric o xalate becomes Ferrous o xalate and Carbon dio xide.
Fe2(CZO4)3 2 Fe(C2O4) 2 CO2
When the hot so lution ofneutral po tassium oxalate isappl ied, the ferrous o xalate i s decompo sed and the fo llowingreaction takes place
Chloro -
platinite become Ferric o xalate ,ofpo tassmm
Ferrous o xalate and
6 3 K2PtCl
,2
Potassium chloride and Platinum.
Fe2C16 3 Pt.
The image fo rmed is in one of the most stable substances known be ing unattacked by o rdinary atmo sphericinfluence, the pictures by this process may be considered to
be permanent.The paper used has to be siz ed to prevent the image
nking into the surface Gelatine or arrowroo t are the
siz ings which are most usually employed. For the latter siz ethe fo l lowing preparation answers — 10grammes ofarrowroo t are rubbed up in a mortar with a li ttle water, andgentlypoured into a l i tre ofwater which has been brought up to
bo il ing -po int. After the l iquid has bo iled a sho rt time
172 Tlie Platinotype Process.
z o o cc.me thylated spiritare added, andthe so lution i s fi l tered .
The so lution should be poured into a dish slightly largerthan the paper to be employed. The sheets are drawn into
the so lution, taking care that no air-bells form. Theyshould be left in it 2 or 3 minutes , when they are taken
out and hung up in clips to dry.
The fo llowing so lutions are prepared acco rding to the
directions given by Piz z ighelli and Hubl
No 1 . Ferric o xalateO xalic acidWater
This so lut ion must be kept abso lute ly in the dark.
No 2 . Chloro-platinite ofpotassium 8 grammes“later 50cc.
When preparing the sensi tis ing so lution the fo llowing isthe mixture
The two experimenters we have quo ted add a certainproportion ofa so lution ofchlo rate ofpo tash to th i s to givemore or le ss deep blacks, but the above maybe taken as theno rmal so lu t ion.
The coating ofthe paper takes place in a feeble light.Ye llow light is the best, but it is hard to see the co lour of
the so lut ion . The paper should be pinned by the cornerson a smoo th board, and the sens itising so lution applied witha piece offlanne l enclo sing a pledget ofco tton-wo o l . For
c. square about 3 cc. ofso lution i s required . Thisquantity sho uld be poured in the middle ofthe sheet ofpape r, and be immediate ly spread over with a circular motion. The rubbmg should be very gentle, and should becontinued until the coating is as uniform as po ssible . The
paper must be dried in from 5 to 10minutes,and i t is ofthe
174 Printingwit/i C/zromium Salts.
The Platino type Company i ssue paper which gives tonesapproaching sepia, with which they issue special instructions .I t is be l ieved that thi s tone i s dependent on the use ofamercury salt in combination with the platinum.
Platinum paper, before and after printing,should be
stored in boxes containing calcium chloride . Such boxes ,ofa very convenient fo rm
,are supplied by the Platino type
Company. The chlo ride may be dried from time to timeover a Bunsen burner.
CHAPTER XXVI .
' PRINTING WITH CHROMIUM SALTS.
As already po inted out in Chapter IV.,p. 3 2, the dichro
mates are acted upon by light in the presence ofo rganic
matter,and the result is to render such o rganic matter
inso luble in, and non-abso rbent of,water. The fol lowing
experiments may be undertaken.
1 st. Let albumeni sed paper be prepared such as i s described at p . 154, preferably omitting the chlo rides, & c .
,
and employing only the albumen,and float it on a 6 per cent.
so lution ofpo tassium dichromate. Ifthe student expo ses
one ofthese pieces of paper in a dried state beneath anegative or an engraving, he will find that on soaking it inco ld water all the albumen that has been acted upon bylight wil l remain inso luble, whilst that pro tected will readilydisso lve . Three or fo ur small pieces ofgelatini sed paper
may next be prepared by brushing over the paper a viscousso lut ion ofgelatine, in which i s disso lved the above propo rtion ofthe dichromate . When dry, they may be fullyexposed to light beneath negatives ofline engravings. On
immersing one ofthem in co ld water, it will be no ticed thatthe pro tected parts immediately begin to swel l, through the
Swan’
s Process. 175
absorption ofwater, whilst tho se po rtions unprotected remainunchanged . On immersing ano ther sheet in hot water,the pro tected ge latine will disso lve away ent ire ly, whilst theres t wil l remain firmly attached to the surface ofthe paper .Ano ther shee t ofexpo sed gelatin ised paper may next bebrushed over with thin, greasy, l ithographic ink, and aftersoaking in co ld water, a wet sponge maybe applied to removeall the ink that will come away. I t will be found that the
non-abso rbent parts retain the ink, whilst the latter rejectit. Ifpo rtions have been only partially pro tected, as in
the case ofwhat is called a half-tone negative, the ink will
be found to adhere thinly on them, owing to the gelatinehaving become only partially non-abso rbent.
One ofthe earliest processes in which a dichromate wasused was that due to Salmon and Garnier
,and i s s imilar in
principle to the powder processes which are to be described.
Po i tevin and Talbo t were, however, first in the field with apracticable application ofi t.
Swan’s pro cess was the first commercially successful ,and a briefo utline ofi t may no t be uninteresting, as it IS
sul l wo rked by Braun ofDo rnach . The o rganic matteremployed is ge latine, and i t is appl ied to the surface of
paper, after having been co loured with some unalterable pigment, such as lampblack, and sens itised with ammoniumdichromate . The prepared paper is next expo sed beneatha negative t ill i t is j udged sufficiently printed.
The student must now try to realise the wo rk that the l ighthas been perfo rming . Tho se parts ofthe gelatine next thenegative wil l have become inso luble to a depth correspondingto the intensity oflight entering, and as there wil l bebut little ofthe negative which will not allow some l ight to
pass through, i t may be considered that the who le ofthe
exte rio r surface ofthe gelatine has become inso luble, whilstthe so luble po rtions remain enclo sed between the inso lublelayer and the surface ofthe paper. Ifsuch a print wereimmersed in ho t water to d isso lve away the unal tered gela
176 Printingwit/z Clzromium Salts.
tine,the viscid solution would remain imprisoned, and no
development ofthe image would be possible. This difficulty
Swan overcame by cementing the inso luble surface to
paper by a so lution ofindia-rubber. On immersio n in ho twater the -
o riginal paper easily strips off, leaving the water
free access to the so luble ge latine . When this i s com
pletely disso lved away, an image in pigmented ge latineremains on the india-rubbered paper, though reversed asregards left and right. This defect, again, was overcome in
one of two ways— either by using a negative reve rsed asregards left and right, or by the fo llowing pro cedure .
Ano ther piece ofpaper,coated with starch or ge latine, was
appl ied to the image , and allowed to dry in contact. The
india-rubbered paper was then mo i stened with benz ine o r
some o ther indiao rubberso lvent, and detached.
I t w ill be wel l to draw the student’s attent ion to the
ason why the po rtions ofthe film ofgelatine becomeinso luble to depths co rresponding “
to the intensity ofl ight,
instead ofbecoming only'
partially inso luble through theirwho le depth . The l ight that
"
i s chiefly effective 1n causing
the reduct ion ofthe dichromate i s the blue. Now,since
the dichromate is ofan o range co lour, i t is‘
evident thatan abso rption ofthe blue will take place, and experimenthas shown that a small thickness ofge latine co louredby i t will prevent any effective ray being transmi tted. In
o rder to cause the reduction ofthe chromium compound,the amplitude
,multiplied by the number ofthe waves, must
be ofa certain constant numerical value ifthe product fallssho rt ofthis constant no change will be effected. On thisassumption it will be readily seen that ~insolubility will take
place only to certain depths, depending on the length ofex
posure and intensity ofthe light. At the'
same time it willbe seen that i t do es no t necessari ly fo llow that the rolzole of
the ge latine o r o ther o rgan ic body becomes inso luble to thatdepth, but that the ratio ofso luble to inso luble matter inc reases as the depth becomes greater. This last po int i s
178 Printingwit/i C/tromium Salts .
suggestions that have been made for the impro vement ofthe ge latine process. Much ingenuity has been broughtto bear on i t
,and i t seems now to have arrived at a
state bo rdering on perfection. Many of the improvements in it have been patented, and thus the wo rking
‘
of
the pro cess has been in a measure restricted to the
l icensees ofthe Auto type Company, to whom mo st ofthesepatents have been assigned.
i The manipulation ofthe
auto type pro cess will be described , as i t i s that which hasgained the greatest success .
In regard to the insolubility ofdichromated gelatineafter expo sure to light, a remarkable fact was no ticed by thewriter. It was found that where the inso lubility ofthe
gelatine caused by light had once commenced, i t continuedin the dark, and that the action was further increased bye xpo sure to what would o rdinarily be non-actinic light.This remarkable property has been util ised in the autotype pro cess to diminish expo sure ; and Marion, ofParis,likewise took advantage ofit in a process known as Mario
.type . An outl ine ofthis process i s as fo llows — A paperis coated with gelatine, rendered inso luble by alum, and
sensit ive by po tassium dichromate. I t is expo sed beneath anegative ; and a sheet ofgelatini sed and pigmented paper,which has also been impregnated with po tassium dichromate ,i s brought in contact with it in an unexpo sed state . The
two are kept beneath pressure in the dark for e ight or
ten hours, and are then withdrawn. The action set upin the impregnated paper by the light is communicated to
the o ther co loured gelatine , and, as it starts from the bo ttomsurface ofthis towards the top, the so luble po rtions areexpo sed to the so lvent action ofwater, when the papersuppo rt is removed. The deve lopment takes place in the
o rdinary manner, and the image is not reversed as regardsright and left. The process is not practised to any extent,but is a curious example ofa catalectic action started bythe impact “
oflight.
Autotype'
Process. 79
All gelatine which has been long in contact with adichromate, when dried becomes inso luble after a timewithout any expo sure to light having taken place. The
probable cause ofthis has been shown at p. 3 2. In ho t
climates the drawbacks to the use ofgelatine in anyfo rm
are that the o rdinary temperature ofthe water i s such as torender i t liable not to set, but to remain in so lution, and if
dried , it rapidly becomes inso luble. Wi th care, ofcourse,the want of setting power may be avo ided, but there isno doubt that the difficulties ofwo rking this process in
the tropics are far greater than in a temperate climate such asthat ofEngland.
The paper when coated with gelatine and pigment istechnically termed carbon tissue, and as such it will bereferred to .
Since the o riginal patent ofSwan many improvementsin the manufacture ofthe tissue have been made, and the
different substances added to the gelatine are only partiallyknown to the public. The Auto type Company, whopo ssess Swan’s patent, together with all o thers which are
essential to the right wo rking ofthe process, supply the tissue
at a reasonable rate, and an amateur canno t do be tter than
procure the needful supply from them in preference to
making it himself. Should he determine to make it:
himse lf, however, the fo llowing so lution should be pre
paredNelson’s No . 2 flake gelatine 100grammesSugar (brown) 10grammesHoney soap 10grammesGlycerine 20cc.
Water 490cc .
Pigment ofa permanent nature i s finely ground, and in:
corporated with a little warm ge latine and glycerine, and
then mixed into the above. An i l ine dyes maybe employed,
though some are apt‘
to render the film inso luble, as are
also . c ertain kinds ofpigments , There are two ways of
N 2
180 Printingwit/i C/zromium Salts.
applying this gelatine solution to paper. A fixed quantity
may be taken in a measure and applied to paper which haspreviously been soaked in warm water, all excess ofmo is
ture be ing blo tted offon blo tting-paper. The paper in thiscase i s placed on a carefully levelled glass plate, and theproper
'
quan ti ty offluid poured on and distributed evenly
o ver the surface by means ofa glass-rod. In co ld weather
the gelatine will set almo st at once, and when firm the
paper i s hung up to dry. In warm weather iced water maybe caused to come in contact with the bo ttom surface of
the glass plate, which will cause the setting to take place
rapidly. Rapidity in setting and drying is conducive to
sensitiveness, and hence must not be overlooked. The
next metho d is simpler, perhaps, and almo st as effective.
A porcelain or o ther dish, A, i s placed on a hot-water tin,B, the water being kept at bo iling po int by a lamp or Bunsenburner. Over the dish is placed a level table, D,
at one
end ofwhich is a ro ller, G, that is on a level with the topsurface ofa glass, E, placed on the table, D. The paper, F ,i s floated o n the warm gelatine so lution contained in the
dish, drawn through it, se iz ed by’
the hands and drawn overthe ro ller on to the plate, E, where it is allowed to remainti ll the gelatine i s well set
,after which it is hung up by clips
to dry. The dish has to be removed each time that paperis floated ifB be lengthened
, the dish can be run backwardsandforwards in a very simple manner.
In making the tissue a great point i s the selection ofthe
paper. It will be found advantageous to use rather aporousk
'
ind, n'
ot o ver—siz ed. A wash ofammon ium hydrate improves it, as
'
all grease i s thereby removed.
Ano ther'
point to be attended to i s the temperature of
the gelatine so lut ion. Ifraised too high, the coating given
t0' the pape r becomes uneven. Much practice is required
befo re pape r can be evenly coated, and it will even thenprobably be found inferio r to that obtainedfrom the manufacturers. Air-bubbles are a cons tant source ofannoyance.
182 Printingwitn‘
C/iromium Salts.
Carbon tissue. In order'
to float the tissue on’ “
the aboveso lution, a dish somewhat larger than the piece to besensi tised
is used ; and‘
it is co iled up in a small ro ll, with the"
ge
latini sed surface outside. The extreme end ofthe tissuefo rming the ro ll i s turned up for a centimetre. In thi s fo rmthe tissue acts as a spring
,and will unro ll itselfifallowed to
do so . Advantage is taken ofthis. The turned-up end i s
brought to one side ofthe_dish and dropped on the so lu
tion. The hand grasping the ro ll is gradually unloo sed,and the t issue, unco il ing itself,~ -pushes the end which firsttouched the so lution to the farther side ofthe dish , and l iesflat on the so lution, all chance ofair-bubbles clingingto
it be ingthus avo ided.
FIG. 34.
After floating for three m inutes,the turned-up end i s
pinned to a lath, by which it is hung up to dry. The drying-room should be well ventilated, and have a constantcurrent ofdry air circulating through it in order ~to causerapid drying, which is so favourable for sensitiveness. Whenquite dry the paper is expo sed under a negative in the
o rdinary manner, taking the precaution, however, to leavea small portion at the external edges ofthe tissue no t expo sed to light, Since thi s gives greater certainty ofadhesion
to the metal plate,or o ther impervious surface , in the subse
quent operations; A mask ofbrown p'
apet placed over thenegative effects this. Owing to the co lour ofthe pigment no
.
change ofappearance in the tissue will be no ticed ifexaminedafter exposure. I t is therefore necessary to resort -t o an;
183
actinometer in orderto judge ofi
the exposure . The simplest
fo rm i s chlo ride ofsilver paper expo sed in the same light as
the tissue through a small aperture“surrounded by a medium
tint ofthe same hue as that which the chloride takes aftermoderate expo sure. Two to three such tints may be required.
By adopting the plan ofunder—expo sing, and leaving in
the dark, o r in non-actinic l ight,as explained at p . 178, the
expo sure,ofcourse, can be materially reduced.
When in . a developable state a shallow tin or o ther dishi s fi l led with water, and a finely
-mulled z inc plate i s placed
at the bo ttom ofi t. The plate must have been previously"
treated with what i s known as waxing compo s i tion, made as
BeeswaxYellow resinOil ofturpentine
These propo rtions are no t abso lute, as the composition
ofthe beeswax varies. ‘ The resin must be added to the
beeswax in such propo rtions that the ge latine film wi llremain on the -plate without cracking or pee l ing, even when
dried in a ho t room, but at the same time will leave the
plate readily (when the applied transfer paper has becomedry)
“ witho ut the application ofanyfo rce .
’The plate is
first rubbed with a piece of flannel,on which has been
smeared at small. quanti ty ofthe fatty body. All excess ofwax
, except a very fine layer, which persistently adheres,must be removed by po lishing. It is no t necessary to
wax the“
plate each time-a print is removed, but this mustbe done whenever the ge latine image shows a tendency to
stick to the z inc plate during transference to the permanent
suppo rt. The plates are freed fromdirt and greasy -matterby the application ofa little turpentine , ammonia,
'
or po tash.
To attach the gelatine surface to the z inc plate the t i ssueis 1mmersedface downwards in the water in the dish
,and
as soo n as it begins to curl upufara’s,the
, _
z inc . plate is lifted .
out'
ofthe water, carrying between it and the surface ofthe
184 Printingwit/i Cliromium Salts.
gelatine a layer ofwater. The plate 15 then placed on a“
small low stoo l (slightly smaller than the z inc plate ), andtheexcess of
'
water squeez ed out by means ofa squeegee. The
squeegee i s shown in the annexed figure . It consists ofa
flat bar ofwood, into which islet a piece ofindia-rubbe r about75 centimetre thick
’
and 2 cent i
metres broad . When al l the su
perfluous water i s thus expelled,‘
the gelatine fi lm i s allowed two are three minutes to expand,and is then placed in warm water ofa temperature ofabout
40° C .
The annexed figure shows the developing trough as
suppl ied by the Auto type Company, and i t certainly is very
Fro . 35.
Fro . 36.
convenient for“
the purpo se. A is a trough, fi tting into a
A certain amo unt ofdexterity is required to prevent the papercocklingat the edges the india-rubber ofthe squeegeemust be broughtto bear with considerable pressure on to the surface ofthe paper, andthe strokes made with it should commence from the centre and finishtowards the ends.
186 Willis’s Aniline Process.
CHAPTER XXVII .
w1LLis’s ANILINE PROCESS.
WILLIS’S anil ine process may next be briefly described. I t
i s dependent on the action ofdichromates on o rganic matter, though the printed image i s given co lour by means of
an i l ine . Siz ed paper is floated in po tassium dichromate,to which a little phospho ric acid has been added . I t i sthen expo sed beneath a transparent or translucent po sitive ,such as a plan or map, and when the image i s perfectlyvisible, i t is expo sed to the action of aniline vapour.Anil ine salts have the property ofstriking a green, black, orreddish co lour when brought in contact with acidified dichromates hence tho se parts which have not been expo sedto light, or have been shielded from it (as is the case withthe l ines ofthe po sitive print), are deeply co loured, therest ofthe paper remaining ofthe faint co lour due to thereduced chromium oxide . In developing these prints
,ani
l ine i s disso lved ln spirits . ofwine, and the mixed vapoursare allowed to come in contact with the print. I t wi ll atonce be evident what an extremely valuable process this i sfor copying engravings
,plans
,and tracings. All that i s
required i s a sens itising so lution, a shee t ofglass to place
o ver the plan, &c . (which, when exposed, sho uld have itsback in contact with the sensitive paper), to keep them in
contact , and the sensitised paper. A rough “
boxwith a l id,on which can be stretched the printed paper, a basin to
contain the ani line so lution, and a spirit lamp to warm it,
complete the outfi t .The prints can be washed, and are then to lerably per
manent .This process was patented by the invento r, Mr. W.
Willis, the father ofthe inventor ofthe platino type process.
Tne‘
Po'
zva’erProcess. 187
Whether in the face ofthe many o ther processe s to effect thesame object the ani line process will be wo rked is a matterof conjecture.
7 There a re ‘ various modifications of thismethod ofprinting by using co loured anil ine dyes, such asro saniline .
’
For some purpo ses they are useful , but as arule, they are better for the reproduction ofsubjects executedin line than for half-tone negatives .
THE POWDER PROCESS:
Reference has already been made to Po itevin’s process,in which orginally salts ofiron were employed to sensitisegelatine , the development being effected by the applicationofplumbago , or o ther impalpable powder. The dichro
mates subsequently were found to answer be tter than the
ferric salts, the development ofthe prints be ing somewhatmore easy with them. A mixture ofgum-arabic
,sugar
,and
a little glycerine, toge ther with a sensitising so lution of
po tassium dichromate,is prepared and poured o ver a glass
plate, or o ther impervious surface, and allowed to dry in a
warm temperature. The plate thus prepared is at onceexposed for a few m inutes beneath a transparent po sit iveand withdrawn. Tho se parts acted upon by light will befound to be hygro scopic in the ratio ofthe t ime ofexposure and intensity ofthe l ight . Any impalpable powderbrushed over the plate will now be found to adhere to thesehygro scopic parts in propo rtion to the moisture which theyho ld . Hence a po sitive, reversed as regards left and right,will result. When the image i s developed it i s coated withco llodion,
and can then be transferred to pape r, &c .,in an
'
unreversed po sition. The so luble dichromate will be washed
out during the proces s of transferring. This process ls
sometimes employed for obtaining images which can be
burnt in on glass o r ename ls . For thosewho wish to try
the process the fo llowing formula for the sensitive com
1 88 Woodbury/type.
pound will be found efficient. I t is due to Obernetter,
ofMunich
Dextrine 4 partsWhite sugar 5 partsAmmonium dichromate 2 partsG lycerine 2 to 8dropsfor every 100cc . ofwaterWater . 96 parts.
It is somet imes recommended to give the glass plate aprel iminary coatingofplain co llodion. The powder must bevery gently applied with a cotton-woo l brush or fine camel ’shair brush .
WOODBURYTYPE.
The Woodburyt'ype process i s an exceedingly ingen ious
method ofobtaining a mould ofa gelat ine print, from whicho ther prints may be obtained. A rather thick film ofsens it ive gelat ine i s prepared, rest ing on a tough film ofco llodion. This i s placed beneath a negat ive, the co llodionside be ing next the image . I t i s then expo sed to light proc eedingfrom a po int
,or to sun -l ight
,arranged in such a
manner that i t always rece ive s the rays in one direction .
Uncontro lled diffused light will no t do , as, owing to the
thickness ofthe gelatine,the image on development would
be i l l-defined. When sufficiently printed, the ge latinepicture i s developed as ifit were an auto type print
, pre-l
senting the image in considerable rel ief.'
When dried, thege latine picture i s placed on a perfectly flat metal plate , anda shee t ofsoft metal (lead, for instance) is pressed on it bymeans ofan hydraulic pre ss . This latter presenting an
exact mould ofthe fo rmer, i s then placed in a press madeas in accompanying figure . Ge latine i s next disso lvedin ho t water and fine pigment or permanent dye addedto i t, and the viscous so lution thus prepared is po uredon to the mould. Paper ofa very even texture, and whichhas been strongly Siz ed, i s placed on the top ofthe poo lofliquid gelatine, and the top plate ofthe press, hinged as
190 Pnoto-litnograp/iic Transfers;
CHAPTER XXVIII .
PHOTO-LITHOGRAPH IC TRANSFERS.
ANOTHER process, to which reference must be made, i s that‘
perfected by Co lonel De C. Sco tt, R E ,and Sir Henry
James,late Directo r of the Ordnance Survey Of Great
Britain. I t also is dependent on the insolubilizj fOfgelatinewhen treated with a dichromate and expo sed to light. Itwill be described in detail, as i t is capable ofproducingprints in printer’s ink, as well as in ink suitable to give atransfer on to z inc or stone. From such transferred prints
the original drawing can be reproduced by o rdinary surfaceprinting. I t may be wel l to no tice the requisites for thesetransferrable prints . First, the image should be made in an
ink which i s readily held by a l ithographic stone or mulled
z inc plate. Secondly, it must be capable Of. a fair amount :
ofresistance to pressure that i s, i t must be to lerably hard
and cohes ive, o therwise the act Ofpassmga paper ho ldingthe image through a l ithographic press would cause aspreadingOfthe ink, and a consequent want Ofsharpness
in all the impressions taken from the stone . Thirdly, the
ink must be ofsuch a qual ity that a very thin coating is
sufficient to leave a sharp and firm impression on the stone
or z inc plate . Fourthly, the paper on which the image i sdeveloped must be tough, and not easily torn or stretched.
These requisites are fulfilled ifthe fo llowing directions are
attended to . The best paper to select is that known as
bank po st paper, which is not highly siz ed. Ifit be, thesiz ing should be removed by immersion in bo iling water,previous to coating it with the gelatine so lution. The
so lution i s prepared acco rding to this formula
Potassium dichromate 44 grammesGelatine 44 to 66grammesGlycerine 2 c c.
Water 1 litre.
The varying quantity Ofgelatine i s due to the fact that
Preparation ofGelatinisea’ Paper. 19 I
some gelatines give much more body to the so lution thano thers. Thus, iffine-cut gelatine be employed it has beenfound in the writer ’s experience that the larger quantity willbe necessary, whils t ifthe harder qual ities ofgelatine be em
ployed the smaller quan tity will usually suffice. The gelatinei s Ofcourse tho roughly softened in halfthe abo ve quantity Ofwater, and then the remaining half, in which the dichromatehas been disso lved, i s added in a boil ing state . The so lut ion is poured into a dish, and placed over the ho t-watertin, as described at p. 1 8 1 . A sheet Ofpaper ofthe propersiz e is floated on
,i t for three minutes, and then hung up by
two corners to dry. This causes the coating to be thickerat the bo ttom co rners than the top, to avo id which reso rtmay be had to the artifice shown in the figure, p. 1 8 1 . In
any case a second coating is required, and this i s given in aSimilar manner. Ifthe paper have been hung up to dryprevious to the setting Ofthe gelatine, the Oppo site co rnersto tho se by which the sheet was first suspended should behung lowest. This securesafairly even coating. The paperin this condition, even when damp, i s slightly sensitive,and therefo re i t should be dried in a room which only ad
mits non-actin ic l ight. I t i s exposed in the o rdinary mannerbeneath . a negative, which should be Ofa l ine engraving
,
and no t in halftint. l When the l ines appear ' Of a welldefinedfawn co lour on a yellow ground, the paper should
‘
be removed to the dark room for subsequent treatment.Ifthe Object be to make a print to transfer to stone or
z inc, the fo l lowing ink should be prepared (though any
l ithographic ink will answer fairly well)Lithographic printinginkMiddle linseed varnishBurgundy pitchPalm Oil
White waxBitumen
Partial success has been Obtainedeven this latter class ofwork.
16 parts by weight8 parts6 partsI part1 part2 parts.
by SirHenryJames in render
192. Photo-lithograp/zic Tra'
nsfers.
The ink and varni sh are first mulled together with a
muller, the Burgundy pitch and bitumen are next me ltedOver a clear fire ti l l all wate r i s driven Off, the wax next
melted, and finally the palm Oil. When properly me ltedthey should readily catch fire
,which shows that certain
gases are be ing liberated . The ink and varnish are now
we ll stirred in to i t, and the mixture run into the po tsfo r storage . Should i t be desired only to make a singleprint, the best o rdinary chalk lithographic ink may be
employed .
Where a l ithographic press i s available, a fine and even
coating Ofone ofthese inks is usually given to a stone bymeans ofa l i thographic ro l ler, the paper bearing the picture
is then placed face downwards on it, and pulled through
the press, by which plan a thin coating ofink is given to
the entire shee t ofpaper. In the absence Ofa press the
ink may be rendered l iquid with turpentine, and an even
fi lm Ofink may be given with a fine sponge.
To deve lop the picture the print is floated back downwara
’s on a dish ofwater, having a temperature Ofabout
50° C . , and i s allowed to remain on i t ti ll the lines are seen
as depressions . I t is then removed on to a sloping board,and a stream Ofwarm water, ofabout 70
°
C.,i s poured
over the surface the so luble gelatine being in a hydrated
condition, i s carried away together with the ink that coveredi t, and the image i s left, fo rmed Ofink resting on slightlyr aised ridge s of inso luble gelatine. [A very soft spongedipped in the ho t water and applied to the surface aids thed
t
evelopment, in fact i t can rarely be accomplished without
,but the most del icate touch is required for thi s part of
the Operation, as the ink on the fine l ines is very liable tobe carried away. The developed print is next washed inco ld water, and then hung up to dry. In thi s state i t i sready for transfer to stone o r z inc, iftransfer ink have beenemployed
“
. I t is beyond the scope Ofthis book to describethe transferring Operations : these are described in o ther
wo rks. 1 A very conven ien t l i thographic press, sui table foramateurs, has lately come under the no tice Ofthe writer ;the figure on the preceding page will give some idea Ofit s
fo rm. I t is cheap and well adapted for this pro cess,as we ll
as for certain pho to -mechanical printing processes.A A i s a cast- iron carriage ofthe fo rm shown. B i s the
bed Ofthe press, which is caused to move in the carriage
bymeans ofa ro ller, to which is attached the arm H. D D
are drawe rs containing the ne cessary plant . 0 i s a l ithographic stone, shown ia po si tion, and held firm by means
Ofthe cro ss pieces Ofangle - iron fitting into the slo ts asshown. F i s a substitute for the usual scraper it consists of
a ro ller, ro und which , as wel l . as round a smaller ro ller, i spassed a band Offlannel. A downward pressure can be
given to the ro ller by an ingen iously devised screw-mo tion,F , which, whils t giving the necessary pressure, yet causes i t
to take the natural bearing Ofthe stone or plate. K K are
the clamps by which i t can be attached to a table , and R i sthe ro ller supplied for inking it. With this imachine the
impressions pulled are exce llent,and i t i s very portable .
Plates made Ofcompo sition s imilar to so lder are suppliedby the manufacturer . They are excessively sen s itive to
greasy ink, and a number Of'
impressions can be pulled Off
without clogging the work. To clean these plates all thati s required .
’ i s to wash out the Old wo rk with a so lution Of
caustic po tash , and then to scour the surface with fine emerypowder. A dilute so lution Ofacid is poured over the plate,and after washing under the tap, and gently warming, i t isready to rece ive a transfer.
Should only one copy ofthe picture be required, theprint
,which should in that case have been printed in litho
graphic ink, is placed in a copying, lithographic, o r typographic press, face up, and a slightly damped piece Ofwhi te
or o ther paper placedover it. When the pressure i s brought
1 Instr uction in Plzotograplzy , published by Messrs. Piper Carter;or in Sir HenryJames
’s work , published byMessrs. Longmans.
P/ioto-litlzograp/ij/ in Half-tone.
"
195
to bear, the ink i s retained by the latter, and a good impression i s thus Obtained. This method has been named bySir H . James as the papyrograph . I t must not be mistakenfor ano ther pro cess, usedfor copying letters or circulars, andknown by the same name.
Various modifications Ofthis process have from'
time to
t ime been propo sed, such as coating the gelatine W1th albumen, but in the writer
’s experience, when a picture i s to beObtained by dissolvingaway the gelat ine, no better processthan the above can be used.
Another process which differs from the foregomgis one
brought out by the writer under the name ofpapyro type .
I t is based on the fact that when light has acted on age latiniz ed surface then water is no t abso rbed. Thus by
exposing a bichromated p aper, prepared as in the aboveprocess, to the action Oflight behind a negative, the lines ,which are represented by transparent glass in the negative,are printed through and the gelatine becomes non -abso rbent,whereas the parts which have no t been acted upon remainabso rbent ofwater. Ifsuch an expo sed paper be placedin water, and be then made surface -dry
,anda ro ller covered
wi th. a fine layer ofgreasy ink be passed Over .it, tho se parts
which are non-absorbent will ‘ take ’
the greasy ink whilsttho se which have taken up water will repel it. I t does no tneed much intell igence to see that in this way a print ingreasy -ink i s Obtained which can be laid down to stoneo r z inc as desired.
PHOTO-L ITHOGRAPHY IN HALF-TONE.
I t will be no ticed that bo th these processes are best fi tted
for reproducing subjects which are represented by lines , and
it i s a different matter to produce prints from the lithogra
phio stone in half-tone . I t is easy to reproduce on the
p rinted bichromated ge latine a print ofa half-tone subject
in different blacknesses Ofgreasy ink, but when such are
0 2
196 Plzoto-engravingandReliefProcesses.
laid down to stone or z inc the parts which should behalf-tints become deep black and the prints from the pressbecome smudges Ofblack interspersed here and there withpatches ofwhite . Ifthe who le surface be grained we havea different resul t. Suppo se for instance that embedded in
the gelatine are very fine crystals which will disso lve inwater, manifestly tho se parts which are most s trongly actedupon by light will have the finest grain, the surrounding
ge latine being unable to expand, whils t the parts whichair has strongly acted upon wi ll swell to a certain extentand leave a more pronounced grain in tho se parts.
Such is the artifice that Q uartermaster-Sergeant Husband
, R.E. ,has adopted in his process called papyro tint.
With his gelatine and bichromate he mixes common saltand o ther such crystalline bodies, and coats paper withthe mixture. When dry he exposes i t beneath a half-tonenegative and soaks i t in water, and then passes a ro ller containing greasy ink over it. The lights and shades are inthis way produced by greater or less fineness Ofgrain, andas a result he i s able to produce excellent pho to -lithographsin half- tone.
There are several processes in the market to Obtain thesame result, but they are allmore or less secrets though itis believed that they all depend mo re or less On the graingiven to the transfer which has to be laid down to stone.
CHAPTER XXIX.
PHOTO-ENGRAVING AND RELIEF PROCESSES.
N IEPCE’
S process , it will be reco llected, was founded on thefact that a bitumen Of Judaea, when expo sed to light,became inso luble in its ordinary so lvents if partiallysaturated. Silver plates were coated with bitumen, and
198 P/toto-engravingandReliefProcesses .
the exposed portions . The‘
soluble portions are then;diSso lved away by a so lvent which is nearly saturated withthe asphaltum. Ifthe man ipulations have succeeded, themetal should be perfectly bare in parts . Steel , copper, o rz inc plates may be employed for this wo rk the two fo rme rare mo re especially suitable for engraving. The mo rdantusually employedfo r these maybe .
a m ixture ofhydro chlo ricacid with po tassium chlorate, which causes an evo lutionOfchlo rine . For z inc
,hydrochlo ric acid alone may be em
ployed, though it is we ll previously to dip the plate in aso lution ofcopper sulphate . For an engraving the biting-lnneed be but very slight, though much ofcourse mustdepend on the nature Ofthe wo rk as shown by the thickness Ofthe l ines . The thicker the l ines the deeper mustbe the biting-ln. For a reliefblock the biting-ln has to becarried to a far greater extent in fact, as deeply as seen inan o rdinary wood-cut. This invo lve s ve ry tedious manipulation after the first biting. The plate has to be warmed,dusted with resin again heated to slightly mel t the bitumen,
so as to allow it to flow down the sides Ofthe bitten-in
l ines . This pro cess has to be repeated till a sufficient depthis attained. When there are larger spaces Ofwhite in theprint
,the metal i s usually removed by a fine saw, o r a grave r.
Rel ief-block making is essentially difficult in almo st eve rystage, and rarely repays an amateur the labo ur he maybestow upon it.
Ehrard, OfParis , has ano ther me thod ofproduc ing en
gravings,which i s also dependent on biting in. He prepare s
a transfer, as for z iiicography, and, after going through theusual manipulations to transfer it to a copper-plate
,he plunge s
i t into an electro -plating bathfor afewminutes,thus covering
the copper with a thin film Ofsilver, the lines ofthe engravingbeing pro tected by the greasy ink. After
‘
a rinse in diluteacid the plate i s transferred to a bath ofmercuri c chlo ride,where the silver i s converted into the double “ chlo ride .
After washing, the ink is removed, and the biting process
Tzzloot’
s Process.
allowed to proceed. The details Of this process are asecret, but what is stated above gives a general idea of
the pro cess. The analogy that exists between this and
FoxTalbo t’s process ofengraving a daguerreo type plate isObvious.
Ano ther process for obtaining the same results,various
modifications Ofwhich have from time to time been an
nounced, is due to Talbo t. I t consists ofprinting the negative on a ge latine film, washing away the unaltered ge latine,andmaking an electro type from i t. In the trade there are
several firms who practise e i ther pho to —engraving or re liefblo ck making, but it is not known which methods theyadopt, as the several pro cesses are kept secret . Amongstthese may be named Goupil, G illo t
,and Dujardin, Of
Paris ; Dallas, and Le itch CO. , ofLondon. Scamoni,
ofSt. Petersburg, also makes very beautiful reproductionsOfengravings, &c . H is method seems to be based on the
building up Ofa rel iefon the negative itself, and then taking
an e lectro type. Fig. 39 i s a print from a pho to -re liefplateby Warnerke, produced by a process ofwhich the details are
no t as yet publi shed .
The processes above described are adapted to the te
production Ofl ine wo rk in contradistinction to half-tonedrawings or pho tographs from nature. The production
Ofpho to -engraving or pho to -etch ings in half-tone i s also
much practised and there are two methods by which theycan be done . One i s by first graining a plate with somegrain, and then transferring to such a plate a gelatine cast in
half-tone, and biting this with some etching chemical . Sucha process was that which Fox Talbo t wo rked in the earlydays Ofpho tography , He gave a grain to a smoo th copperplate by pouring on to it aso lution in ether ofcampho r andresin. This gave a thin layer ofresin and campho r
,and
a gen tle heat expe lled the latter and left the fo rmer on
the plate in small granules . A stronger application ofheatcaused the resin to adhere to the copper surface. On such
202 Pfioto-coZ/otj lpe Processes.
the one fact already po inted out at p. 175, that gelatine“
likeo ther simi lar bodies, when impregnated with po tass iumdichromate, become s incapable ofabsorbing moisture after
full expo sure to l ight ; and that where light has partiallyacted, there i t becomes only partially abso rbent, when com
pared with the amount i t wil l abso rb when entirely guarded
from light. Suppo se we prepare a film ofgelatine with
which has been m ixed some po tassium dichromate , byfloating a warm so lution of the mixture over a smoo th
surface , such as a thick glass plate, and when dry expo se itbeneath a negative in which we have different degree s oflight and shadow, as in a landscape o r a portrait negative
on immersing the film in co ld water, we shall have a pictureimpressed in which the different degrees of shadow are
represented by different degrees ofrelief. Ifthe back ofa
similarly treated ge latine fi lm be exposed to light previously
to i ts immersion, the reliefafterwards will be found to be
much sl ighter. This is evidently a necessary consequence. If
over e ither ofthese surfaces, when all superfluous mo isturehas been removed, a smoo th soft ro ller carrying a fine layerofgreasy ink be passed,
”
i t will be found t hat the greasy inkwill adhere to the parts expo sed to l ight in nearly exactproportion to the intensity of light
'
which has acted on
them.
With the film in which the rel iefi s high the ink wil l takeless readily, because the ro l l er, even when to lerably soft, willfail to come in contact with the expo sed parts . With the
film having but small rel iefthe difficulty will no t be found.
Ifsuch a film as the latter be now placed in a printingpress, an impression from it maybe o btained, but i t willbe found that as regards right and left the pictures will bereversed . A reversed negat ive is therefore necessary. Theoretically the number ofimpressions which can be pulled fromthe surface is not l imi ted, ifthe surface be kept damp , andif
'
a
fresh application ofink be given by the ro ller. I t will be
found, however, that after each pull"
there is a tendency of
Preparation ofPrintingSurface. 203
the'
unexpo sed ge latine to adhere to the paper, and thus tospo i l the printing surface . In order to preven t this it hasbecome customary to introduce into the ge latine somesubstance which will harden i t. Certain gum resins, alum,
chrome alum, and kindred substances effect thi s hardening,and one or o ther ofthem i s to be found in the fo rmulae given
for most ofthese processes. Albert,ofMun ich, may be
said to have fi rst discovered a thoroughly wo rkable pro cess,based on the above principles , and we shal l briefly give ano utline ofthe method he adopted as be ing a typical one,and unencumbered with any of the large number of
modifications introduced at various time s by o ther experimenters .
‘
A piece ofplate glass some 2 centime tres in th icknessis coated with a gelatine mixture made as fo l lows
I .
10grammes.80cc .
Po tassium dichromateWater
These are disso lved separately and mixed warm. The plate18 then coated and dried by heat, 5 or 6 ho urs
’
expo sure toa temperature ofabout 60° C. be ing sufficient to effectdesiccation. The plates are now expo sed back uppermo stto light for about a quarter ofan hour
, the gelatine filmsresting on a smoo th black surface, after which they re
ceive over the fi rst a second coating made as fo llows
Po tassium dichromate 3 grammes.Water 49 cc .
To No . I i s added 60 cc. ofwhite ofegg, and after
204 P/zoto-collotype Processes.
heating to 60°
C ,No . 2 i s m ixed with it, and the so lu
t ion i s fi l tered through co tton -woo l. This coating is
dried,and the plate i s ready for printing. The expo
Sure depends upon the quality of the light ; i t mus t becontinued til l the who le of the details are visible on
the gelatine, and much of the success depends uponthe depth to which it is carried. When j udged suffi
ciently printed, the back ofthe plate is again expo sed to
light to such a degree that the resulting reliefwhen the
film i s wetted will be small . The film. i s now washed to.
re
move all exce ss ofthe dichromate, and i s again allowed to
dry. The dried plate i s next placed for 5 minutes faceuppermo st in a dish containing a 25 per cent. so lut ion of
glycerine in water. The back is then embedded on the bed
ofa lithographic press by means ofplaster ofParis, andlightly
‘
rubbed over with l inseed oil, and again slightlydamped with water. A soft ro l ler, charged with greasy ink,is then passed over the surface, when it is found that aperfect print appears on the surface. The plate, the surfaceofwhich is in c ontact with a piece ofpaper, is now passedbeneath the.press, and an impression pulled. Such a pressas that in . fig. 38 may be employed.
Mr. Ernest Edwards introduced an important modification ofthe above by mixmgchrome alum with the ge latineto harden the gelatine film. He only uses one coating tothe glass plate, and when dried strips i t from the glasssurface,’ and prints it in this condition. He retransfersthe
_
filmto a pewter or o ther me tal plate , and pulls an im
pression from it'
when thus supported. By this devicethe danger
‘
of destroying the printing surface, owing to
the pos sible breakage ofthe glass plate, i s overcome, and
in consequence the co st of production IS dimin ished.
Fo r a full description of the process, which is named‘ Helio type ’ by the invento r, the student i s referred to
ano ther wo rk.
l
I nstruction in Pnofograpny . Piper Carter.
206 Elementary P/iotograp/zic Optics.
I t is found experimentally that the sines ofthe angleswhich the ray makes with the no rmal at the surface ofthe
two media have a fixed ratio to one ano ther, and that thisco efficient is dependent on the media through which the raypasses . Thus from air to
'
o rdinary flint-glass the co efficientis about and from the flint glass to air the recipro calabout T
l? o r 6 6. Applying plane trigonometry to this expe
rimentalfact, i t will be found that there i s a limit to the angleat which a ray oflight can pass from anymedium to one
less dense, since the l imi t of the s ine ofan angle is un ity.
When the ray strikes the surface at this particular angleo r at a greater the rays are reflected back, and the lim i tingangle i tselfi s called the cri tical angle, or angle ofto tal reflec
t ion, for these two media. A reference to this is made ina subsequent chapter.Instead ofthe surfaces ofthe glass
"
be ing paralle l we mayhave them inclined at an -angle to one ano ther, and in thiscase the refraction at each surface will fo l low the same law.
An object which is really at K, fig will apparently o ccupy
the po sition K’, which is
Fm, 4,equivalent to saying that
K a ray of (monochromatic)light projected in
_
the direction K A wo uld have adirection c E after passingthrough the prism. Ifthe
projected beam ofl ight in
the direction K A be white, it will be found, as already no ted
in the second chapter, that on emerging from C it is spl it upinto rays ofthe different rainbow tints . Ifwe take any three
distinctive rays in the red, yellow, andblue, we shall find that
the red i s least refracted andfalls in a direction R, fig. 42,'
the
blue mo st andfalls at B, and that the ye llow occupies the
intermediate po sition.This difference in the index o r co
efficient“
ofthe refractive power ofthe media for differentco loured rays gives the phenomenon known as dispersion.
Dispersion. 07
It is found by experiment that the angles fo rmedby the directions ofthe different rays of light vary ac
.cording to the compo sition ofthe glass employed for theprism ; that with one specimen, for instance, the angleformed by R andY does no t bear the same ratio to the angle
F IO . 42.
fo rmed by Y and B that it does when '
another specimeni s employed. I t is owing to thi s difference in dispersivepower ofvario us glasses , that it has been found pos
s ible to cause the component rays ofwhite l ight to be
nearly equal ly refracted, and yet to show no appreciable
co lour, due to dispersion. It will be seen in fig. 43 that,by employing oppo sing prisms ofdifferent compo sition ,
the
dispersion may be almo st entire ly overcome. Thus itmay happen that by placing a prism B ofthe dimensions ,and in the po sition shown
,the rays o riginally forming
FIG . 43.
white light, and which were decomposed by the prismA,might be so bent
,owing to the difference in the dis
persive power of two media, that they emerge from B
paral le l to each o ther,instead ofeach ray fo rming a defi ~
ni te angle with its neighbour, and that still the o riginal ray
208 Elenzentary P/zotograp/iic Optics.
may be refracted. Supposing B and A to be ofthe samehomogeneous medium ,
i t is evident that the same result
would no t be obtained. Ifthe distance between B and A
were dimin ished till the adjacent surfaces touched, the paral
lelism ofthe rays emerging from B would still be obtained,and
,owing to the small dispersion ofthe rays in A
, an inci
dent ray ofwhite l ight would emerge as white light. The
two media we have been suppo sing to be employed are
only hypo the tical . Unfortunately up to the pre sent date no
two media have been found who se dispersive power can be
util ised so as abso lutely to correct one ano ther.
Suppo sing we have a series ofprisms and their frusta
j o ined together as shown, i t i s evident that the surfacesmay
FrG . 44 .
be worked at such angles that the rays ofl ight proceedingfrom an object at any distance from them may cut in one
po int andfo rm an image ofthat object. In the figure 44 we
have suppo sed the luminous object to be infini tely distantandto fo rm one single image. By rounding offthe angles the
same result may stil l be obtained andwill fo rm a lens. The
curve that a glass would take'
, to give such theore ticallyperfect results, would be practically unsuitable , owing to the
difficulty ofgrinding it and also because i t would only beco rrect for a particular distance and direction ofobject .In practice lenses are worked to spherical surfaces, asbe ing mo st convenient, and being capable ofapproximate
accuracy.
We wil l first glance at the inaccuracy that the spherical
2 IO Elementary Pnotograpnic Optics
used (supposing spherical aberration e l iminated) therewould be no plane , xx, on which the two objects wouldappear at all defined. The effect ofa diaphragm, o r
as i t is technically called, i s to narrow bo th pencil s ofl ightso that ne ither of them is much out offo cus at any po intintermediate between the foci ofthe extreme rays. See
F IG . 48.
fig. 48. This will be entered into further on In thischapter.
Supposing that the rays from the near object fo rmed anangle with the axis ofthe lens
, and tho se from the distantobject co incided with i t, a larger diaphragm might be em
ployed ifthe plane on which the images ofthe objects haveto be rece ived makes an angle with the axis ofthe lens ,fig. 49 . It will be seen that the swing-back ofa cameraserves this purpose.
F IG. 49.
I t is to be observed that nearly the same results can beobtained by placing the diaphragm behind the lens insteadofin front, fig. 50 and also that the siz e ofthe diaphragmde termines the brightness ofthe image, for only a portion ofthe lens is utilised.
The U se ofMe D z'
ap/iragm.
Wi th a lens such as shown there is a difference In the
resulting images when the diaphragm is placed in front orbehind the lens. In bo th cases we have disto rtion, but thedistort ion in one case i s the reverse ofthat in the o ther.When the diaphragm is in front ofthe lens the image of
a square would be barrel-shaped . When i t is behind the
F IG. 50.
curvature would be reversed, fig. 51 . It would be useless in
e i ther case to take an architectural subject wi th such a lensunless the building o ccupied but a small propo rtion ofthe
picture. The reason of this di sto rtion will be apparentwhen i t i s remembered that the margin ofthe lens, i ts surfaces be ing po rtions ofspheres, will cause greater refractionthan the central po rtion. When the diaphragm is in frontofthe lens i t i s the margin ofthe lens which g ives the
F IG. 51 .
image ofthe co rner ofthe square. The image ofthe centreofeach side is fo rmed by a po rtion of the lens whichis more central, and therefore i s less propo rtionally bent.When the diaphragm i s behind the lens different portions of
the lens are used to fo rm the image,and consequently the
distortion i s reversed. By placing a lens on each side ofthe
diaphragm i t is evident that distortion due to this cause9 2
2 I 2 Elementary P/zotograp/zic Optics.
may be entirelyp vercome, and thus we get what is called adoublet lens . I t will befound thatwi th certain lenses, ifweattempt to obtain a sharp fo cus ofho riz ontal and verticall ines near the margin ofthe fo cussing screen, we shal l faile ither the one or the o ther will be indistinct. This is dueto .astzgmatz
'
sm, a defect also caused by the spherical fo rmgiven to the surfaces oflenses .
Lense s have various shapes given to them the fo llowingare the different fo rms
M is the double convex N, a plane -convex o , a con
cavo -convex P, a double concave Q , a plano -concave R,
ameniscus. Lenses in which the concavity is greater than
the convexity can have no actual but. only a virtual focus,
FIG. 52.
as may be seen by making a diagram. All such,when
combined with o ther lenses, in which the convexity preponderates,
l will e i ther increase the focal length o r give avi rtual fo cus to the combination. In pho tographic lensesthe chiefuse ofconcave lenses is, bymaking them ofsuitableglass , to secure achromatism .
The principal focus ofa lens i s the po int where rayswhich enter parallel meet on emergence . As an examplewe may refer to fig. 47.
The o ptical centre ofa lens is that po int in the axis of
the lens through which lines j omingany po ints In an objectand the ir images would intersect.
The material in which the lenses are worked must be taken intoconsideration in determiningthis.
2I4 Elementary P/zotograp/iic Optics.
EC, the distance apart ofthe images ofthe bases ofthe'
two
rods, to be 6 centimetres.Then E D+ G E c B ::C E C F , which is the equ ivalent
focal distance.
30centimetres.
I t is,therefo re, this distance along its axis from the
ground-glass ofthe camera to the optical centre ofthe lens .
The student will readily devise the means ofsetting off
the distance thus found on the brasswo rk.
I t would be out ofthe scope ofthis work to touch on
the higher mathematics ofoptics , and the fo llowing fo rmulaeare only true when the thickness ofa lens may be neglected.
The relation ofthe conjugate foci to one ano ther i s expressed by the fo llowing fo rmula
I
f
Where it i s the distance ofthe optical centre ofthe lens fromthe ground-glass, n i s the distance ofthe optical cen tre of
the lens from the object to be pho tographed,fi s the equivalent focal distance . From this i t will be seen that ifu i s
very great, then . .
I.
“
5 so smal l that it may be neglected, andu
there remains o f. That is, the image ofan object at a
great distance will be at the .equivalent fo cal“
distance .
Applying the above formula, suppose we have a lens
wheref 30centimetres and u 40 centimetres
That is o : 1 20 centimetres , or the distance ofthe ground
glass from the centre ofthe lens must be 1 20 centimetres
to bring it into fo cus .Let i t be requ i red that u should be 72 times greater than
Cnoice ofaLens. 2 15
which is the same as saying that the Image must be7
1
2
the s ize ofthe object.Then
0 :
Suppose, as befo re,f 30 centimetres, and i t is required'
to dimini sh the image ofan object . to it ofthe s iz e oftheo riginal
30(4 I )centimetres,
a no 4 x 375 150centimetres,
or the ground-glass must be centimetres and the object150 centimetres from the lens.By similar reason ing, ifthe object is to be enlarged 4
times, it will be found that the above distances must bereversed.
In choo sing a pho tographic lens the purpo se for whichir is required must be kept in view, for i t will be evidentthat the requirements necessary may be different. In alens for taking portraits we have , for instance, certainpropert ie s which are no t essential, and even might be dett imental in a - lens for taking landscapes. Wi th the formerthe obj ects to be pho tographed are generally within a fewfee t ofit, and there are a variety ofpo ints s ituated in different planes which ought to be impressed with sharpness onthe pho tographic plate, and that without anydistortion. The
last desi de ratum puts the employment ofa single lens out ofthe que stion unless a small stop he used , and it is evident thata double lens must be used. Starting with this, i t is quiteevident that the curves ofthe surfaces ofportrait lensesmust vary from tho se for landscape wo rk
,and must be so
designed as to be capable ofdel ineating po ints in differentplanes no t far from the lens itself. It will be found that this
2 16 Elementary P/cotograpnic Optics.
can be. secured by combining lenses ofthe same ordifferent
fo cal lengths,separating the pairs by a long interval. Thisl imits the extent offie ld and necessi tates the employment ofobject glasses of.wide diame ter in o rder to cover a sufficientarea. In practice the lenses are so far separa ted that theamount of surface ofthe photographic plate which can
be uti l ised for some purpo ses,F IG 54~ scarcely exceeds the diameter
ofthe lens itself. Again,rap id
i ty is an essential quali ty of
a good portrait lens, and the
curves ofthe surfaces ofthe
lenses, and their separation,must be so adapted that
,with
out the use ofany diaphragm,
they shall give a fairly sharpimage ofa figure or part ofa figure when placed at a
reasonable distance. Spherical aberration i s a positive ad
vantage for some ofthese requisites. Fig. 54 gives an idea ofthe curves and also the amount
Flo ss. of separation which is given to
the lenses ofa Petz val po rtraitcombination, on the pattern of
which many ofthe modern ones
are still constructed. The dark‘
shaded po rtions show the crownglass
,and the l ight shaded-
por
tions the fl int glass lenses . ~
In one ofthe beautiful por~
trait lense s introduced by Dal-lmeyer we have a decided varia
tion from this model . The advantage ofth is lens, fig. 55, is
that two components ofthe back combination are capableofbe ing slightly separated, giving a greater depth (thougha more diffused) focus than o rdinarily obtainable.
2 I 8 Elementary P/zotograpnic Optics.
and the separation between . the two lenses to be about2 inches .
I t may be useful to give a rule for ascertaining approx
Fm. 57.imately the focal length of anypair oflenses when combined.
Multiply the focal length of
one lens by that of the o ther,and divide by the sum of theirfo cal lengths less the distance of
separation. In the above case wehave
20x 20 400
40— 2 38
The diaphragms for this combination o ccupy a posit ion half-way between the symmetrical lenses, and there
fo re give no distortion. This lens covers an angle ofabout
The next lens, fig. 58, is what is known as a wide angledoublet, in which the separation between the lenses is very
small, and their foci considerably shorter, in proportion tothe area ofthe circle that i t i s to cover.Some ofthese combinations are made so
as to cover a circle whose diameter subtends an angle of 90
° from the opticalcentre . The objection to these lenses i sthe unequal i llumination and the smallstop that is obliged to be employed withthem, and their consequent slowness.
The fo l lowing diagram (fig. 59)‘
showsa section of the ‘ triplet lens,
’in which
the place o rdinarily o ccupied by the dia
phragm i s replaced by a 3rd compoundmeniscus lens. There were certain advantages connectedwith this lens at the t ime when i t was introduced, but,s ince the manufacture ofnon-distorting doublets giving
F IG. 58.
F lare Spots. 2 I9
a fairly flat field has been perfected, they are compara
tively obso lete. It i s, however, a good illustration ofthe.
ingenuity with which Opticians
aimed to meet the requirements of
pho tographers.
In the doublet lens the po sit ion ofthe diaphragm Is Impo rtant,o therwise— as can well be understood— the second lens will notco rrect the disto rtion ofthe
‘
first.
In the case ofa double t in whichbo th lenses are symme trical, the
diaphragm should naturally o ccupya po sition half-way between them.
Ifthe fo cal length ofthe front lens be diflerent from that of
the back , the diaphragm’
must o ccupy a po s ition“
propo rtional to the focal length ofthe lense s.Wi th certain classes ofdoublet lenses as formerly con
st ructed there was formed a fogged central patch on the
expo sed plate . This was due to what is called a‘ flare
spo t,’ which is a circular patch of light seen on the
ground-glass immediately in a line with the axis ofthe
lens. I t is,in
.
reality, an image ofthe opening in the dia
phragm. Ifglass were perfectly transparent, such a defectcould not exist but , owing to i ts reflecting ligh t from its
surfaces, i t has a reali ty which is often very troublesome.
The surface ofthe lens reflects the aperture in the diaphragmand fo rms a distinct image ofi t, and ifthis image happento co incide with the fo cal distance ofthe lens, the flare spotis sure to make i ts appearance. By slightly altering thepos ition ofthe stop this defect is o vercome . But aswill havebeen no ticed befo re, the po sition ofthe d iaphragm in adouble t lens i s ofimpo rtance for el iminating disto rtion ;hence by curing this defect disto rtion might be introduced .
By previously altering the distance of the separation of
the two lenses, bo th evils may be avo ided. At the best it
FIG . 59 .
220 Elementary'
Pekingrapnz'
c Optics .
seems,“however
,that the flare spo t isreallyonly distributed
over the entire area which ‘
the lens covers . This reflectionfrom the surface seems to account in a measure for the ve i lon negat ives, which i s often apparent when using certain
’
slow len ses where bright objects have been photographed,and the exposure pro longed to enable the de tails in darkshadow to be capable ofdevelopment. The ve il i s probably the pho tograph ofthe
'
illuminated lens.We must again revert to the diaphragm, or
‘stop,
’in
o rder to give some further idea of i ts use, and also of
the necessity which may exist for using one oflarge or
small aperture .
I
In the case of a single lens we havealready shown that the po sition ofa stop affects the shapeofthe d isto rtion
,depending whether it be placed in front
hr rear of the lens . I t may now be stated— and the
reason will be apparent on examining the previous figures—that on the distance of the diaphragm from the lens
is dependent the amount ofdistortion, as is also the siz eof the picture which - the lens 15 capable of defining
,
whilst at the same time the flatness ofthe field is also“
in
a great measure due to a large distance being maintainedbetween - them. In constructing a lens, then, an Opticianhas to hit a mean in order to give a satisfacto ry result.From these remarks it will be evident that a lens whichembrace s a wide angle should give least disto rtion
,because
the diaphragm must be necessarily clo ser to the lens thanwhen the angle i s curtailed . I t is for this reason that theemployment ofa wide angle lens
,with a plate ofa siz e
larger than that i t was c onstructed to cover, i s found to
yield mo re satisfactory pictures than if a lens capableofembracing a less angle be employed. Thus a wideangle landscape lens intended to be used for a 40x 30centime tre plate, gives mo re accurate pictures on a
20x 1 6 centimetre plate than does a lens embracing amo re moderate angle when used for the same
'
siz edplate.
222 Elementary Pnotograpnic Optics .
it from a po int. In practice I minute ofarc is takeh as
the l imi t.’
When the diame ter ofthis disc, as viewed froman o rdinary distance for examining a picture (40 to 50
centimetres) subtends more than a minute ofare, the
object will appear to be out offo cus, whilst ifless i t will bein focus. Hence we may argue that the smaller the apertureofthe diaphragm the greater the depth offocus there will be,since the foci of nearer objects and distant ones may allbe made to fall within this l imi ting angle by diminishing i t.A reference to fig. 48 will aid the student in comprehendingthis . Takinga di sc of
°
25mill ime tre diameter, which is abouta minute ofare as seen from a distance of50 centimetres,as the greatest admi ss ible diame ter ofdisc ofconfusion, atable i s readily constructed ofthe nearest po int which willbe in focus when any aperture ofdiaphragm i s employed .
Suppo se we know the equivalent focus ofthe lens in question to be 25 centimetre focus, and that we are to use an
aperture of25 centimetresTaking the formula which will give an approximately
true value
when the distance is in focus, the nearest part ofthe foregro und which can be cons idered sharp will have a focuswhich is longer than the equivalent focus by
°
25 centimetre,for
Cent. Cent. Millimetre .
25“025 t x
x '
25 centimetreI I I
‘
2521 25 25x25
°
25I
2525
z; metres.
That is to . s ay, all parts ofthe picture lying beyond 25
Advantage ofSnort F oci.
metres will appear to the eye to be in focus. The followingtable has been constructed on that basis
Focal Length ofLenses in Centimetres
Intensity,o r
RelativeApe rture
ExposuresRatio
Distance ofnearest distinc t Obj ects in Metres
I 9 I 162 252 36-3 644 100-5
27 6 I 108 169 243 43‘
I 67-2
4 2 1 4 6 8-2 12-7 18 3 324 505
9 12 3 2 17I6 2 4 42 65 9
-3 164 255
The annexed fo rmula will approximate ly give the nearestpo in tp which will appear in fo cus when the distance is accurately focussed, suppo sing the admissible disc ofconfusionto be '
025 centimetre
P 4 1 xf2xa,whenf the fo cal length ofthe lens in centimetres,
a the ratio ofthe aperture to the focal length.
The result is in metres.In the application ofthe forego ing fo rmula the stu
dent should no te the advantage ofusing a lens ofshortfocus in l ieu of one of long focus, viz ., that more of
the fo reground can be placed in the picture without anydetriment to it through fuz z iness. ’ I t can also be shownthat an e nlargement from a small negative is be tter thana picture”
ofthe same siz e taken direct as regards sharpness ofde tail . Suppo se, for instance, we wi sh to comparefor sharpness a picture taken with a lens 50 centimetresfo cus with an enlargement ofthe same siz e, from an o riginalnegat ive taken with a lens ofonly 10 centime tres focus,bo th having the same aperture ratio , say 7
1
6 . The negativein the last case would be only s, the s iz e (linear) oftheformer. To compare the two the disc ofconfusion
"
in”
th'
is
224 Elementary Pnotograp/iic“
Optics .
latter should only'
be °005 centimetre diame ter, and thisshould give the distance ofthe nearest distinct object, since,when enlarged 5 times i t will give a disc of
‘025 centimetrediameter, which we have already taken as the limit ofdistinctness. Calculating as befo re for the lens ofsmallerfocal length,
'
5 Z 10 005 2 x‘
I centimetreI I
10 10+ “
I
IO‘
I metres,
that is,after enlarging a picture to the siz e given by a lens
of50 centimetres'
focal length, an object » 10'
r metres willstill appear in fo cus. In looking at the table , i t will befoundthat with the direct picture ofthe same siz e the nearest objectin focus will be at 505 metres distance. Calculation showsthat the gain in an enlargement compared with a directnegative 15 Inversely ' proportional to the focal lengths ofthelenses .
'
This,ofcourse
,refers only to an aplanatic lens, and
care must be taken to distinguish between the advantagesto
_
be=gained in enlargement by the use ofa smaller lens
,
wi th the disadvantages that ensue from the de terioration inthe relative values oflight and
i
shad'
e .
The student should remark that in double t lenses the
apertures In the diaphragms do no t show accurately theavailable aperture ofthe lens . In order to ascertain theirco rrect value, a distant object should be fo cussed
'
in the
camera, In order_that the focussing screen may be at the
equivalent fo cus ofthe lens this screen i s then removed andreplaced by a glass over which is pasted any opaque paper.
A candle is bro ught near the Centre ofthe opaque screen in.
which a small ho le has been punctured . The front com
bination ofthe lens is illuminated by the rays oflight comingthrough the o rifice, and the diameter ofthe disc oflightseen on the front ofthe lens gives the available aperture of
the‘
lens when used with that diaphragm.
226 Apparatus.
e andfi which run in the slo ts as shown . Revert ing tofig. 60 i t will be seen that the camera has what is known
pm 61 ,as the ‘ bellows form
,the be l lows
B be ing attached to A and alsoto the swinging framewo rk D . E
i s connected with R by means of
a rod, pass ing through the side
ofthe framewo rk, and terminatedby a clamping
.
screw K. R can
be made to approach o r recedefrom A by means ofa slow-mo tion
screw turned by the handle x. D i s connected with M bypivo ts which wo rk in the brass plates H, and since 0 is fixedas regards the vertical plane, i t is evident that D can movethrough any small angle about H,
without in anyway interferingwith the o thermovements ofthe camera, andthe angle canbe maintained by clamping the screw,
which wo rks in a slo tas shown. Thus
,then
,a swing away from the verti cal plane
i s secured . The mo tion ofD in a ho riz ontal plane i s securedby pivo ting the frame M on to R. Ifthe clamping screw Kbe loosened, M,
and therefo re D,can be moved through any
small angle in a ho riz ontal plane,and can be fixed in that
po sition by tightening K. The double swing mo t ions are
therefo r e secured. F is a bar with a long slo t cut in i t, soarranged that clamping screws in c andA can fix i t and giveaddi tional rigidity to the camera.
~ When R has been movedalong the tail-board Q , so that c touches A where the
clamping screws M and K are loo sened, the latter i s freeto turn up against the ground glass G. When a small pinat s i s wi thdrawn from P
,thi s board
,be ing hinged as shown ,
fo lds round the turned -up tail -board and Q is kept inpo sition by means ofa small snap spring fixed to the bo ttomofthe camera.
The camera itselfcan be attached to the stand by the
tail-board Q , in which position the greatest length ofthe
picture i s horiz ontal, or by E when the he ight ofthe picture
ReversingBack. 27
has to be longer than its breadth. A camera 2 1 x 1 6 centime tres ofthis fo rm, when packed in a leather case , weighsabout 6 kilogrammes, o r 14 lbs . Forwo rk in the studio wherethe diminution ofwe ight is no obj ect
,a rigid form ofcamera
can be adopted . Such a
form we give in fig. 62 .
This is a camera adapted
for taking cartes de Visite,and it wi l l be no ticed that
the alteration in fo cu s i ssecuredby a different arrangemen t to that in thelast . Thefront part,which
carries the lens , sl ides outside the back part, the movement be ing effected by a pair ofracks fastened on the baseboard, o n which a long pinion wo rks. Some pho tographersprefer this mo tion to that given by the screw,
since the hands
do no t in terfere w1th
the positIon of the
body whi lst viewingthe image on the
screen.
I t will also be no
ticed that there i s along carrier for the
dark slide s,and that
the dark slide ismorethan double the
length n ece ssary to
secure one picture.
The object ofthis isto be able to givetwo expo sure s on thesame plate , and thus to economise time.
I t i s very convenient to have attached to the camerawhat is known as a ‘ reversing back.
’
For this adj unct i tQ 2
F IG . 62.
F IG .03 .
228 Apparatus.
is necessary that the camera shou ld be square in section.
Fig. 63 gives an idea ofit. I t will be seen that the slide s fitinto the back, which can be placed with the gro und glass andplate having their greatest length ho ri z ontal or verti cal.By this plan the camera can be util isedfor taking picture s of
e ither shape without any alteration except ofthe back itself.
For some classes ofviews a pano ramic camera is a veryuseful piece ofapparatus to employ. For Instance , where
the view embraces I 20°
any lens would be incapable ofgivingthe who le picture
,
unless at least twoviews were takenfrom the same spo tand afterwardsjo ined . The faultin such a picturewouldbe that therewould be two o r
mo re fixed po intsof sight
,which
must inevitablygive a mo re or lessuntruthful com
plexion to it. In
a pano ramic ca
mera the eye is suppo sed to travel round the view, the po int
ofsight altering at each movement ofthe eye . There i s something to be said for this kind ofperspective, s ince the anglethe eye sees distinctly at one t ime i s very small in compari sonwith what is delineated with an o rdinary lens . Some ofthe
magn ificent views in Switz erland by Braun,ofDo rnach , were
taken by such a species ofcamera, and they certainly aremo repleasing than they would have been bad the po int ofsightbeen abruptly altered. The accompanying figure (64) givesan idea ofLiesang
’
s pano ramic camera . In all cameras ofthis description it i s necessary that the ro tation should take
F IG . 64.
230 Apparatus.
the o ther, one portion carrying a thin blackened and hingediron or tin plate . A sensitive plate i s put in each halfofthe
slide, the sens itised surface be ing outwards. The blackened
plate prevents the passage of light from one to the o ther.
The plates are placed in the camera as usual, and o pened
for expo sure as with the o rdinary slide .
I t will be seen that for every couple ofplates one double
back is required , and i t will seldom be convenient to carry
mo re than three ofthese, on account oftheir we ight.
Ifi t be decided to use a changing box, there i s none
be tter than that manufactured by Hare . In order to use it,
it i s necessary to have
a dark slide especiallyconstructed , the pecu
~
liarity ofwhich consistsin i ts having a movable
end-piece through which
the plate passes into the
ho lder. The plate box
itselfhas a lid, whichis longer than the top of
the box, and is capableoffo lding, andin i t is a slo t through which a plate can pass inor out ofthe box. The box i tselfis fi tted with groo ves , thepo sitions ofwhich are marked on the outside ofthe l id byan ivo ry scale. In o rder to fi ll the sl ide
,i ts end is s l ipped
into a gro ove which borders the slo t , and when i t is home aspring is forced on one side, and this opens the end ofthe
dark slide, and the slo t in the shutter i s uncovered. The
l id ofthe box i s now moved till the slo t in i t is over aplate as registered by the scale. The box is now gently inverted and the plate passes into the dark slide . This lattercanno t be removed till the shutter once mo re covers the s lo t,andthe act ofremoving it clo ses the shutte r over the
’
opening .
This is a very s imple method ofchanging a plate even inbright sunshine
, and i s always successful providedthe plates
FIG. 65.
Warneree’
s Roller Slide. 23 1
are carefully cut to the proper s iz e. The method ofre
turn ing an expo sed plate to the box i s se lf-evident. The
weight ofa do z en plates and this changing boxshouldno t be
much mo re than that ofhalfa do z en plates in double backs .Exposingsensitivepaper in tlze camera.
— Warnerke’
s wasthe first ro lle r slide which was in the marke t, and as such
we de scribe i t. Several o ther patterns have recently been
brought out.F IG . 66.
A band ofsensitive tissue i s ro lled round one ofthe .
movable ro llers A, andafter passing o ver F F, which consists ofa couple ofro und bars anda flat blackened board, is attached
to the o ther ro ller A. The band can be made to pass fromone to the o ther by turning a thumbscrew placed at D D
, and
can be fixed at any time by the clamping screws c c . It i sthus evident that after an expo sure has been given to one
part ofthe tissue, ano ther po rtion may be brought fo rward torece ive a fresh expo sure . The ro llers &c . are enclosed in abox B, which answers to the o rdinary dark slide
, the sensitive surface being pro tected by an o rdinary shutter.I t may be mentioned that a capital substitute for ground
glass may be made by coating an o rdinary plate with the
fo llowing varnish
EtherMasticSandarac
232 Apparatus.
After disso lving these resins, benz ine is added l ittle byl ittle t il l the grain becomes sufficiently pronounced.
In o rder to ascertain ifthe face ofthe sensit ive platein the dark slide o ccupies the same po sition as the gro undglass surface ofthe focussing screen ,
a bright object sho uldbe focussed accurately on the latter. The slide sho uld next
be fi lled with a piece ofground glass as ifit were a‘ plate , ora plate may be used. Ifthe image re tains the same definition it may be presumed the focussing screen i s co rrectlyplaced. In wet plate pho tography it is well to test thepurity ofthe silver wires which are in the dark slide .
The usual contamination i s copper ifa drop ofdiluten itric acidbe applied to the wire an absence ofgreen co lo ration
,due to the formation ofcopper ni trate, may be deemed
conclusive that the silver is to lerably pure . In any case, however, it is a good plan to give the wire a coating ofshe l lacvarni sh. Ano ther po int about the dark slide which should
be alluded to i s that i ts front should no t be less than 5 inchaway from the surface of the sensi tive plate. A nearerapproach is apt to cause markings .
Camera stands.— The camera-stand next requires a fewremarks , as the comfort ofworking
Fm. 67. depends much on the form adopted.
The essential s ofa stand fo r landscape wo rk consist ofrigidity, l ightness, and compactness when fo ldedup. The annexed diagram give s aform which is convenient, thoughperhaps rather heavier than is de
sirable . It is on Kennett’s principle,with a modification introduced by
Lane . The inverted top is shown in the diagram,fig. 67.
A i s a circular mahogany disc to which is attached a brasshinge B ofthe fo rm shown. On this hinge wo rks the whole
ofthe brass framewo rk to which the legs are attached.
0i s a screw which passes through the centre ofthe frame
234
ingdevice, fig. 69 , thi s can be secured. The camera-standis screwed to the small ho le, and the camera itselfi sattached by a screwto some po int in the slo t . When the
ho le is vert ically beneath the optical centre ofthe lens, and
the camera is turned, it moves round the optical centre of
the lens.l enses — As regards the cho ice oflenses, i t is very diffi
cult to give advice . Ifthe student is confined to the cho iceof one lens for landscape-wo rk, he should unhesitatinglyprocure a double t-lens, which has no perceptible flare-spo t ,and which embraces an angle ofabout since with i the can take bo th landscapes and architectural subjects . If
he can afford ano ther lens, perhaps a landscape wide-anglelens i s next to be recommended, be ing exceedingly useful{in mo st positions and giving great brilliancy ofpicture .
The mo st complete battery oflenses wo uld be a rectilinear‘
doublet, embracing an o rdinary angle ; a wide-angle doublet,to embrace about a couple ofsingle landscape lenses,one embracing about 50
°and the o ther about
Fo r portraiture, the mo st useful lens is one which willgive a cabinet siz ed picture , and it may be supplementedby one oftho se quick-acting lenses, which are familiar
'
to
all po rtraitists ofthe day, for taking instantaneous pictures
ofchildren, fire . I t should be no ted that many fine portraits and groups have been taken with the o rdinary landscape do uble t-lens, which, though slower than the portraitlens, yet i s sufficiently rapid to be usable .
I nstantaneous Shutters — In connection with lenses i t i snecessary that a wo rd be said regarding the means ofgivingrapid expo sures . In the market there are many numerous socalled instantaneous shutters which can give expo sures fromthe fi
lm second upwards. I t may be well to slightly touch
upon the rules which should regulate a properly made shutter.Ifthe shutter be attached to the lens , first and foremostevery action should be a double action, that is, ifone part ofthe instrument moves up, ano ther part should move down.
Drop Sliutter. 235
The reason for th is i s that the centre ofgravity ofthe who le
apparatus (including in this the camera, the shutter, and
the legs) should remain as nearly as po ss ible unchanged inpo si tion befo re, during, an
‘
d after expo sure . When ' the
centre ofgravi ty shifts, there i s ofnecessity a vibration se t
up in the camera, with the result that the picture will not
be sharply defined. Again, except in FiG . 7o .
the final clo sing of the shutter, no
part should be arrested wi th a jerk, forthat will entai l vibration.
~ Thus suppo s ing we have a pair of shutters
open ing from the centre, the passageto andfrom the po sit ions at which theywill commence to clo se again shouldbe as gentle as po ss ible . Ano therpo int is that the who le aperture ofthe
lens should be opened as rapidly as
possible, and the principal part oftheexpo sure take place when the who lelens -i s engaged in do ing its wo rk .
The effective exposure may be taken as
the sum.ofthe apertures into the timeduring which such aperture i s utilIsed.
I t wil l be evident, then, that the longerthe full aperture is employed the greaterwill be the effective expo sure in a giventime . Hence two shutters may be
employed which o pen and clo se the
aperture of the lens in equal times, and yet one may
give halfas much more expo sure to the plate than the
o ther. The cheapest shutter i s that known as the dropshutter, and the maxims above laid down can be broughtinto play with i t. The principle ofa drop-shutter i s thepassing ofan e longated aperture cut in a board over thefront ofthe lens. The longer the aperture in the boardproportionately to the . aperture ofthe lens
, the longer the
236 3
Apparatus.
latter is uncovered. Ifgravi ty be he ld as the moving powerfor the shutter, no great rapidity ofexpo sure can be givento the plate but gravity may be aided by an elastic spring
ofvarying tension, and so the times ofexpo sure may beal tered. A shutter ofthis description should no t be attachedto the lens, as there i s only one part which moves
,no
counterbalancing movement taking place . In practice i t iswel l to attach the shutter by a velvet bag to the lens, andhold the shutter during expo sure .
F IG . 7I .
T/ze tent — There are numerous patterns ofdarktents in the marke t, some very simple and some complicated .
These tents are intendedfor wet plates, but they can alsobe conveniently used for the deve lopment ofdry platese ither in a room or in a shady place .
The tent that will be described is one ofRouch’
s
form whi ch has been slightly modified by the writer. Whenclo sed it fo rms a shallow oblong box. When opened
,the
lidfo rms the front ofthe tent,and the clo th is extended
over the breadth ofthe box by means ofmovable iron rods.
238 Apparatus.
in the top ofthe tent. The legs ofthe ten t are offthe
o rdinary tripod form,and are fi tted on to a square frrame
which i s secured to the bo ttom of the ten t by thuimb
F IG . 72.
screws . In order to wo rk in a tent of this descripttion
an assistant is necessary, unless all the apparatus can be
placed in a light spring truck This is seldom feaSIlble.
On Me Picture. 239
Ifa well beaten and to lerably smoo th road be traversed,an assistant can wel l carry this tent and legs, whilstthe operato r can carry the o ther apparatus
,supposing
the s iz e ofthe plate to be used no t to be larger than 2 1 and
1 6 centimetres . The writer wo rked with this tent in Egyptfor some months andfound i t very conven ient and to lerablycoo l . This would nor have been the case had the coverbeenmade ofindia-rubber shee ting, as is often recommended.
One thickness ofblack twilled calico , and two ofo range,
tammy, were the material s with which this tent was covered.
A white cover might be an improvement .The annexed figure shows a very convenient dark room
which was designed by De la Rue fo r the Transit ofVenusexpeditions. It was adaptedfor wo rking dry or wet plates .Ofcourse a small room may be made , but ofwhateversiz e i t should be capable ofefficient ventilation . The fumesofammonia are pernicious to be inhaled for long, and thehypo sulphite bath is apt to give offsulphuretted hydrogenwhen ge latine plates are fixed in i t.
The l ight which should be admi tted to a dark room wil lbe discussed in a subsequent chapter.
CHAPTER XXXIII .
ON THE PICTURE.
IN a text book ofthis class it i s impracticable to enter intothe discussion ofall the rules which should go vern the
composition ofa picture . I t wil l suffice to po int o ut a fewof. the leading ones which should be followed. In com
pari son with the painter, the pho tographer is sometimesunder a disadvantage, in that he i s unable to choo se a po intofview to represent some particular feature, in which eve rything that is objectionable to artistic feel ing may be left
240 On t/ze Picture.
out or modified, or in which some extraneous object maybe introduced in order to give proper harmony to the picture. Thus the painter may render
‘
a distant landscape in afavourable aspect ofl ight and shade from some particularlysuitable spo t, though the foreground which may be at hand
may be to tally unsuitable for picto rial effect. The latterhe may discard for one which may be better fi tted for hispurpo se
,taking it from any o ther lo cality, providing it is
no t incongruous . The photographer, on the o ther hand,
i s rarely at l iberty to use this artifice, unles s he re so rt to thelabo rious process ofprinting from two or mo re negatives,although when the object i s attained the result amplyrepays any labo ur that may have b een expended. I t needscarce ly to be said
,when combination printing is re so rted
to , that the greatest care isrequisite to avoid incongruity, oran inartistic massing oflight and shade . When confined toa single negative, there is no thing for the pho tographer to dobut to make the bes t ofhis landscape,including his fo reground. This usually entails a sacrifice to a certain extent ofe i ther one
“
or the o ther, and i t is the possession ofthe knowledge as to where the sacrifice i s to end thatmarks the difference between the successful artist and the mere manipulato r.Bes ides focussing the object by the lens
,as will be pre
sently described, there is the fo cussing ofthe picture as awho le that is, the securing ofthe necessary harmony oflightand shade. In a
'
good and arti stic pho tograph the object onwhich the subject ofthe picture i s to be buil t sho uld standprominently o ut in the print
, the eye should instinctivelyrest upon it without be ing distracted by o ther parts .Thus sweeps ofshade may lead up to a mo re highly lightedpo rtion in which should be the principal object, or asweep oflight may lead the eye to a dark o bject whichthen should o ccupy the same prominent po s i tion. In anegative as it is deve loped this may often be unattainable,but by judicious masking ofparts during printing thisharmony may generally be secured, providing the taste
24a 072 tka'
Pz'
czfure;
nearest po int ofthe stern “occupies about '
a Similar distanceofthe
t
length ofthe picture. , The landscape being sub~
sidiary to the boat, he has caused the horiz on line to be
about 5way up the picture , andin order'
to break unifo rmity,he has -
'
so arranged that the boat should no t be sym:
metrically placed in_
regard to the c entre ofthe plate. The
lines ofboat also make an angle with the .hori'
z on, and
these are again balanced by the thwarts; &'
c . It would
have been a very easy matter to have made the pic ture
wantihgin harmony,byplacmgthe came ramore to the ri'
ght;
Km.74:
and causing the l ines ofthe boat to run paralle l to thehorIz on In which case the boundary of the small poo l“
ofwater In which it is lying would have had the'
samedirection. The keel of the boat might al so have beenplaced nearer the bo ttom ofthe picture, and the generalmass ofi t have o ccupied a central po sition In this casethere would have been a symmetrical p icture , the generallines running paralle l to the ho riz on andat right angles to i t,the result ofwhich would have been that the eye wouldbe partly satiated with it, and there would have been littlevarietyand much monotony. As i t is the pi cture, which
lmtaéz'
lz'
tjf. in ii Picture 243
Can only be faintly represented by the woodcut ; i s pleasantto lo ok at.Instead ofa boat be ing the object to be del ineated, we
mayhave it as an accesso ry to a landscape . As an example,we
have at View,fig
'
. 74, taken on the Thames byWo odbury . Tlie
object ofinte rest is undoubtedly the village beyond, with itsc hurch , and middle distance fo rmed by the trees . Ifthe
boat were taken away there,would have been a large space of
F IG‘. 75.
bare sho re , unbroken by any object to rel ieve i ts mono tony ;The boat
,however, happened to be there, and the artist
’
has
Se iz ed the chance to make a picture. No ti ce how it i s made
subsidiary to the general landscape . I t does no t o ccupysuch a prominent po sition as in the last example . I t iskept abo ut 3
1 from the edge ofthe picture, and the keel
occupie s a l ittle over 9; ofthe distance from the bo ttom, and
the l ine ofthe village i s placed about a}; up. Were the boatbrought lower down or mo re central , i t would have appearedto have been the ‘mo tive ’
ofthe picture I t i s evidenthow such a picture. might have been spo ilt from a want oi
R 2
244 On Me Picture.
knowledge ofart rules as i t i s, it is a beautiful example of
artistic pho tography.A third example ofa
’
study ofboats is given to show .
certain o ther po ints'
which are often neglected. We have
here, fig. 75, a specimen ofa picture that might have been
readily spo ilt. It should be no ticed how the lines ofthe
masts, sails, and pier are parallel, and were the figure re
moved from the side ofthe boat, and the small skiffmadeto lean in the o ther direction, the effect would have been
to give the idea that the boats, &c ., were tumbling out ofthepicture, and a sense ofinstability would have been created.
The oppo sing line ofthe mast ofthe small skiff, the inclinat ion ofthe figure, and the small po st in the fo reground,balance the general lines, and no impression ofinsecurity isleft. The general composi tion, too , ofthe picture shou ld
be no ted . The lines fo rming the extremities ofthe spars
fall on the body ofthe skiff, while a sense ofsupport to the
outer line ofthe large sail is given by the po st. The l ine
fo rming the top ofthe post and the top ofthe pier alsoapproximate ly passes through the cap ofthe man and the
top ofthe mast ofthe skiff. The pIcture i s then built, as itwere, on diagonal lines. A slight change in the position of
the camera would have altered all this. Again no te thatthe general mass oflight i s oppo sed to the black hull oftheboat, in tensifying the interest with which the boat, evidently
the principal object In the picture, is regarded.
The accompanying woodcut, fig. 76, taken from a photograph by Woodbury, well i llustrates the treatment ofanold water mill. In this. case the angle ofthe wall, that i s,the base ofthe corner ofthe mo st prominent piece of.ma
somry, i s placed about way up the picture . Had it beenplaced lower i t would have been aggressive, whils t ifplacedhigher it would not have given sufficient so l idity to the
mi ll. The water-wheel base, the object of interest, i splaced nearly centrally in the breadth of the picture, as
from,
the subject there i s no danger of symmetry, which
46 [0h
picture would have been out ofp roportion to'
the bak to
p lay the part ofan accesso ry. The distance, or what might
be called the ho riz on l ine , is drawn about é way up the
p icture, but beingso broken by the shr ubs and smaller
trees it i s invested with no impo rtance, and consequently
need not be dwelt upon as fo llowing any particular rule.
The outside ofthe trunk . ofthe oak i s placed iway from the
right hand edge ,had it o ccupied a more central position
the pIcture would have appeared cut in two by it. As i t
is, the dark foliage behind it fi lls in the side ofthe pic'
FIG . 77.
4.
t ure,and there
i
" i s not feel ing that the oak 15
‘
out 0place; “ Had:the fo l iage been
‘ light there would have been adanger that the eye mignt have been offended, but this i s
_
o ne ofthe cases in which the positionofthe camera“
mustbe made subservient to the operato r. The who le force of
the picture i s given' by the light, which breaks against the
t runk ofthe oak andaswith the trunk,so with the branches,
care has been taken to prevent any single bough cutting the
picture into two divisions. No tice, too , the stability given
by the straight stems ofthe tree s, in the distance .
247~
I'
n the next picture,~
fig. 78,
we have the distance, or
perhaps mo re strictly speaking, the middle distance as
the po int ofinterest The ho riz on line is kept in the
weake st part, the centre, of the“
picture. The trees in
the foreground are so grouped that they frame the
viewwi th dark masses, rel ieved by the l ight fo liage ofsome
ofthene arer bushes and'
shrubs. The foreground fini shesat a distance ofabout i from the bo ttom. More ofItwould
“
take awayfrom the
value ofthe
distance, as it wou ldplace it in the
weakest part ofthepicture— viz . , cen
trally ; less of itwould have ren
dered the picturebald, and havecut off part of
the deeper shadeswhich are so valuable in giving the
effect of distanceto the stream be
yond. This picturewould have beenspoi lt hadz the ca
mera been so placedas to give mo re topfo liage, since the
‘
bough which ‘
now partially c rosses thepicture at about § the he ight, would have caused an uglydiv ision, and also
Tg
the tops ofthe distant trees and the skywould have appeared. This latter, in views such as thatunder criticism, i s objectionable, as patches ofwhite givethe eye an inclination to wander offtowards i t, and i twouldhave beenfian'
, insufficient p recaution to have printed in,
248, fl e e/Tie. Picture.
clouds from another “
negative, owing to‘
the difficu lty thatwould exis t In subduing at the same time the l ights on the
leaves ofthe near trees . As i t i s, the,
picture is in -picto rialfocus. By placing the stream to the right o r left, the
balance would have been wanting, and, its general direct ionwould have been altered to such an extent as to
_
havegiven -a feeling that . i t was a subsidiary part ofthe p ictureinstead ofan essential.
The next example, fig. 79, Is intended to show a picturetaken on the diagonal not on an absolutely straight line, but
F IG. 79.
one in which the general direction ofthe picture is on the
diagonal. The po int of interest is the extreme distance of
the stream, and acco rdingly it is placed in one ofthe strongest po si tions in the picture, viz . gway in bo th directionsfrom the margins, and it contains the highest light, as seenin the water. This brilliant l ight is repeated in the clouds,and mo re faintly still
:it is echoed
’
in the ro cks, Where
it takes approximately the same fo rm, though it i s“
repeated
in a lower tone and ofdifferent dimensions. The . picture
might easily have been spo ilt by placing the .distance in a
250.
generati veontortrgofthe clouds fo l lowed in_
any'
degfee‘
the
cpntour ofthe“sky-line. In a wo
'
odcut it i s impo ssible -to
give all the expression that i s‘
to befound in the pho tograph ,but the st
_udent -m
_
ay gain a fair knowledge of. the ruleswhich have been fo llowed.
As regards the introduct ion offigures into a .landSCape ,It may be necessary to say a few wo rds . It should be clearlyunderstoo d that the one must be made subsidiary to the
o ther , that Is, ifthe po rtraits ofthe figures are requ ired theymust bemade the principal objects, and the who le landscape
i
FIG 8 1
tunst be made subservient to them. On the o ther hand,if
a landscape Is to be photographed, the figures though prominent, y. et should o ccupy such a po smon as to be subo rdimate to i t, though they may enhance and give the ‘forte
points to the picture ; and above al l things care must betaken that the figures compo se as we ll with each o ther aswith the landscape.
Robinson, in his ‘Pictorial Pho tography ,’ a wo rk which
every pho tographer should possess , says : The figure mustbe Ofthe subj ect as wel l as . in i t in o rder that un ity may
Pictor ial P/zotograp/zy. Piper Carter.
Cloud and {landscape 2251
cbc preserved ,e it must
"
be fiused'
.with a'
purpose to give life
to ~a' scene, or to supply an impo rtant spo t oflight or
dark to give . balance, or to bring o ther parts into sub
o rdination, by e ither being blacker or whiter than tho se‘
parts and that what 13 to be avo ided is the '
indiscriminate
dragging In offi gure s into scenes in which they have no
bus ine ss ; andWhere"
they do no thing but mischief.’
We have’
sucha n’
example in the cut'
below,taken from
a pho tograph by , H .
"
P. Robinson, called( “
BlackberryGathering,
’
fig. 8 1 . The landscape i s one which is mo stunpromi s ing in i ts aspect the sombre bank ofblackberry
bushes alone, would fo rm a melancho ly, gloomy, picture ;but by placing these figures, as they are, some 4
way from
the left hand side, the contras t of light they offer to
the deep shadow behind them at once attracts the eye,
and leads it gradually up the winding broken path beyond.
The spo t oflight, in fact, affo rds the exact balance required
to what o therwise would be an uninteresting picture. Itaccentuates everything, as i t were, and gives the ‘fo rte ’
po int , which is such a desideratum.
Again, in this study we have an example ofthe valuethat a sky ,D
ives to a picture . I t should be no ti ced how the
lines ofthe clouds balance the lines ofthe hill. Ifthe left
a nd dark cloud be covered up how wanting in vigour is
the composition. It will be seen how Robinson, out of
such 1unpromising materials as a blackberry bank , a couple
offigure s, and a good c loud negative, has been able ,to build
up apicture which Is technically perfect andfull ofinterestand repo se.
In the next il lustration, fig. 82, which isfrom a pho tograph
by the same artist, we have a capital example ofthe co rrectgro uping offigures to fo rm a picture In which they are the ob
jec ts ofinterest, andthe landscape mere ly subsidiary, thoughessential . L ike the typi cal picture by Sir D avid Wilkie ,‘ The Blind Fiddler,
’ this,which is named
‘Ho liday-“
mak ing
inj the ,
Wgods,’ i s built
,
up on a series ofpyramids, the. base
252 3
b e ing curved. I t should be no ticed‘
how one pyramid runsinto ano ther, each corner be ing differently supported as
“
,
for example, the right-hand co rner ofthe big pyramid i ssupported by the basket, and the left-hand corner by the
arm of the recl ining lad. . Tracingthe compo sition allthrough, i t will be seen the l ine s have been artisti cally keptin view
“
and the figures po sed accordingly. The straightl ines ofthe distant trees contrast with the fal l ofthe pyra
m idal lines, and give a fi rmness which would o therwise bewanting. The neares t object, too , i s
"
made the most distinct,
F IG . 82.
whilst the darkest object, which is the figure ofthe boy,cuts acro ss the highest light, givmgJust sufficient contrastand no more. Had the lad’s head been raised higher theeffect would have been to fo rm two patches
_
ofwhite, frome ither ofwhich the eye would have wanderedby the attraction ofthe o ther. Again, leaving the main group, the twosmall figures lead the eye instinctively to the distant gladebeyond, so that over every part ofthe picture we are led"tosome fresh beauty.
Very different -are the groups so often seen as po sedby many photographersfi Either the
“
heads ofthe standing
254: <s fagPiafa'
refi
all times’
is impossible ; The‘
gréat po int"
with the rphoto
grapher i s to know when and how he may transgress Without spoilin
'
gj he treatment ofhis subjec t.1 . Ifthe -
object ofinterest be on the fo reground, i tsbase should o ccupy a po sition offrom 3} to gthe height of,the picture ifi t be in the distance its base should be about
5way up the picture .
z In a general landscape : the ho riz on lines should
o ccupy a po sition aboutgway from the top or the bo ttom
ofthe p icture with'
the latter a , cloud n egative will probably be required.
3. It is advisable that the general l ine ofa picture
should run on a diagonal or take a pyram idal shape.
4. A long obtrus ive l ine should never be permitted to
intersect the picture i t should always be broken up as far
as po ss ible .
5.
-A picture should never be cut in two by a dark ob
j ect against a light background or by a light object againsta dark background.
6. Ifthe general features ofa picture have a wedgelikeform, care should be taken that the wedge i s suppo rtednear the po int, in o rder to give the idea ofstability.
7. The general l ines ofa picture should be balanced byoppo sing lines, for the same reason as that given in 6.
8 . A la rge patch ofone approximately unifo rm tintis distastefu l to the eye, and should be broken up , if
po ssible.
9 The object ofinterest should be picto rially focussedby a general sweep oflight (ifit be a dark object) o r of
shadow (ifit be a light object), thus causing the eye to fall
naturally upon i t.I O . Avo id mono tony, whether in constant repet ition of
lines, l ights, or shades , and never allow a picture to be
symmetrical on the right and left ofi ts centre . A repeti
t ion ofa high light once or tw1ce in a lower {one i s, however,much to be
“
recommended See figs . 79 and
Fwax ingtaéPictu re. 2557
3
As“
regards'
8 and‘
9 ,~it must bebo rne iri
'
mindthat in theprinting ofc the picture
‘
a‘
great power is'
placed in the photo
grapher’s hand ; by Ia j udicious masking ofparts he can
cause pictures which would be inartis tic to become merelyinoffensive, and he may give an atmo spheric effect o ther
wi se unattainable by remembering that shadows in the dis~tance tend to become lighter, whilst high lights tend to
become darker. Tissue paper stretched'
on. the back ofanegative, and a limited use ofthe s tump, will be found to
be powerful aids to the production ofan artistic p icture.
“
An artis tically educated pho tographer instinctively s ees
the mo st favourable aspect ofthe subject he may wishto de l ineate . When he perceives that he is in a favourable situation, the camera should be . erected , and the
mino r “ details of the compo sition must be“ attended to ;
Then i t’
i s that he must exercise to the rfull extent his artisticknowledge. Knowing the p osi tion his principal subjectshou
'
ld occupy in the p late, he must no te the differentsubordinate objects that are also seen on the gro und glassofthe camera. Ifthe subject require figures to be
'
introduced to give forte po ints he should no te where, andhow,they should be arranged. He
~
should also no te the mostfavourable time ofday for taking the view,
bearing in mindthat one ofthe chiefcharms ofapicture is a propermassing
ofl ight and shade, which as a rule can only be secured bysunlight fal ling across the picture, and not coming from
behind, or from the front of, the camera.‘
S As regards the abso lute manipulation ofthe camera thereis?not much to learn beyond fo llowing a few simple rule"After selecting the View
,the angle which is to be taken in
should be roughly measured, and the lens selected accordingly. When this is determined, the view should be
‘broiJghtapproximately on to the ground glass ofthe camera. It
require s a certain amount'
ofpractice to forma co rrect pic?.torial estimate ofan inverted image, .and i t improbable ,
byturning the head in such a position t bat the . lin§ -,lQ in.ing
6 . On Me“
Picture.
the eyes 7is nearly vertical; a more correct idea ~
cangbe
fo rmed than by keeping. it in the usual po sition.
'
At fi rstno diaphragm should be in the lens, as the general sweep oflight and shade can be better studied. When this IS satisfacto ry and the lines ofthe picture are the best that can beobtained , a diaphragm may be inserted w i th an aperture of
the largest siz e which will . admi t ofa good general fo cusbeing .The object ofinterest must, however, ;be
that which 'is most sharply~ defined on the ground glass, andi t is sometimes
'
advisable to'
.sacrifice the sharpness ofthe
o ther po rtion'
in. o rder to attain this, and when the characterofthe '
sweeps o f‘
l ight and shade is no t as good as co uldbe desiredgi
'
t may also sometime s be necessary to adopt thisartifice to . secure the proper attention ofthe eye to thatpo int. I t is not intended to imply that a picture out of
focus is more artistic than one sharply defined . Tho ughthe eye sees only one po rtion ofa landscape at
.a time in
facus, the remaining portion be ing blurred, ye t, be i t remembered, the pho tographic print, when properly viewed,o ccupies the position of the natural landscape
, and the
same difference offocus away from the object of interesttakes place naturally and, in the pho tograph, as in nature,the eye may wander to the po ints oflesser interest, and stil lfind a charm in the minute details.
One ofthe essential suppo sitions ofperspective i s, that thepicture plane shouldbe verticalandthehue ofsightho riz ontal.Nevertheless, in focussmga landscape -taken me rely for pic,torial effect it' usually does not signifywhether the camera betilted downwards or upwards , or whe ther the gro undglass bevertical, so long as the top and~ bo ttom ofthe pictures areparalle l with the horiz ontal line, though in architecturalsubjects, as we shall presently see, the po ints canno t be neglected. For a simple :lan
’
dscape, then, i t will be found that thepower ofobtaining a good focus is well within the
,hands of
“
the operator. From what has been already said at p. 209 ,
;it is seen that the focus ofnear obj ects is longer than that of
258 072 t/zePicture:
po rtance to preserve the paralle lism ofvertical l ines, thesensitive plate must always be kept in a vertical plane. In
case the axis ofthe lens has to be tilted the swing-backmust be used ti ll this is attained. When this vertical pl aneis not adhered to we shall have the vertical l ines which are
paralle l in nature converging in the picture. Ifwe look ata cube lying on a horiz ontal plane the feeling to the eyes i sthat the vertical lines are paralle l (perhaps because the
mo st natural position for natural movement ofthe axesofthe eyes is in a ho riz ontal rather than in a vertical plane )if, however, we tilt the cube the impression at once vanishes ,and they will seem to converge . Hence to give the idea tothe mind that an object i s standing on a horiz ontal plane ,the vertical lines must appear parallel to the eye, whe therseen in nature or seen in a picture. Now
, i t can readilyand easily be demonstrated that by tilting the camera with
out u sing the swing-back, vertical l ines must converge, hencethe resulting picture would be untrue . It will perhaps aid
the student in regard to the use ofthe swing -back to re
member that for theore tical purposes the lens,whe ther
i ts axis be t i lted or not, may be replaced by a pin-ho lein an o paque card, and that the image rece ived through the
pin-ho le must be theo retically co rrect when rece ived on a
vertical plane .
As regards the exposure to be given to a picture there i s
one golden rule to fo llow ‘Expose for the shadows and let
the lights take care ofthemselves,
‘ that is, the de tail in the
shadows must be developable . By the j udicious use of
different developers, page 67, effects may be given whichwould be unattainable were one fo rmula alone to fo l low.
Thus,ifthe picture be full ofgreat contrasts in lights and
shades, a stronger developer swept over the plate, carryinginto the sink the greatest part ofthe free n i trate ofsilver,will lessen the differences be tween them, whils t a weakdeve loper, ofwhich none is allowed to flow over the plate,is mo st suitable when the contrasts are l ittle. Again, even
259
during expo sure someth ing maybe done to give harmony to
the negative by shading the lens with a piece ofblackenedcard from tho se parts, such as the sky and clouds, which are
mo st quickly impressed on the plate.
As regards po rtraiture the variations in l ighting that can beproduced in a we ll -appo inted studio are so numerous that itwould be impo ssible to treat ofthem in a limited space . For
outdoo r po rtraiture an angle ofa wall facing the no rth witha background fo rmed by a blanke t is suitable for producingpictures that can be vignetted.
Indoo r portraiture can be attempted where a windowwith a northerly light i s available, and where white screensare at hand to l ighten up that po rtion ofthe face which isin shadow. Ordinarily the principal light should make anangle ofabout 45
° with the vertical andh oriz ontal planeswhich intersect in the axis ofthe lens .
The student should refer to Picto rial Pho tography,’ for
a comprehensive description ofwhat to do and what toavo id. The rules given for landscape pho tography
,however,
in a great measure apply, particularly paragraphs 3— 5, 7— 10.
CHAPTER XXXIV.
P H O T O -SP ECT R O SC O P Y .
ONE ofthe branche s ofscience into the service ofwhichpho tography has been impressed is that ofSpectro scopy,and the aid it has g1ven dates from nearly the early days of
the daguerreo type . In the researches at present made withthe spectro scope i t plays such an important part, that
i
arather detailed de scription ofthe apparatus necessary and
the me thods employed will be g1ven.
Pho to -spectro scopy, however, has two aspects in one itis the study as to the sensitiveness ofcompounds to theinfluence ofdifferent portions ofthe spectrum in the o ther
5 2
260 Fad e-spectroscopy.
the study ofthe spectrum itself. The first may be con
sidered an essential prel iminary to the second, and willtherefo re be examined first.
In the loan co l lection ofscientific apparatus at SouthKensington is exhibited the instrument with which Hersche lmade his class ic researches on various substances whichwere acted upon by light. His experiments were undertakenbefo re the days of the co llodion process, and his mea’usoperana
’
z'
consis ted in giving washes of one or mo reso lutions to paper, and then submitting the sensitised paperto the so lar spectrum. The accompanying diagram
,fig. 84 ,
gives an idea of the prismatic arrangement be adopted.
A is a flint-glass prism, capable ofturning on an axis,D B
F IG . 84.
a lens ofabout z 4-inch focal length c the screen on whichthe sensitive paper was placed. The sunlight was re
flected byamirro r into the prism,the image ofthe sun after
passing through A was e longated into a spectrum,and
brought to a fo cus on c by means ofthe lens B. One
portion of the spectrum was caused to fall on the l ine,E E ; the part icular ray be ing found by examining thespectrum by means ofa piece ofcobal t glass
,which cuts off
all rays excepting two . The cage was covered with a blackvelvet clo th. I t will be no ticed that no sl it was employed, butthat the spectrum was really fo rmed by a series ofoverlapping image s of the sun. The spectrum was thus of
necessity an impure one, and in the results obtained withi t this has to be taken into considerat ion. For a detailed account ofthe experiments carried out by Sir John
26 2 Paolo-spectroscopy.
Co llimato r, c . The lens,D, ofthe co ll imato r i s so placed
that its equivalent focus falls accurate ly on the exterio r of
the slit. This should be obtained by trial , and i t i s ve ryadvisable that the sl it arrangement should be attachedto an inner tube , which can sl ide in that to which the
lens is attached. The rays proceeding from the illumi
nated slit travel in such a manner that when certainrequirements, which wi ll be entered into presently, are
fulfilled, the lens ofthe co llimato r is perfectly fi lled, o r atall events the rays ofl ight fo rm a central disc on the lens, DThe light then trave l s to the prisms, E
,by which it is
refracted and dispersed,and then i t reaches the lens
,F , of
the camera,G, by which lens the spectrum is thrown on the
plate , H , where i t is focussed. As difficulties frequentlyarise at fi rst in the adjustment, &c ., ofthe apparatus
,a few
hints may no t be out ofplace. In o rder to secure goodresults the focal length of the condensing lens, D
,when
divided by its diameter, should never exceed the length ofthe coll imato r when divided by the effective aperture ofi tslens. Should it do so
,i t will be seen that the co llimating
lens wil l be more than fi lled , and reflections from the sidesofthe tubes might interfere with definition. Again
,it i s
useless to have prisms which canno t rece ive all the raysproceeding from the co llimating lens. The ir he ight should
,
therefo re, be at least equal to the effective aperture ofthe
co ll imat ing lens,and the faces should be longe r, s ince they
are placed obliquely. In all cases the centre ofthemirro r,the axis ofthe condensing lens, and ofthe co llimato r should
be in one straight l ine. To effect this , i t is better at first-
to
remove the condenser from the train ofapparatus . The
image ofthe sun,when thrown on the sli t, shouldgive abright
diffused line occupying the centre oft/ze[
collimatingr lens, D .
The po sition and height ofthe hel io stat must be changedti ll this i s obtained. The condens ing lens, B, may now be
inserted and moved_ t il l the rays ofl ight form a circular
disc,fi l ling the centre ofthe lens, D, when a sharp image
l nz'
mnnz Deviation. 63
ofthe sun is thrown on the sli t. When this IS obtainedthe prisms may be placed in position. By the aid of
an ordinary small telescope the angle ofminimum deviation may be obtained. Suppo se i t i s required to pho tograph the po rtion ofthe spectrum about the line G. The
pri sm would be placed roughly in po sition, and that linewo uld be observed . It would be found that by turningthe prism in one direction, the line would appear at firstto trave l in one direction, but that when a certain po intwas reached i t would begin to
'
travel in the Oppos ite direc
tion. The po sition the prism o ccupied when the change
in direction-ofthe apparent mo t ion ofthe l ine took placewo uld
'
be the position the prism should occupy, in o rderfor that particular ray to be refracted in the angle of
minimum deviation. I t is often insisted upon that thisangle i s ofsupreme importance, but it can be shown thatwhen a co ll imato r is used, and not a distant sl it alone, thatthe resulting spectrum will not give erroneous impre ssions .I t must be adm itted, however, that the angle ofmin imumdeviation i s a po sit ion which is convenient ifnot ‘
necessary.
After the first prism is adjusted, the second and o ther prismsmay be adjusted in l ike manner. When once they are fixedthe pos it ions can be marked by l ines, ifon a slate table, orby small guide s screwed into the wood, ifa board be used.
The camera should be finally placed in position so that theray for which the prisms have been adjusted should o ccupythe centre ofthe focussing screen or sensitive plate .
Ano ther contrivance 1 for always secur1ngthe m inimumangle ofdeviation i s shown in the accompanying diagram.
Cut out, in sto ut card or brass, pieces having the fo rm A A A,
also D and K.
’
Care must be taken that the bases ofthe
triangles are ofun ifo rm lengths, and slightly longer than thebase ofthe prisms
,which should be ofuniform angle, and
preferably ofequal siz e. D should be let into the board 8 s,
The principle ofit is due to Mr . Browning, the Optician , but we
first saw this easy adaptation ofit,to ordinary work in the laboratory
ofDr. Huggins.
264 P/z’
o/o-spectroscopy.
’
so that its top surface is flush with it, and there'
should be'
agroove cut beneath the slo t, b, to allow a pin, of
-a diame ter
equal to that ofthe slo t, to trave l along it. The slo ts a a a
in A A A and K are also placed over the pin. The firsttriangle i s attached to the board, 8 , by a pin at B. The
remaining triangular po rt ions, and also K, are attached to
each other at c c c, and are free to move over the board 5.
F IG . 86
The axi s ofthe co llimato r is placed at right angles to the slo t,e,and, by touching K, the armsfrom the triangular po rtions
move about. the pin, the slo ts, a a a,guiding the mo t ion at
the same time the parts move separately about c c c, and thewhole system turns about B. It will be seen by thi s thateach base ofthe triangle moves through twice the angle of
the preceding one, as also does K. The direction ofthe linejo ining the po int in K
,answering to the middle po int of
the base ofthe triangle, and the middle point ofthe base
ofthe adjacent triangle, determines the position ofthe axisofthe lens, F, ofthe camera . Such a boardmay be used
as a pattern by which to set the prisms for any partic‘
ula
part ofthe spectium, or the prisms themse lves may be set
266 P/zoto-spectroscopy .
enables it to be raised a quarter ofan inch at a time,thus
allowing at least I 5 expo sure s to be taken on such a plate,
When the length ofthe spectrum i s small,an inner frame to
Fla gs. carry a quarter plate may be used, allowingsome t o expo sures to be made on the sameplate, a matter ofno smal l convenience.
I t i s usually prescribed that thefo cus shouldbe obtained by placing a transparentglass plate1n the place ofthe ground glass and viewingthe spectrum by a high-powermagnifyinglens.This latter should be attached to a sl idingtube , so that when the endofthe tube is p lacedagainst one surface of the glass plate the
o ther surface immediately opposed should bein the plane ofits fo cus. By this means therays which have to be fo cussed on the innersurface ofthe plate can be viewed by the
magnifier, and the plate ofglass moved backwards andfo rwards by the screw mo tion ofthe
camera, till the l ines appear sharply defined to the eye.
Theo re tically, this i s a perfect me thod, but practical ly, i tfails iftwo portions ofthe spectrum which are far aparthave to be
“
pho tographed at one operation, for the necessaryincl ination
,ofthe surface ofthe glass away from a plane
perpendicular to the axis ofthe lens (the deviation beingeffected by the swing-back), i s so great that the axis ofthemagnifying lens is thrown complete ly out ofthe d1rect1on of
the rays oflight, and there i s necessarily no image observed.
Unless a micro scopic arrangement be employed, it i sbetter to obtain a fair fo cus on very n ice ly gro und glass, o ron a plate coated with very fine ge latine bromide emulsionsand then to makethe final adjustments by trial.
A good method ofobtaining the co rrect distance oftheco ll imating lens from the slit may here be indicated bymeans ofan eyepiece. Make an ink mark or a scratch on a
piece ofglass, place it against the eyepiece, which should be
Lnlztfor t/ze Spectroscope. 267
surrounded by a draw-tube, and fo cus the ink mark or
scratch, which sho uld be next the eyepiece , by altering thedraw- tube. Now place the draw-tube of the eyepieceagainst the j aws ofthe slit (which whould be rather widelyopened), and view some a
’t'
stant object through the collimat
ing lens . When this appears quite sharply defined,the
defini tion be ing obtained by altering the distance ofthe slitfrom the co llimating lens , this adjustment i s Comple te.
In the case ofa lens only partially corrected, the fo cusmay vary. In one po sition the image may appear perfectlysharp but surrounded by a blue fringe, and in ano ther bya red fringe . When the mo st refracted rays are to be
pho tographed, that fo cus should be-cho sen in which the red
fringe i s seen whilst,ifthe least refracted, that in which the
blue fringe i s apparent.It must be reco llected that the greatest accuracy in al l
these po ints is requ1s1te ln o rder to obtain the best results.When o ther than the sensitive salts ofsilver are to be
examined,a single prism will suffi ce for the dispersion,
u sing a camera lens ofabout 1 8 inches focusIt is better then to open the sl it to such a degree that the
Fraunhofer l ines broaden out. One or two distinguishinglines , such as the Hand5 lines, will stil l be traceable, andfromthese, and by a comparison with a picture taken on a silvercompound, with a mo re clo sed slit, the l imits ofthe spectrumimpressed on the sensitive sal t under c onsideration may beaccurate ly obtained. For obviously, when the sli t is say amillimetre wide, there i s but little impurity in the spectrum.
Wi th this apparatus, as in that above, it i s advisable to
employ a condenser.We have hitherto been suppo sing that sunlight was to be
used as the source ofillumination, but sunl ight is not anecessity . Skylight will answer for investigations on the salts
ofsilver,as will also an o rdinary gas jet, or an incandescence
lamp. In fig. 85, instead of employing B to fo cus thesun’s rays
,they may be employed to focus on the slit
268 Pane-spectroscopy.
any one ofthese source s oflight an e lectric arc l ight mayalso be employed, usmg the Vio let in po sitive po le as thepart focussed on the slit, but for delicate work this i s somewhat difficult to employ, as there are a series ofcarbon
bands in the vio let and ultra vio let, which may be misleading. To ascertain the po sitions ofthe different parts ofthespectrum, a spectrum should be pho tographed ofthe lightfrom burning magnesium wire, or ofthe base ofthe blueflame ofthe gas jet or ofa candle. These . will give l ines,the po sitions ofwhich are known, and with which the
p ho tographed continuous spectrum may be compared.
Fto . 89 .
For the student who uses sunlight or skylight, an o rdinary
mirro r may be employed. I t is preferable to use one
s ilvered on the front surface, as the“
double reflections are
thereby avo ided. Themirro r should be ofcarefully selectedpatent plate, and be silvered on the best surface , and the
silver should be brought to a fine po lish. I t is convenient
to use a hel io stat'
when sunlight is to be employed, and the
fo rm given in the accompanying figure, designed by Stoney,answers well wi th a little care.
A i s a small French clock, round“
the drum ofwhich,
and also round a small wheel fixed on the instrument,
Lawyer'
s Arrangement. 27I
ape rtures are cut en éc/zelon, the top Ofthelower one be ing inaccurate continuation ofthe bo ttom ofthe upper one , and
so o n. When the top aperture ofthe moveable plate is in
fron t ofthe sl i t, only the top part ofthe slit is uncovered,andwhen the bo ttom aperture is infront ofthe slit the bo ttompart ofthe sl it i s unco vered. At B i s a rack and pinion,which is used to adj ust the distance ofthe sli t from the col
limating lens, inserted at the o ther extremity Ofthe tube 0At D i s a train ofprisms (the number in which can be
altered at pleasure), set to the angle ofminimum deviationfor the mean ofthe rays which i t may be de sirable toexam ine . E i s a camera, some 6 feet long, furni shed witha lens ofthat fo cus , and the usual means offo cussing.
At F is inserted the dark slide. capable ofcontaining a plate6 x2 inches. The spectro scope and camera are rigidlyconnected one with ano ther, the prisms and co l limato r
,
C, be ing fastened to an“iron plate, T, suppo rted on a so lid
pillar, s. Thi s complete s the pho to -spectroscopic arrange
men t. In order to compare the spectrum ofa me tal withthat ofthe sun, Lockyer adopted the arrangemen t shown.
K is an e lectric lamp, be tween the po ints ofwhich the metalto be examined is vo latil ised
,
by means Ofthe electric current passing be tween them. The po ints are so placed that
the in terval be tweenthem lie s in a continuation ofthe axis
ofthe co llimato r c . At H is a small lens,the distance
between A,H,and P be ing so arranged that A and P are
conj ugate fo ci ofH. In some case s the place ofthe lamp is
o ccupied by a Ruhmko rffco il, and the me tal vo latilised bythe heat ofthe spark. G is ano ther condens ing lens on to
which the so lar rays are thrown by means ofa helio stat, andplaced at such a distance from A that its principal focusthe fo cus for paralle l rays— is at P. By this arrangemen t
the lens H will throw a perfect image e i ther ofthe sun, o r
ofthe electric arc,on A.
When the portion ofthe so lar spectrum requ ired has
been accurately fo cussed on the plane to‘
be o ccupied by
Photo-spectroscopy.
spectrum falls on the sensit ive plate, but in this case immediately above that ofthe metal .
It will be seen that it i s quite po ssible to compare the
so lar spectrum with two metals by increasing the number
ofaperture s en éclzelon in the sliding plate to three, iftheso lar spectrum be taken with the middle ofthe slit. Two
examples of the pho tographs obtained by Lo ckyer are
annexed, fig. 92. The first shows the co incidence of
some ofthe bright l ines (near H) ofthe spectrum ofi ron,
with the abso rption l ines in the so lar spectrum,and the
second shows a similar comparison between calcium,
aluminium, and the sun.
The material s which are used in the spectro scope or for
the reflecto rs i s a matter ofsome impo rtance. Glass i susually employed for the prisms and lenses, but for s tudying
the ultra vio let a quartz train is a desideratum,since glass
absorbs the extreme rays ofthat portio n ofthe spectrum.
W. A. Miller studied the abso rpticn ofdifferent transparentso l ids
,and although undertaken at an early date in the
histo ry of pho tography the results are even now useful.The following are some ofthe results obtained by Miller.
Termina Relative
Name ofSubstance tion of lengths of RemarksSpectrum Spectra
Iceland sparFaraday
’
5 opticalglass Pale yellowFlint glassWindow sheetglassHard Bohemian glassPlate glassCrown glass Greenish
Reflection from Polz'
snea’ Surfaces. 275
He used a quartz train for the purpo se ofhis investigation.
The line B in the extreme red is 84 on his scale,and the
line H in the extreme vio let 100. In every case the com
mencement ofthe pho tograph was 96 5 near G on his stale,silve r iodide be ing the sensitive salt employed.
Dr. W.A. Miller also investigated the reflected light fromdiffe rent po l ished surfaces . Ofmetals and alloys he tried
the following platinum, go ld, s ilver, mercury, lead, copper,tin, cadmium,
z inc, aluminium. For pho tographic rays noj udgment could be fo rmed ofthe perfection ofthe reflect
ingpowe r from the co lour ofthe me tal. The fo llowing isa summary ofhis results
Golai— Reflects all rays, even the most refrangible,equaHy.
Spectrum extends from 96 5 to 1705 .
[ earl — Similar to go ld, some parts of the spectrumbe ing mo re strongly reflected.
Spectrum extends from to
Sz'
lner .— Impressed spectrum strong, from 96 5 to
a breakfo r 1 °
5 ofthe scale , and then recommencement, continuing to 159 5 ofthe scale.
M ercury— Strong at each end of the pho tographed
spectrum,but weak in the middle ; extent, 9 6 5
to 159 5.
Platinum — Same as mercury, but less intense.
Zine — Same as mercury,but less intense.
— Same as mercury, but less intense.
Cadmium — Same as mercury but less intense than thelast three.
Coupon— Deficient in strength for last half(most t e
frangible) ofthe spectrum extent, 96'
s to 159 5.
B rass — Same as copper, but weaker.
Steel — Intense spectrum extent, 96 5 ,to 159 5.
Speculum metal — Intense in least refrangible half of
T 2
276 P/zoto-spectroscoj fiy .
the pho tographed spectrum, but weak in the most te
frangible half; extent, 965 to 159 3 .
At the present time diffraction gratings, used by refleet ion, are much employed. If the grating be flat, the
co llimato r and camera remain the same as when prisms are
employed, and it is well that the plane ofthe gratingr shouldbe at right angles to the axis ofthe camera lens. A specialfo rm ofapparatus is employed for this purpose. I t shouldbe no ted, however, that the brilliancy of the differentspectra exhibited by different gratings is by no means alwayswhat theo ry wo uld give, and i t may be that some gratingsused by reflection cut offpo rtions ofthe spectrum e itherby want ofreflectionfrom the metal employed or by peculiarity in the ruling. Gratings maybe obtained with lines varyingin number per square inch from to or evenmore . The advantage ofa grating is that the wave lengthsofdifferent l ines can be determined from pho tographs ifi tbe placed as indicated above. The vio let and ultra vio letdo not o ccupy the ino rdinate length which they do in aprismatic spectrum. Nor are the red and infra red con
densed as they are when prisms are employed. Excellent copies oftransparent gratings have been made , andthese are used by transmi tted light, and are perhaps mo reaccurate in giving the true i lluminating value ofthe differentparts ofthe spectrum than are the reflection gratings.Concave gratings have been introduced by Ro land
, and
the advantage with them i s that they need no camera lensnor co ll imator. The slit is focussed by the concave surfaceandthere i s no lo ss ofsharpness through want ofachromatismofthe lenses employed.
I t should be no ted that the earliest published pho to
graph ofa diffraction spectrum,at. all events ofany scien
tific value, was due to Dr. H . Draper. He fi rst publishedit in England in the ‘Phil. Mag.
’
for Dec . 1873. The l ines
278 Effect ofSpectrum on Salts ofSilver
the action ofthe spectrum in No . 1 i s due to the action of
diffused light. I t next remained to trace the action on the
different silver compounds existing in this paper, which waso rdinary siz ed saxe paper. Paper was prepared as befo re,but washed in common water till nearly all excess ofsilvernitrate was el iminated, and i t was then given a wash of
po tassium ni trite, an abso rbent ofiodine. Such paper wasexposed to the spectrum,
first, coming through chromate,second, unshaded . The print obtained is that shown in
No . 2, by which it will be seen the same limi ts were reachedas before, but that there is no t that abrupt descent ofsensitiveness
“
near G ; evidently some cause ofthe extremesensitiveness near this po int had been eliminated, and
apparently that could only be the silver n itrate and the
presence ofthe po tassium ni trite . To test the matter further, paper was prepared in the same manner, but befo reapplying the po tassium nitri te it was soaked in commonsalt and water and washed. This would effectually removeall traces ofsilver nitrate, converting it into silver chlo ride .
Expo sure for five minutes to the spectrum gave the resultshown in No . 3, in which i t will be seen that whilst themo strefrangible portion took a grey co lour, the small po rtionbelow G became pink , the line ofdemarcation between thetwo being well defined. I t now seemed probable that
the pink part ofthe spectrum was due to the chloride and
the grey to the iodide.
To further investigate the matter, the same paper without iodide was floated on silver nitrate and expo sed to the
spectrum, with the result given in No . 4 , a very faint traceof action be ing visible where the paper was expo sed fora quarter ofan hour to the spectrum transmi tted by the
po tassium chromate.
Iodised paper prepared as in the first experiment waswell washed and simply exposed with the result to be seenin No . 5. Finally
,paper was prepared and washed, then
immersed in a weak so lution ofpo tassium iodide, washed
Efeet on Silver Iodide. 279
we l l and flo oded with po tassium ni tri te, and the result isgiven in No . 6. Fig. 1 co incides with the observations madeby Sir J . Hersche l;
'
on paper srm1larly prepared, in 1 842,
and described in the ‘Phil . Trans .’ for 1843, and he classesthis spe ctrum as due to the silver iodide. I t will be seenthat the prin ted spectrum due to silver iodide is that given
in No . 8, and that the tail extending to the least refrangible
end i s really due to the action ofthat region on the organicsal t (and perhaps chloride) ofsilver present in the paper.Further , i t will be seen that the greater part ofthe darkeningin No . 1 ofthat tai l is due to the action Ofthe different raysafter or whil s t d iffused light has acted or is acting on thatOrganic compo und . Confirmatory experiments were madewi th pure si lver iodide in co llodion, with excess ofsilve rn i trate , and also without such excess, with the result shownin No . 8 .
Thus, then, we may say that the parts ofthe spectrumcapable ofdirect action on s ilver iodide are shown inNo . 6.
In these experiments different acid developers provethick and pro ceed to give the same results as did the alkaline and o rganic iron developers.I t may be necessary to point to the different materials
employed.
Five grains ofcommercial cadmium iodide was disso lvedin an o unce ofco llodion, and this was also used with asilver n i trate sensi ti sing bath . The pyroxylin fo rming the
co llodion was carefully se lected. Before taking into use it
had been precipi tated from so lution by water, washed inalcoho l, again precipitated, and washed and dried, and thenredisso lved in equal parts ofpure ethe r and alcoho l at therate of7grs. to each ounce . Such a so lution after prolonged expo sure when impregnated with nitrate ofsilver
gave no reduction ofthe salt.The emulsions ofsilver iodide were made by disso lving
6 grs. ofs ilve r n i trate in alcoho l, adding this to co llodion,and gently adding the equivalent to 5 grs . ofsilver nitrate of
2 80 Efl'
ect ofSpectrum on Salts ofSilver.
F IG . 93 .
1 . AgI -l-AgNO, on pa r (prin t). 2. AgI on pape r washed from excess ofAgNO, and treated with NO2 (print) 3 . A 9 1 on paper washed
.
from AgNO.
soaked in NaCl, washed from excess and exposed w1th KNO, (pn nt). 4. Paper
282 Ej ect ofSpeetr zim on Solis ofSilver.
and silver iodide fo rmed in i ts place . If, then, a film of
iodide ofs ilver in co llodion (whe ther prepared from an
emulsion or by the bath pro cess) be washed from s ilverni trate , and be then immersed in a weak so lution ofpo tas
sium iodide (i t must not be strong or i t will d isso lve out the
silver iodide from the film)or o ther so luble io dide, i t maybeseen that there will be no thing but s ilver iodide in the film,
all impurities be ing decompo sed. Ifthe film be washedwell with d istilled water, and again immersed in the bath ,or flooded o ver with some sensitiser, such as po tassium n itrite ,sodium sulphi te, beer, pyrogallic acid, &c . , it maybe exposed
wi th the certainty that only pure si lve r iodide i s under examination. I t is necessary to make these remarks, s ince
the whole ofthe util ity ofthe research depends on the use
ofthe pure substance, the co llodion be ing abso lutely inertas regards the silver salt . The s ilver iodide emuls ion madefrom the purified po tassium iodide proved to contain no thingbut the pure iodide, but that prepared with the cadmium
and o ther iodides, as will be seen, proved untrustworthy as
to purity.
One wo rd also as to the neutral or alkaline developer
employed. I t has been customary to state that si lver iodideis unamenable to alkaline development. This is
,however,
no t the case . The ferrous oxalate and the ferrous citroo xalate bring out a distinct image, as does pyrogallic acidand ammon ia, when no restraining iodide i s employed. In
all dry plates prepared with the iodide and o ther silve rhalo ids, the iodide i s deve lopable (though i t give s a weaklyimage compared with that due to o ther salts) by the alkalineor o rganic i ron deve loper.A plate was coated with cadmium iodised co llodion, and
placed in the bath for a couple ofminutes, and expo sed tothe spectrum. The re sult is seen inNo . I O. The development took place by the acid developer. Plates similarlyprepared and washed and then similarly expo sed alsogave as resul ts No . I O. When using ferrous oxalate, the
Efeci on Silver Iodide. 283
cadmium emuls ion also gave the same result. Platescoated with a film ofthe same co llodion, washed, and thenimmersed in a weak so lution ofpo tassium iodide or cad
mium iodide, again washed clean with distilled water, andfinally, treated with silver nitrate, beer, pyrogallic acid,po tass ium n i trite, when developed by the acid or o ther
methods gave the results in No . 8. The purification ofs il
ver iodide by this treatment cut offthe smal l tail on the
least refrangible s ide ofG, seen in No . 10. When the pure
silver iodide prepared by the aid ofthe pure po tassium
iodide was used, No . 9 resulted. A plate was next coatedwith co llodion iodised with the pure po tassium iodide, im
mersed in the bath, washed, and then placed in a so lution
ofcommon salt (I gr. to 5 with the result that figuress imilar to No . 1 1 were obtained.
A plate similarly treated , except that po tass ium bromide
was substituted for the common salt, also gave as a result
No . I I . There was no marked difference whether the plate
was developed by the acid developer or by the ferrous
o xalate. It would be useless to describe the many o ther
experiments wh ich we re made, all tending to prove that thetrue action ofthe spectrum or silver iodide in collodion isthat given in No . 8. No deviation from i t has been obtained,unless impurity in the pyroxyline or in the so luble iodide
was proved to exist.Wi th ge latine emuls ions ofyellow silver iodide, when
rendered sensi tive by the use ofpo tassium n itrite or silvern i trate , the same actionwas found to ho ld good, and the
same may be said for plates prepared with albumen as avehicle, when all silver was converted into iodide, and thesensitising was effected by po tassium n itrite o r some o thersimilar sensi tiser.We next come to the iodide ofsilver when held in situ
by paper. The same method ofpreparationwas adopted asthat given abo ve for the printing experiments. When paperwas exposed with the excess of silver nitrate, on acid
284 Efecl ofSpectrum on Salts ofSilver.
development No . 1 2 was obtained . When developed by ano rganic ferrous developer, No . 1 3 was obtained No . I O was
obtained when similar paper was washed and salted withcommon salt, and washed again, and then sensiti sed withpo tassium n itrite .
Nos. 1 2 and 13 are worthy ofattention. It i s seen in
No . 1 2 that the iodide has much greater power ofattracting
freshly deposi ted silver than have the impurities present withi t in the paper. On the o ther hand, No . I 3 shows that theferrous oxalate deve loper has mo re power ofreducing theimpurity (or rather the reduction i s better seen) than it hasthe iodide .
When silver iodide paper is prepared andwashed, andtreated with a weak so lution ofpo tass ium iodide and re
sensitised by.po tassium n itrite, No . 8 is obtained.
No . I 4 shows the action ofthe spectrum on pure iodidewhen the exposure i s very pro longed I t appears as if
the sensitiveness on the mo re refrangible side of G haddimini shed. This is not the case , however. The pro longedexposure causes a commencement ofwhat is called a reversalofthe image due to oxidation, which will be no ticed furtheron, and the maximum effect has, therefore, apparentlyshifted to the least refrangible side ofG,
as shown. Thisi s impo rtant
,since phenome na which have been described
and figured by o ther investigators can be shown to be causedby this reversing action.
What has been no ted regarding the action ofimpuritiesin the s ilver iodide po ints to a method ofascertaining if
an iodide or iodine i tselfi s pure. I t i s bel ieved that themerest trace ofimpurity may be recogni sed by this methodofspectrum analysis .
Silver B romide.
When paper is immersed in a 10per cent. solution of
po tassium bromide, then dried and floated on a 10per cent;solution ofsilver ni trate, and exposed to the action ofthe
‘
286 Ej ect ofSpectrum on Salts ofSilver .
refrangible side. When the co lour transmi ttedby these three
forms i s taken into account, these differences are to be
expected. Whether the si lver bromides were expo sed witha slight excess ofsilver n itrate, or with a sl ight excess ofso luble bromide, no difference in the spectra resulted.
FIG . 94.
15 AgBr-l-AgNOq , on paper (print) 16. Green AgBr in co llodion with or
W ithout AnIOS (pr in t). 17. Orange AgBr in co llodion gelatine with o r withoutAgNO3 (print). 1 8. Grey AgBr in gelatine (print). 19 . AgBr 011 pa r washedfrom AgNO developed with acid or ferrous citro-oxalate develope r {diveloped}20. Grey AgBr m gelatine . developed alkaline or fe rrous oxalate developed) .2 1 . Orange AgBr in collodion or ge latine
, developed alkaline fe rrous o xalate or aciddeveloper (developed). 22. Green AgBr in collodion, developed ferrous oxalatedeveloped).
We next come to spectra developed on the differentpreparations of silver bromide. No . 19 repre sent s the
action ofthe spectrum on silver bromide paper, preparedas above, which has been washed. Whether deve lo pmenttook place by acid deve loper or byferrous ci tro-o xalate,
’
no
Ej ect on Silver C/iloride. 287
difference was observable. When the paper was washedand treated with po tassium bromide and then expo sed, wehave as a. resul t No . 2 1 .
When a plate i s coated with co llodion containing cadmium bromi de , z inc bromide, o r po tassium brom ide, andplaced in a strong silver nitrate bath
,and developed with
e i the r acid developer on with ferrous citro -oxalate, we getcurve s s imilar to No . 2 1 . The same figure also represents
the action ofthe spectrum o r co llodio -bromide emulsionstransmitt ing o range light by anykind ofdevelopment. This
appl ies equally whether the plate be exposed wet o r dry, or
whether expo sed in the presence ofsilver nitrate or o therinorganic sensi tisers .No . 20shows the results obtained when using gelatine
bromide plates with the silver bromide in the grey mo lecularstate, whether expo sed with an ino rganic sensitiser, or without, and whether developed with an acid, alkaline, or o rganiciron developer,No . 22 represents the action on the blue-green mo lecular
fo rm ofsilver bromide in co llodion, when deve loped and
expo sed under similar circumstances to the preceding case .
I t will be remarked that the direct visible action ofthe
spectrum and the developed image co incide.
The effect ofimpurity in the bromide i s not so marked
as it i s in the iodide. The presence ofiodide except in
minute quanti ties is rare the halo idmo st frequently presentas an impurity be ing the chlo ride. When the spectrum
On the chloride is considered, i t will be seen that such an
impurity is hardly po ssible to be detected, as the spectraimpressed on i t are somewhat similar in general character
to those on the bromide .
Silver Cliloride.
Paper‘
was impregnatedwith a 10 per cent. so lution of
sodium chlo ride and s ens itised on a 10 per cent. so lut ion
ofsilver ni trate . Paper thus prepared was expo sed to the
288 Ej ect ofSpectrum on Salts ofSilver.
spectrum in a damp state, and also in a dry state, and the
visible impression reco rded. No. 23, fig. 95, shows the action .
When the paper was exposed for twenty seconds to diffused
Fro . 95.
23. AgCl-t-AgNO, on paper (prin t) . 24. AgCl
-l-AgNO, on paper, slight preliminary e xposure print) . 25. AgCl on paper washed from exce ss ofAgNO,
(p rint) . 26. AgCl on paper washed and treated withNaCl andwashed again, alsocollodion chlo ride ofsilve r, also yellow form ofAgCl in gelatine (prin t). 27. Greyfo rm ofAgC lin
(ge latine print). 28. AgCl in collodion in presence ofe xcess of
AgNO, or Na 1 develo ped ; ferrous citrate , o r acid development (developed) .29. Yellow fo rm ofAgCl in ge latine acid or ferrous citro-oxalate development
(deve loped) . 30. Grey fo rm ofAgCl in gelatine, ac id, or fe rrous c itro-o xalate de ~velopment (developed) . 3 1 . AgCl in collodion given a short preliminary e xposure ,acid, orferrous c itro-oxalate development (developed) .
light a different curve as shown in No . 24 was fo und ; an
approach -to the ' same curve being also shown wi th veryprolonged expo sure without the pre liminary action oflight.
290 Ej ect ofSpectrum on Salts ofSilver.
exposed'
with an excess ofsilver ni trate, o r with anexcessofthe so luble chloride, unless it be one ofgeneral sensitiveness. In o ther words, the spectrum seemed to act on the
si lver chlo ride in one and the same manner. No . 27shows
the printing action on the chlo ride when enveloped in
gelatine. The emulsion was fo rmed in the usual manne r
habitual amongst pho tographers, each ounce ofemuls ioncontaining abo ut 25 grs . ofconverted silver nitrate. No . 27has reference to this emulsion after it was heated to its bo ilingpo int for halfan hour, and when treated with ammonia ;when used unbo iled it took an impression similar to No . 26 .
When these same preparations ofthe chloride in gelatine
are expo sed for a short time t o the spectrum and developed
With ferrous .citro -oxalate'
developer, or with gallic acidand si lver, we get Nos. 29 and 30, the former express ing theresult of the unbo iled emulsion which transmits yellowo range light, and the latter number that on the bo iled
emulsion which transmitsb lue~grey light.
Silver chlo ride in collodion by whatevermeans prepared,and whether expo sed with an excess ofsi lver n itrate , or an
excess ofso luble chloride, gave No . 28,the fo rmer be ing the
result ofexpo sure ten times longer than that shown by thelatter. The mode ofdevelopment had no effect on the
spectrum developed.
The washed paper gave on development the same re sultas that shown for the direct action oflight
, viz . No . 27.The mode ofdeve lopment had no effect on the result.
The washed paper subsequently treated with a so lutionofsodium chloride and again washed
,when expo sed to the
spectrum gave on deve lopment with e i ther gallic acid; andsilver nitrate, or with ferrous citro -oxalate, the same figureas that obtained by the direct action oflight
,viz . No.26.
When a briefprel iminary expo sure to white l ight wgiven to either the pape r o r the different emulsions
,fig
. 3 1
was obtained on development. On looki
it will be seen that
M ixtures ofHaloid Salts. 29 1
feaclhed between H and11. Acco rding to many autho rs themaximum is near G, whilst, acco rding to o thers , i t is in theu l tra-violet. We carried out about 200experiments on thec hlor ide with sunl ight and
‘
with the electric l ight,and in no
case has it’
been found poss ible to alter the po sition ofthe
maximum .
M t/zzods ofoétainiugIllixtures ofSilver I odideandB romide,
Silver I odide and C/zloride, dy e.
To te st mixtures ofthe iodide and bromide, paper wasprepared by immersing it in a so lution ofpo tassium "iodide
and -po tassium bromide, the proportion ofeach being so
arranged that there should be definite propo rtions betweenthem,
suppo sing that each salt was entirely decompo sedby the silver
,n i trate. Unfortunately this is never abso lutely
the case,‘
and hence the results obtained with the papermust be rece ived with some caution. Chemists
.
know that'
silver bromide or si lver chlo ride canno t exist in the presence
ofa so luble iodide , nor can silver chlo ride in presence ofaso luble bromide; Hence when we have an iodide and
bromide impregnating paper, the silver iodide will first be
fo rmed, and. then the bromide or, again, with iodide and
chlo ride, the si lver iodide will fi rst be formed and then
the Chlo ride ; and, finally, with bromide and chlo ride, thebromide wi l l first be fo rmed and then the chloride .
It was necessary to make these remarks, as a right con
ception ofthe results might not be taken on casually lookingat them.
The same remarks apply with equal fo rce when asens it ive film ofthe double salts 18 prepared by the o rdinarysilver bath, when very short immersion is given to the plate .
The only true way ofobtaining definite results seems to beby means ofseparate emulsions, in which a definite amountofso luble chloride , bromide, or iodide i s fully convertedinto silver chlo ride , bromide, or iodide, and then to mix
these emulsions, after proper washing, in the required pro~u 2
292 Ej ect ofSpectrum on Salts ofSilver.
po rtions . I t was in thi s manner that the emulsions whichwill be discussed presentl
‘
y were preparedWe would here call attention to a somewhat remarkable
behavio ur ofsilver iodide . I t i s we ll known that ifsilveriodide ,
be prepared with en exces s ofso luble iodide, i t i sto tally insensitive to light. Thus ifwe prepare (say) anemulsion m co llodion with an excess ofiodide, and wash i tthoroughly 1n the usual manner, and after redisso lving the
pe llicle resulting from the washing o perations, expo se i t inthe camera
,no amount ofdevelopment will bring out an
image . If, however, to such an emulsion but a drop ofabromide o r chlo ride emuls ion be added sens itiveness willappear. This seems to be due to the last trace ofso lubleiodide be ing converted into silver iodide
Mixtures ofSilver Iodide andB romide.
EqualEquivalent Proportions ofIodide and B romide.
Paper was soaked in a so lution ofequivalentp ropo rtions of
so luble iodides and bromides, and, after drying, was sensi
tised on a 10per cent. so lution ofsilver n itrate for such atime that the back ofthe. paper became tho roughly damp,
The s ilver‘
_nitrate so lution was acidified in o rder to prevent
the fo rmation ofany sub-salts.A strip ofsuch p aper was expo sed to the spe ctrum whi
‘stmo ist, and the printing action no ted. The result is given inNo . 3 2, fig. 9 6 . Similar paper was washed and treated wi thpo tassium nitri te, and expo sed whilst mo ist ; the . effect of
the ac tlon ofthe spectrum i s seen in the same figure .
Paper was next washed, and po rtions were treated witha so lut ion ofpo tassium bromide, and again washed . Stripsof these two specimens were dried and expo sed to the
spectrum,and in bo th cases the printing action i s seen in
No . 3 Similar papers, in a mo i st state, were also expo sedwitho ut any deviation ofthe re sult. Again, paper whichhad been prepared as above was allowed to dry with the
exces s ofsi lver -ni.trate on i t, and exposed , andN0. 33 again
294 Ej ect ofSpectrum on SaltsofSilver.
approximately resulted as also it did when the washedpaper treated with po tassium ni trite was dried.
The difference between curves 3 2 and 33 i s veryremarkable, and at first sight might not seem to admit of
explanat ion. A study ofthe experiments described , how
ever, affo rds a clue to the apparent incongruity ofthe results.
Acco rding to text-books on chemi stry, bromine will displaceiodine in combination, whils t iodide displaces bromide.
Later researches seem to modify the first statement to a
certain extent. Bromine wil l only displace a defin ite propo rtion ofiodine when i t i s
_
in excess ; but for our purpo sewe may take the text-book statement as practically corre ct.When the paper was expo sed wet with ei ther silver nitrateor po tassium n itrite (o ther halogen absorbents gave the sameresult) the iodine and bromine liberated by the action of
l ight would be at once absorbed by them in the one casesilver iodide (or bromide) and silver iodate (or bromate)beingfo rmed, andin the o ther po tassium iodide (or bromide)so that each ofthe two kinds ofsensitive sal t would have itsfull action. When the paper was washed and expo sed in adry,
state the result would be diflerent, and the questionwo uld arise, what would become ofthe io dine and bromineliberated by light ?
Ifsilver iodide be expo sed to light and treated with atrace ofbromine, the sub-iodide combines with the bromine,and al l trace ofthe action oflight is destroyed. Thus whenthe mixture ofiodide and bromide is exposed to light,iodine and bromine being liberated, the bromine will at onecombine with the sub-iodide and destroy it. Thus
,
Agzl Br AgQIBr,
the only factor remaining be ing the sub-bromide, which idevelopable .
_Now i t may be said that the iodine 1
should also destroy the‘
sub-salts but i t isthat, in the presence
'
ofl ight,i t has .no pow
the sub-iodide, since it is immediate ly again
Ej ect on M irtures ofSilve'
r Haloids. 295
the mo lecule.
‘ Iodine can destroy the sub-bromide mo lecule
,andfo rm a new saturated mo lecule thus,
An r 1 An rI.
Whethe r the two mo lecules AgQBrI and AgQIBr. have thesame value i s a moo t po int, but the evidence tends to showthat such is the case . Ifthe equivalents ofbromide and
iodidewere equal , that is, ifthe brom ide and iodide ofsilverwere equally distributed, suppo sing bo th the above actionstook place, the lo cality ofthe Spectrum in which the iodideand bromide are equally sensitive should show an almostentire destruction ofa developable image, and also ofa
printed image.
This lo cal ity i s doubtless about G, andwhen we come
to analyse the curve‘
No . 33 we see that there i s very smal leffect
'
about G, whilst there IS an increased effect between:G and F. Now, to test the matter further, paper preparedwith washed silver bromide was expo sed to l ight ti ll itdarkened tho roughly, and such paper was treated with avery dilute so lution of iodine, and then expo sed in
i
the,
spectrum,with the result given in No . 55, in which i t will beseen that the new mo lecule i s more sens it ive to the greenbetween G and F than above G ; in fact,
‘
we have very li ttleaction comparatively at G and above i t. In thi s case wehave then a paper prepared in which there is an absoluteimitation ofthe action that . takes place in the mixed iodideand bromide. I t canno t be said that by this treatment awe
haveAg212+Ag2Br, , smee the mo lecule fo rmed by light isAn r
,and the addition of the iodine is simply to form
AgoBrI , which 15 very different from a Simple mixture . Thisexperiment then seems to show that this new mo lecule ismore sensitive to the blue -green than it is to the vio le tThe po int then comes as to how, when the o riginal paper is
It must be noted that the iodide is much less sensitive oflightwhen no absorbent ofiodine is present. This is fully
.
accountedfor bythe immediate-recombination of, at all events, a portion ofthe iodineliberatedwith the sub iodide molecule.
’
296 Ej ect ofSpectrum on Salts ofSilver.
exposed to the spectrum,we have not. only afall ofsen si
tiveness at G and beyond it, but also a greater sensitivene s sin the green. Now,
silver iodide, as has already been shown,i s no t in the least sensitive to beyond a ve ry small region
below G ; therefo re, in the green the.
only componen t of
the mixture ofbromide and iodide that can be acted upon
IS the bromide .
'
As we see when bromide is acted upon
one atom of bromine ls l iberated from the mo lecule thus,
An r2= An r Br
The l iberated atom ofbromine immediately attacks themo lecule of iodide in its immediate neighbourhood and
fo rms a newbromo -iodide mo lecule l iberatingiodine . Thus
Br Ag,1, =Agn r -i I,
and the iodine either escapes o r el se forms the mo leculeAggBrI thus
,
AggBr I An rI .
Here then i t is probable that we have a new saturatedmolecule formed by the action oflight, which on fo rmationis susceptible ofbeingacted on by light in its turn. Whe theriodine o r bromine is liberatedfrom this newmo lecule we areno t at present prepared to state, but it i s our belief thatit i s the iodine , since density in development by the alkal inemethod is readily obtained when experimenting with it.
To sum up, the difference in shape be tween curve s 3 2
and 33 seems to depend on the destruction ofthe sub
iodide when fo rmed,and its conversion
‘
into a newmolecule,which is sensitive to the blue -green, the same new mo lecule
“
be ing fo rmed by the l iberation ofthe bromine from the
mo lecule of silver bromide when the sub-bromide i sformed. In the case ofthe paper which is dried in the
presence ofsilver n itrate and po tassium nitrite the sameresult o ccurs . Bromine and iodine attack these
‘
salts whenin a crystalline state with difficulty, and hence will in preferencefoim the new molecules as befo re.
298 Ej ect ofSpectrum on Salts ofSilver.
’
We get the maximum co rresponding wi th the maximum of
pure silver iodide, No . 39 . The same paper developed.
with ferrous citrate shows a slight dip near G, No . 40. The
difference in 39 and 40 i s seemingly due to the fact that
silver iodide has more attractive power for precipitat ingmetallic s ilver than has the bromide (a fact which is wel lknown) and that the bromide i s mo re amenable to reduc
tion than i s the iodide.
No . 4 1 i s well wo rthy ofattention. I t i s the resul t of
the expo sure ofa plate to the spectrum. I t was prepared in .
the s ilver ni trate bath and expo sed in the presence offreesilver nitrate . Taking No . 4 1 alone, it might be suppo sedthat .we h ad a similar case to that which we have recen tlycons idered, since we find an extrao rdinarily (apparent)greater sensitiveness in the green than in the vio let, and yetwe have the image fo rmed in the presence ofan excessofsilver n i trate
,which would be against the theory pro
mulgated. A short expo sure, howeve r, clears up the,dis
crepancy the maximum i s found to be at G, and in thiscase the dip in the curve ofNo . 4 1 i s caused by the reversingaction alluded to .
No . 4 2 gives the curve obtained bythe above mixture ofthreeparts ofiodide to one ofbromide ; when emulsified ingelatine the bo ttom curve shows a sho rt expo sure.
One Part ofI odide and T[tree ofB romide.— We now
come to a mixture ofone part ofs ilve r iodide to three of
bromide. The printed spectra are no t described since theyc orrespond nearly with Nos. 32 and 33 .
Ifwe compare the curves in No s. 43 and 4 1 we see a
strange s imi larity between them, but ifwe take . into con
sideration the curve due to a sho rt expo sure, i t will be(found that the cl ips about G are due to two different causes ;the dip near G i s caused by the fo rmation ofthe new mo lecule. No . 43 is also the c urve s hown by paper preparedwith the above equivalents ofiodide and bromide, and alsoofthe same in co llodion when deve loped by an o rganic
MixtureofIodide and C/zloride. 299
ferrous salt. When developed by acid deve lopment, the
curve in the mo re refrangible region is a l ittle more pronounced in character.No . 44 shows the same equivalents emulsifiedin gelatine
and developed by ferrous oxalate.
The different equivalent proportions of bromide to
iodide, it will be no ticed, show themselves in the curvesmore particularly when a comparison i s made between Nos.
36, 42, and 44.
M'
xtu i e ofI odide and Chloride.
Tlirce Equivalents ofI odide to One ofClitoride. —Whenpaper is prepared with three equivalents ofsilver iodide to
o ne ofchlo ride and washed and dried, or ifexposed in thepresence ofdried si lver ni trate or dried po tassium ni trite,we have the curve shown in No . 45, fig. 97. If, on the o therhand, we have the same paper expo sed mo is t, wi th si lvern itrate or po tass ium n i tri te, we have the curve shown inNo . 46.
The reasoning applied to the mixture ofiodide and bromideapplies with equal fo rce here, the results be ing modifiedfo r the shift ofmaximum ofthe chlo ride which l ies about1 Hit . In N0 45 the mo st refrangible part ofthe spectrumas far as G 15 ruddy, be tween G and F a pink colour, ;and
beyond that grey. Thi s difference in co lo ur indicates (asit does in all o ther pho tographed spectra where differentco lours are impressed or deve loped) a diffe rence ofcom
pound acted upon. Acco rding to our theo ry the mo leculeacted on beyond G in the vio let and ultra-vio let ,would beAg212+AgICl, and between G and E Ag2ICl+Ag2Clzalone. The grey he re i s probably due to the o rganic silvercompound fo rmed in the paper.No . 47shows the same equivalents ifcontained in paper
or collodion, and when expo sed to l ight in the presenceofmo ist silver n i trate or o ther ino rganic sens itiser and
developed by acid o r ferrous citro oxalate developer, the
sl ight modification due to the fo rmer develo per no ted above
300. Ej ect ofSpectrum oli’
Salts ofSilver.
st i ll ho lding good. No . 48 ishows the same paper o r co llodion emulsion washed and deveIOped with ferrous citroo xalate . No . 49 shows the same when emulsified in
Fro . 97.
45. 3AgI+ AgC I+AgNO, on paper, o r dit to washed, bo th dry (pr in t). 46.
flgl+Agu +AgNO3 wet , o r: n gI -t-AgCl-t—KNO2 we t (prin t). 47. 3AG I +AgCIn NO , Or 3AgI + AgCl+KNO, o n pape r, deve loped wrth gallic acid o r fe rrouscrtro-oxalate (develofied) . Washed 3 Agl+ AgCl On paper , ferrous c itro -o xalatede veloper (developed). 49 . 3 Ag
¢l+ AgCl in gelatine , develo ped ferrous oxalate« 50. Agl+AgCl 111 gelatine developed . ferrous oxalate (developed)
st . Agl+ 3 \gCl paper, washed (print) . 52.
‘
AgI-3 QgCH—AgNO , we t (print).
53.AgI+ 3 4gClm gelatme , o r on paper, developed with'
fe rrous citro-o xalate o rac 1d developer (developed) . 54. ac id developer (developed) .55, A
§Br e xpo sed to light treated with I exposed to spectrum (print and alsodevelo ea).
302. Destruction ofthe Photographic Integer
gelatine plates can be obtained asWhen using pure bro’
midealone, the
’
s’
ensitiveness be ing preserved by a shift ofthe
maximum to the green.
M ixtures ofSilver Chloride and B romide.
There 15 no th ing special calling for remark in a mixtureofthese two sensi tive salts. The printed spectrum and the
developed spectrum seem to be a combination of the
spectra impressed on each individually, a slight pro longationtowards the least refrangible end taking place.
M ixture ofSilver I odide, Bromideand Chloride.
When these three salts are combined together we havespectra which are very simi lar to the spectra produced oniodide and chlo ride
,or iodide and bromide , with a pro .
longation towards'
the red
CHAPTER XXXVI.
ON ' THE APPARENT DESTRUCTION OF THE ACTION OFLIGHT ON THE PHOTOGRAPH IC IMAGE.
THERE i s a phenomenon which we may now treat of, sincethe last chapter has described the o rdinary action ofthe
spectrum on the sensitive image. Several years ago1
the writer made experiments on the causes of the
apparent de struction of the pho tographic image, and
later ou2 showed how these causes explained the phe
nomenon ofso larisation, o r reversal of the pho tographicimage. The image on the daguerreo type plate was longknown to be de stroyed by the action of the vapour of
PhilosophicalMagaz ine, June 1878.
11nd , September 1880.
iodine, bromine, or chlorine. In fact i t was no um
commo n th ing in early days for an operator t o re sensit ise adague rreo type plate after exposure and befo re deve lopmentbv the means ofone of these agents, the first image be ingto tally de stroyed. The action in this case is very simple.
We have,
Silver sub-iodide and iodine give silver iodide.
AGzl I Agz lz ,
or,
Silver sub-iodide and bromine give silver iodide and silver bromide .
2Ag21 2Br AgzI2 An rz .
The re are , however, o ther agents which the writer foundde stroyed the pho to graphic 1n1age. It was fo und that all themine ral acids, and bromides and chlorides ofcertain dyadmetals (such as cupric bromide and chloride ) were equally
effective , and that substances which readi ly parted with
oxygen also destroyed the image.
The chemical equations relating to the dyad bromides
and Chlo rides can be we ll expressed thus
Silver sub-bromide and cupric bromide give silver bromide2An r 2CuBr2 2Ag2Br2
and cuprous bromide.2CuB.
W ith the mineral acids the chemical reaction i s no t soeasy to
,
explain. The effect i s probably due.
to the con
version ofthe sub-bromide ofsilver into bromide by the
abstraction ofone atom ofs ilve r. Thus,
Silver sub-bromide Silver bromide and silver.2Ag2Br An r
2 2Ag.
The atom ofme tallic silver is here supposed to be taken upby the acid.
If a sensitive film of silver bromide in a co llodionemu ls ion be placed after expo sure in solutions ofpo tassiumbichromate, po tassium permanganate, hydro xyl, & c .
, i t isfound that the action ofl ight on a plate is also destroyed.
1304 Destruction ofthe Photographic Image.
Experiments showed that -this was due to the oxidation ofthe si lver sub-bromide. This silver salt is unsaturated
,and
bes ides,me tallic silver is capable ofattracting other bodies,ofwhich oxygen i s one. When i t has combined with i t,unless such oxygen can be removed, the image becomesurrdevelopable, as there is no nucleus on which the me tall icsilver precipitated
“by the act‘
ofdevelopment can be de
po sited. Oz one has the same effect as the l iquid so lu tions.Ifa plate which has been expo sed and subjected to
the action ofthese oxidising agents be treated with nascenthydrogen, the image becomes developable, showing that
when the'
oxygen has been removed the sub-salt re turns toits fo rmer act1v1ty.
These preliminary experiments led the writer to invesrigate the action ofl ight on films which had been exposedto white light, and again exposed in so lutions ofmany ofthe se destructive agents in the presence ofwhite l ight andofthe spectrum
,and also when expo sed rn presence ofthe
alkaline halo ids under similar conditions . The que stion tobe so lved was
,whether the action of light aided this destruct ion, and whether i t applied also to the visible image.
Sir‘ John
'
Hersche l has no ted in the ‘Philo sophicalTransactions ’ for 1840 that ifpaper . he prepared with rst,
acetate of lead ; z ud, po tassium iodide ; 3rd, nitrate of
silve r, and be then darkened in the sun, and again be
washed o ve r with po tassium iodide, on re exposure to the
sunthe paper lightens agarn.
The experiment may be tried in a variety'
ofways . The
simplest,p erhaps,is to salt o rdinary ungl
'
az ed pape r with aro -
per-cen t.~-so lution ofcommon sal t, andwhen dry to float
on a so lution of silver n itrate ofabout the same strength;and then to dry and expo se it to the daylight to blacken.
When the blackening is produced, ifthe paper be slightlywashed and then be treated with a 5-per-cent. so lution of
po tassium rodide (slightly acidified with nitric acid) in the
dark,and while still damp be expo sed beneath a negative to
306 Destruction ofthe Photographic Irnage.
tut‘
edfor the iodide, the same resul t obtains. Silver iodide,ifprepared with an excess ofso luble iodide, or if, afterpreparation with excess ofsilver, i t be treated with a so lublebromide , is insensitive to l ight ; and the explanation of
thi s perhaps may be found in the fact already stated .
I t has been usually held that a so luble iodide, such aspo tassium,
can destroy an invisible impression made byradiation ; but this i s no t the case in the case ofmo nad
metals, ifi t be treated with the iodide in the dark. If,how
ever, any iodide ofa dyad, such as cupric orferric iodide, be
employed, which readily l iberates an equivalent ofiodine,the destruction i s accompli shed in the dark. The leastfavourable iodides for such destruction are the monads
Ifa plate prepared with silver iodide have a pre liminary
exposure given i t, and then be expo sed for a considerable
time to the image'
formed in the camera, a reversal oftheimage will take place as before. If
, however, such a plateafter washing, be treated with an aqueous so lution ofpyrogall ic acid , po tassium ni trite , orany o ther deoxidising agent,such reversal ofthe image will not be obtained no r will i tifi t be exposed in a ce l l containing such a substance asbenz ine , or if
.
expo sed in dry hydrogen. From this welearn that
,to obtain reversal , oxygen must be present in
some fo rm or ano ther, and that, ifa substance readily takingup oxygen be in contact with the silver=salt, a reversal canno tbe readily obtained.
An interesting co rroboration ofthe above statement i sto be found in the expo sure ofan already expo sed plate ina ce ll containing a dil ute so lution of permanganate of
potash , bichromate ofpo tash, or hydroxyl, when it will befound that the reversal takes place with the greatest facili ty.
The same reversal s may also be o btained by using any of
the mineral acids in a diluted form.
l
It must , however, be remembered that the solutions must he verydilute , or the whole effect ofthe preliminary exposure will be destroyed,since these o xidisingagents are active in the dark, but act more readilyin the light.
Diference between Gelatine and Collodion . 307
The abo ve experiments show,then,
that'
a reversal maybe obtained by the presence ofthe iodides or bromides (andin a mo re feeble manner, by that ofthe chlo rides), and alsoby oxidising agents and mineral acids ; whilst the presenceofa deoxidis ing agent, or the expo sure ofthe plate in amedium free from oxygen, prevents the occurrence ofthe
phenomenon .
Wi th the bromide of si lve r we have rather differentphase s ofthe phenomenon to consider. The developmentcan be c arried out with the alkaline or the ferrous oxalatedeveloper, a mode which is more easy to carry out than thedevelopment by precrprtatron ofmetall ic silver from an
aqueous so lution ofsilver ni trate . For experimental purpo ses , fi lms containing silver bromide may be ofco l lodion
or ofgelat ine ; but the behaviour ofthe silver salt in thetwo vehicles is somewhat different, and has to be cons ideredseparately. Co l lodion i s, or should be, a strictly neutralsubstance ; that is, i t i s mere ly a medium in the po res ofwhich the silver-salt i s entangled and kept in po sition, andhas no cfi
'
ec t o n the progress ofdevelopment o r'
on the
actionofl ight, beyond that which maybe due to its physicalqualities , its chemical consti tution remaining unchanged.
The micro scope te ll s us that a co llodion film is essen
tially po rous, andfree access ofthe atmo sphere to the silveris thus obtained . Gelatine, on the o ther hand, i s a substance readily acted upon by o xidising agents and by thehalogens ; and consequentlyi t may have an efi
’
ect on the
progress of development and on the action of l ight, itschemical consti tution becoming altered . It is a homo
geneous film, and no t po rous in the‘
ordinary sense ofthe
wo rd, and i s a pro tective agency against the atmo sphere totho se silver-salts which may be embedded in it .
Ifa film containing silver bromide,whether in gelatine
o r co llodion, have a preliminary expo sure given to it, andthen be treated with a so luble
“ bromide ofan alkali, such asofp o tassium
,and be again expo sed to light in the cameras
X 2
308 Destruction ofthe Photographic Image,
it wil l be found that there i s not such a rapid reversal ofthe
image as with the iodide, but that longer expo sure is requiredto effect it, the reason be ing that bromide ofsilver preparedwith a large excess ofso luble bromide i s stil l sens it ive to
l ight. If,therefo re , the light decompo ses the so luble bro ,
mide o n the plate, l iberating enough bromine to fo rm freshbromide ofs ilve r with the sub-bromide fo rmed by the pre
liminary expo sure, that freshly fo rmed bromide, be ing sensitive to light
,is again reduced to the sub-bromide state by
the same rays which fo rmed it. I t will be evident, however,that reversal should take place mo re rapidly with the so lublebromide present than without it and such is the case .
I t i s use less to treat a .silver bromide film with a so luble
iodide , since si lver iodide i s immediately fo rmed , and the
reactions that take place are similar to tho se already described.
Ifb romide of silver in collodion be expo sed to the
camera without the presence ofany o ther substance , areve rsal take s place. Roughly speaking, the reversal takessome sixty time s more expo sure to the light than is requisite
to produce the_maximum o rdinary effect. To i trace the
cause ofthis reversal i t is only nece ssary to treat the filmwith a 5 per cent. so lution
'
ofpo tass ium nitrite, when it willbe found that the reversal does no t take place . The sameho lds true when the film is treated with any deoxidisrngso lut ion, o r ifthe plate be immersed in benz ene or hydrogen.
The cause,then
, ofthe reversal in this case is evidently anoxidation and this may. be further verified by treating .
the film, after a prelrminary expo sure, with bichromate of
po tash , hydro xyl , &c . i t will then be found that the reversaltakes place much more rapidly than when these o xidisingagents were absent. The same may be
'
said ofthe mineral
acids.Ifs ilver bromide be held in a gelatine fi lm, the action
ofl ight is somewhat different, Ifthe plate be expo sed in
the camera for ,a sho rt time, say a few seconds, the image .
3 IO Destruction ofthe'
Photographic Image.
gelatine fi lms with bichromate of po tash shows that'
the
reversing action i s very much increased by the presence of
the oxidis ing agent.Having treated of these reversals of the image in a
general way, it now remains to show which radiat ions are
effective in pro ducing them. For testing this, spectrumpho tography was resorted to , a special dark sl ide havingbeen co nstructed capable of ho lding a cell which would
contain the plate, toge ther with the l iquid, gas or vapourwhose action i t might be des ired to test. Three flint-glassprisms were u sed and the lens ofthe c amera had an equiva
lentfocus ofabo ut 2 feet, the co ll imating lens being ofthe
same fo cus. ‘ The t ime ofexposure was, as a rule , three
minutes to the sunlight or to that ofthe e lectric arc, carebeing takenin the lattercase that an image ofthe po sitive po lefell on the sl i t so as to give a continuous spectrum. The
action ofpo tassium iodide on silver iodide will fi rst bedescribed.
A plate was expo sed after being sensi tised, and afterwashingwas immersed in a cel l containinga r-per
-cent.
so lution ofpo tassium iodide and expo sed to the spectrum.
The result i s shown in No . I the same rays which cause
an image to be formed in the"
usual manner likewise causeda reversal (do tted curve, No . I ).A plate similarly prepared was exposed in a r-per
-cent.so lution ‘
ofpo tassium bromide for the same length oftime,
with the resul t that a reversal was obtained in the blue and
l ikewise in the red,but much less marked in t he latter
(No . The se two experiments tend to prove that, inreality, i t i s the bromide that is acted upon to some extent,and the effect i s n o t entirely due to the silver-sal t . Thiswas particularly manifest in the c ase
'
of the iodide and
bromide sl ightly acidified with a mineral acid, andwasmuchless marked when the so lution was alkal ine— ln the latter
case, the r eve rsal ,taking place in the blue
,and no t in the
red regions ofthe spectrum.
Reversal ofthe Image. 3 1 1
To see ifthe silver-salt had_
anymarked eflect on the
rapidity ofoxidation, a silver-iodide plate was washed, given
the same p re lim inary exposure, and then placed in the
spe ctro -pho tographic apparatus without any surroundingfluid . A reve rsal was obtained in the blue, but no t to any
F10. 98.
Plate showing reversing ac tions ofiodides bromides, and oxidising agents onsilve r iodide .
N .B. The curves below the line show the reversals o r positive images : thecurves above the line show the ordinary ac tion, or negative images ; the ordinatesapproximate ly repre se nt the amount ofac tion.
thing like such an ex tent as when placed in so luble iodideso r bromides. The reversal, therefo re , when the plate is expo sed in the latter is partially due to the action ofradiatio n
on the bromide , andpartly to that e xerted on the silver-salt
itse lf.A
'
silver-iodide plate, t reated as befOre ,’
was next expo sed-in a weak so lution ofpotassium bichromate, when there wasa strong reversal in the red (No . and no action whateve r
m the blue . Permanganate ofpo tash was next substituted
for the bichromate ; and the same reversing action was
found, with the addi tion ofa negative image in the blue
(No .
With,hydroxyl the same phenomena were observed as
wi th the pe rmanganate , the reversal taking place a little
further into the green (No . Studying the abso rption due
to these . three oxidising agents, i t would appear that thereversing action '
is due to the action oflight on the salt of
and not .to the action oflight on the medium in avhich the
Wi th mineral acids a reversal was always obtained in thered andm the blue, a portion ofthe spectrum in the green
andyellow remaining'
unreversed (No . Now the action
ofthe se acids is no t a strictly oxidising action, but i s probably a remo val ofthe loo se atoms of
“
the silver which goesto fo rm the subiodide, and leaving silver iodide behind asthe re sul t ofthe action. The results
‘
ofthe ac tron ofacidsdo no t, therefore, vitiate the above deduction. A plateexpo sed in benz ene or in nitri te of po tash showed no
reversal even with a very pro longed expo sure . I t shouldheremarked that the action o fpermanganate and bichromateofpo tash when very feeble i s sometimes to give feeblenegative images in the red and blue in lieu of positiveimages also po sitive images in the blue , andfeeble negativeimages in the red (see do tted curves in Nos. 4 and Butthis i s to be accounted for by the fact that the dilution of
these oxidising agents is so extreme that the reduc in'
g'
action
on the unaltered sensitive salt is far greater than the
rapidity ofthe oxidation:
3 14 Destruction ofthe Photographic Image.
so l uble bromide. A reference to Nos. 7and 8 will show'
that
t his is the case; but that at'
the same time the features whichare so marked with the action ofbromide on silver iodideare present . No . 7i s the curves due to silver bromide inco llodion which had rece ived a preliminary expo sure and
was then exposed m a 5 per cent. so lution ofacid po tassiumbromide . I t will be seen that the curves in Nos. 7and 2
are similar,showing that the principal action is due to light
acting o n the so luble bromide in the pre sence ofan acid .
No . 8 i s a simi lar plate expo sed in an alkaline so lution of
KBr, in which there is a modification ofthe curve. The
last loop'
i s probably due to the silver sub-bromide i tself,since the
"o xidation ofthis salt by o xidising agents o ccupies
approximately the same po sition (see NoNo . 9 shows the effect ofpermanganate ofpo tash ; and
when i t is compared with No t o , which rs the curve due too xidation by bichromate ofpo tash, it will be manifest thatthe chiefoxidising action lres m the red and ultra redofthespectrum.
No . 1 1 also shows the effect ofbichromate ofpo tash on
s ilver bromide after a p rel iminary e xposure, the plate inthis case be ing a gelatine plate. I t will be seen that thebichromate to tally arrests all action in the blue
,whilst it
rapidly causes a reversal in the red.
No . 1 2 shows the effect ofmineral acids on silverbromide , by which i t will be seen that a maximum of
reve rsal takes place in the red and in the blue. As befo restated in regard to the iodide
, the ac tiono fthese acids canscarcely be regarded as an action ofoxidation.
No . I 3 shows the -phenomena du e to over-exposure of
s ilver bromide, by which it Wil l be seen that reversal takesplace in the blue and not in the red. Comparingthis withNo s. 7, I I , and 1 2, the effect ofextraneous matter in causs
inga reve rsal i s verymarked.
Co llodion plates expo sed in benz ene, or m aqueousso lutions ofpyrogallic acid, po tass ium nitrite, and 'sodiuni
sulphite gavé no reversal whatever.
Orthochromatic Photography . 3 I 5
Gelat ine plates expo sed in benz ene gave the phenomena
shown in No . 1 3 , whilst with the o ther media no reversal at
all was obtained.
The explanation ofthe apparent contradiction shown by
the behaviour ofa gelatine plate exposed in benz ene hasalready been given.
The act ions ofmany o ther l iquids and gases 1 have l ikewise been tried but it was thought that the examples givensufficed, s ince they all po inted to the same conclusions
,
which may be summarised as fo llowsrst The reversal ofan image i s due, in the majoritv of
case s, to the oxidation ofthe subsalt ofsi lver which fo rmedby the fi rst impact ofl ight on the expo sed salt ofsilver.
z ud. The oxidation i s due to the action ,oflight, therays oflowe r
,refrangibility be ing the most powerful accele
rators ofoxidation.
3rd. Reversal ofan 1mage maybe due to the presence of
any halo id ofan alkal i, the reversal in this case being partly
due to the action oflight on such a halo id, andipartly due
to the tendency to oxidation ofthe subsalt ofs ilver
4th. The presence ofa mineral acid tends powerfully tocause a reversal.
CHAPTER XXXVII .
ORTHOCHROMATIC PHOTOGRAPHY.
WE now come to the.
effect ofc ertain dyes on sensitive salts
ofs ilver, but as the salt to which the application rs mo stgenerally made is the bromide, the remarks that will bemade wil l be principally applied to i t.
In 1 873 , Dr. H. W. Voge l , ofBerlin, found that the addition ofcertain dyes, such as co rall in, magdala red, gavemo re than one , or changed the po sition ofthe, maximum of
Oz one was most marked in its o xidisingproperties, andgave acurve very similar to No . 12 bo th with the iodide andbromide ofsilver.,
‘
3 16 0rt_ _
hochromatic Photography .
sens itiveness in the spectruml Voge l experimented wi th theco llodion processes, since at that date the newer ge latine process was no t wo rkable . The application to ge lat ine plates
,
however, was anatural outcome ofthis, andwho firs t appliedit is do ubtful. Vogel, Eder, and the wri te r all experimented
in this direction. Ede r, however, carried out a very extens ive serie s ofexperiments with differen t dye s, and gave avery comple te discussion ofeach . As a matter ,offact
, the
two typical dyes which are at present mo st employed are
eosin and cyanin blue .
The fo l lowingfigure 1 will give an idea ofthe relative
Fro . 100.
No . 1 is the effec t ofHoffman '
s vio let, gentian viole t , me thyl viole t and acidvio le t. No 2 shows the effec t ofbenzaldehyde green, e thyl green ,
meythlgreen , ando ther Varie t ie s. No . 3 is iodine green. No . 4 , cyanin blue .
'
No . 5 is eosin and themembe rs ofthe eosin group. No 6, rose Bengal and ammonia. No . 7, coeru le rn.
No . 8, chrysanilin. No . 9 , eo sin and silver chloride . No . 10, eosin and cyanrn.
Taken from a paper readbefore the Society ofChemical Industry,June 30, 1887, by C. H. Bothamly, and is a repro~duction from Eder’s paper.
3 18 Ort/Zoc/zromatz'
c P/zotogmp/zy .
Afterwards the plates are dipped in a bath made as below
Erythrosin or eosin (t ofwater) 25 partsAmmonia 4 partsDistilledwater I75 parts.
They are kept in this for 1 m inute to 1% minute, and are
drained on a po rous tile and dried in the dark.
The addition ofa few drops ofa saturated so lu tion of
cyanin blue to the above carries the sens itiveness ofthe
plate further into the red ofthe spectrum (see NO . 10 fo r
fig. a nd was the combination firs t made, he believes, by'
the write r in 1882.
As to the practical use ofthe addition ofthe se variousdyes something must be said.
Ifwe take'
sunlight and pass i t through one o r two
prisms and view 1t on the screen, we find that the yellowregion i s by far the brightest, and the blue and vio le t regionthe feeblest in luminosi ty. Yet i t i s these latter regionswhich have the principal effect in pho tography o n o rdinaryplate s. The fo llowing curve ofthe lumino sity ofthe spec
F IG . I OI .
trum ofsunlight is taken from a paper by the writer whichappeared in the ‘Philo sophical Transactions of the Royal
3 19
«Society for 1887, and i s for a sun ofmedium altitude. The
he ight ofthe o rdinate measures the lumino sity at any partienlar part ofthe curve. From this it will be seen that the greatest
effect on the eyes i s produced by the rays which lie be tweenthe green and the yellow. If
, therefo re, a pho tograph can
be taken-with some body which is mo re sensitive to thi s partofthe spectrum than to any o ther, manifestly it will givea truer effect ofthe i llumination ofthe object seen than if
the blue rays are the active ones . Thus in a yellow floweror in a sunse t sky there i s very li ttle blue, but a great deal of
yellow, the latter being very luminous. Ifthese two oh
jec tsb'
e
'
photographed, one with a compound which i s mostimpressed by the yellow, and again with one which is mo stsensitive to the blue, i t is evident that the fo rmer will givea mo re truthful monochrome print than the latter.
For an exact representation in mono chrome , perfecttruth can only be attained when the curve ofsensi tiveness
of the compound to the spectrum fo l lows the curve of
lumino sity ofthe spectrum, and at present such a compoundhas no t been, nor, ifan opinion may be expressed, will iteve r be , found, but an approximation made be made by an
artifice .
Suppose we look through yellow glass at objects l ightedby sunlight
, the bluew ill be nearly entire ly cut offand the
vio le t entirely, but the general effect ofa co loured landscape
o r drawing, so viewed, will have the same general effect of
i llumination though the co lours themselve s will“al l be
ye l lowe r. Ifwe take the i llumination ofthe spectrum t rans
mitted through ye llow glass it wil l be ofthe character shown
in fig. 101 in the lowest curve, from which it will be seen thatthe place ofmaximum illumination i s very little changed,though the vio let and blue are absent . Ifwe attempted to
take a photograph through such a yellowglass on a wet plate
we shoirldget no effe ct, though ifweused a ge latine-bromideplate , which is sensitive to the green, an impression would
be‘ made, but the predominating co lour value wou ld be
«320 Orifice/aromatic
in the green. If, however, we employ a plate dyed with
cyanin, erythro sin (one ofthe eo sin group), or a mixtureofthe two
,. a reference to fig. 100will show that a strong
effect will be made by the green and the yellow, so that apho tograph may be taken through a yellow glass which willfairly repre sent in monochrome the illuminative value ofthedifferent co lours.
Ifa landscape be taken with these dyed plates (whichhave been named isochromatic or o rtho chromatic ) withoutyellow glass intervening there is very l ittle differencebetween the result a nd that obtained on an o rdinary plate
,
the reason being that the objects reflect a large proportionofwhite l ight
,and. that the blue light in this reflected light
is that which is mo st effective (see fig. If, however, a
yellow glass be placed in front ofthe lens, the pho tographwill differ considerably in appearance. The blue ofthe sky
,fo r
instance, in the resulting print , will be rendered a grey insteadofwhite, whilst a white cloud will be white . The greens ofthe trees and grass willbe mo re harmonious, and so will anyye llow object which may be in the View. Mr. B . J . Edwardshas intro duced a very neat arrangement for introducing ayellow screen between the o bject and the plate . With adoublet -lens , where Waterhouse stops are used, he fastensbetween two thin cards
,in which is cut a circular aper
ture, a thin sheet of ge latine dyed with aurine o r someother similar co lour. The film i s so thin and even that noserious deviation takes place in the rays ofl ight passingthrough the lens, when such -a card take s the place ofthe
stop. The writer adopted the plan usually recommended ofplacing in front ofthe lens a tpiece ofo range glass,
" but only
one piece out ofsome twenty-was sufficiently true in its sur
face to be capable ofbeing util ised, whereas by'
employingMr. Edwards’s plan no difficulty was experienced.
For copying co loured pictures , as already hinted,\ these
o rtho chromatic plates are excessive ly valuable , alwayS'
takingthe precaution ofusingsome yellow medium as
'
iridicated
322 n /ztfor too Dar/é Room,
the localities ofabsorption and chemical action in the spectrum is exact, and does no t fo llow a quasi law enunciated byKundt, which can scarcely, as yet, be fully accepted, andwhich wo uld have made only a general co incidence betweenabsorption and chemical action po ssible.
Thus, in every case ofchemical action taking place by thespectrum, it may be laid down that the body acted uponabsorbs in the same localities. This is a law which mus"fiolo
’
gooo'
, since it i s part ofthe law ofthe conservation of
energy.
CHAPTER XXXVI I I .
LIGHT FOR THE DARK ROOM.
THE two previous chapters will have paved the way for[consideration ofthe l ight which is admi ssible in the so
called dark room.
A reference”
to fig. 94m Chapter XXXV. will show that agelatino -bromide plate is sensitive a very long way towardsthe red, starting from the blue ofthe spectrum.
The accompanying diagram taken from ano ther wo rk 1
by the autho r will i llustrate the kind oflight which comesthrough glasses ofdifferent kinds, together with the lightwhich passes through certain dyes . No . 9 is added, as atone time there was an idea that a so lution ofquinine cut off
all chemically effective light. The diagram at once dispo sesof this . Looking carefully at Nos. z o
'
and 2 1 , in fig.
’
94,
i t will be seen that the most efficient l ight is that coming at
the extreme red ofthe spectrum. No 2, fig. 102 , shows that
(
rubyglass would be an exce llent l ight fi lter were the rays
in the blue absent. A combination ofruby glass NO 2 and
No . 6 (stained red) accomplishes this and leave s the portio n
in the red alone . Stained red by itselfi s excellent, and
Instru ction in Pbo/ogmplzy . Piper Carter.
Lag/at zkrouglz Ruby Glass 323
Stained _red glass by itse lf. allows the passage ofmo re light
altoge ther than does the ruby glass.
will show the proportion oflight whY 2
.The annexed diagramich .passes. The areas
324 n fitfor Mo
.
DomeRoom?
ofthe curves give the intensities ofthe l ights.“
That ofthenaked l ight was 2639, that transmi tted thro ugh orange glass268, and that through ruby glass o nly 1 15. In o ther wo rds
,
stained red glass only .-al lows T16 .
of the to tal l ight to
pass,but ruby glass (medium) only allows abou t 15
13
.
When the light is dull,as in winter, i t i s evident that such
a diminution oflight is so great that there will no t be
sufficient fo r the proper manipulation ofplates duringdevelopment, and recourse must then be had to artificiallight, ofwhich we shal l presently speak.
FIG. 103 .
Co loured glass, however, i s not an e ssential paper when
ofa proper co lour and stained throughout answers equallywell, though it cuts offmore ofthe original light . Thus acombinat ion ofchryso idine-dyed paper-andmagen ta-stainedpaper, Nos.
’
3 and4 , fig . 102, will be efficient. T he two dyes
should no t be mixed, but two sheets ofpape r, one dyed withthe one andthe o ther with the o ther, shouldbe superimpo sed .
The'
worstfeature in aniline dyes, however, is thei r certainty offading in white l ight . The best medium ofal l is the common
o range paper which is usedfor packing purpo ses . Two thick »
326 Aotz'
nomolry.
is one foo t off, film ofa candle will not visibly affect it in
the sho rt time necessary to place the plate in the deve lopingdish and pour over the deve loper. Whenthe developer haswe tted the plate it becomes comparatively insensitive.
When e lectric lighting is available for the dark room the
lamp may be surrounded with an o range paper,and i s the
mo st charming light to wo rk with.
For plate s which are prepared with erythro sin a deepred light alone should be used a combination ofruby glassand o range paper maybe used in this case .
CHAPTER XXXIX.
ACTINOMETRY.
AMoriGST the earliest methods ofcomparing the chemicalenergy ofdifferent lights i s that known as Bunsen and Ros
coe’s, its value having been o riginally po inted out by Professor Drape r, ofNew Yo rk. The process i s dependent onthe fact that combination takes place between hydrogen andchlo rine when the mixed gases are expo sed to the ac tion of
light. The two gases may be evo lved by the electro lysis ofhydrochlo ric acid, and then they are in the right propo rtionsfor recombination. Such a mixture ofgases , when expo sedto sunl ight, combines with explo s ive vio lence, though in diffused light the recombination takes place gradually, and inpropo rtion to the intensity ofthe light, and to the time duringwhich they are expo sed to i t. This affords a me thod of
securing a registration ofthe intensity of the light, forthe hydro chlo ric acid fo rmed may be co llected in water,and the amo unt may be estimated by various chemicalmeans. As for general use this method did not proveal together satisfacto ry, and Bunsen andRoscoe abandoned
it for one which will be described in detail. Professor Drape r
Action ofn /zt on Ferric Oxalate. 327
had also po inted out that ferric oxalate when exposed'
to
l ight gives out carbonic acid, and, in 1 859, Mr. H . Draper,ofDublin, turned this fact to practical use by e laborating
a system ofwhich the fo llowing is an o utline .
The chemical reaction on which the method is founded
is this
Ferric o xalate Ferrous o xalate Carbonic anhydride.Fe
23C20, FeC.O, 2C02.
The light vibrations are able to spl it up the ferric oxalatemo lecules, andfor each mo lecule so shaken, one mo leculeofcarbonic anhydride i s liberated. Mr. H. Draper’s apparatus consisted of500grains ofa standard so lution offerrico xalate held in a glass cistern, rendered opaque by japanning
, the light be ing admitted by leaving unco vered one
square inch ofthe cistern. After expo sure to the l ight forany desired time the amount ofcarbonic anhydride disengaged was known by the difference in we ight befo re and
after expo sure, the lo ss du e to evapo ration be ing checkedby comparison with a similar cistern containing distilledwater. There are one or two objections to be no ted as
regards the accuracy ofthi s method. The ferric oxalatebe ing a co loured solu tion, i t i s uncertain to what depth thelight penetrates into i t, and it has yet to be proved that with
equal intensities oflight acting for the same time through
the same aperture, double the same amount of chemicaldecompo sition i s produced after passing through two units
ofthickness, as i s produced after passing through‘
one . In
all apparatus ofthis kind,too , the surface reflection has to
be taken into consideration, as also the material ofwhich
the transparen t parts ofthe cistern is constructed. Ifwe
couldbe assu red that the value ofthe ultra-vio le t: rays increased in the same ratio as the blue rays, the apparatuswould suffice, but we have reason to think that th is is not
the case. Hence this construction must be taken as yie ld
ingan approx imation to the true results rather than as the
328
true results themselves.-We consider that the best results
would probably ,be attained by throwing certain definite
po rtions ofthe- spectrum on some
_mediurn,
and noticing
the results ofeach. This would give a true idea ofthe
relative amounts ofphotographic energy existing. in eachpo rtion. . It canno t be too deeply impressed on the s tudentthat these pro cesse s do not measure the actual energy in theimpinging rays oflight , this 15 altogether a differentmatter,into which we canno t enter here .
Professo rs Bunsen and Ro scoe conce ived the fi rst
practicable method of measuring the actinism of day ,
light and sunlight,by the expo sure of sensitive silver
chlo ride paper to the ir action for certain lengths oftime .
After an e labo rate investigation, they came to the conclu
sion‘that equal quantit ies ofthe intensity ofl ight into the
time ofinso lation (exposure) co rrespond, within verywidelimits, to equal shades ofdarkness producedon chlo ride0fsilver paper ofunifo rm sensitiveness .’ Starting with thisidea theycarried out a labo rious research into the preparation ofa paper that should be unifo rmly sensitive, andwhich
might, therefo re , be considered as a paper of standards .en51t1veness The fo llowing IS a sho rt részcme
’
ofthei rwo rk ;Choo sing sodium chloride as the so luble chlo ride, theyfound, rst, that a paper will not give unifo rm results whensimply floated on the
'
solution ; but that it must be iminersed in it 2nd. That the stronger the so lution the
greater sensitiveness would be given to the paper. It was,therefore, necessary to fix some reasonable l imit to the
strength, and this they fixed at a 3 per cent . so lut ion. 31d,
that using the sensitising so lution of silver nitrate above agreater strength than 6 per cent. gave no difference ln the
results as regards sensitiveness , but that below that strength
It rapidly diminished. 4th. That the presence ofthe salt
resulting from the decompo sition of the sodium chlo rideand s ilve r n itrate had no effect on the sensitiveness, and
that . at o rdinary temperature and mo isture the sensitised
330 Actz'
nome'
try .
Mr. Jordan but practically it i s found that the openingnecessary for the production ofa readable t int in full sunshine, with paper passing slowly befo re it, i s so smal l thatthere are mechanical difficulties in the way ofsecuringaccuracy ; and when i t is considered that one end ofthe
aperture would have to be at least 100 times the width of
the o ther, the impo ssibil ity ofobtaining a proper gradationwith a slip ofpaper ofreasonable width is apparent. To meetthis difficulty a m ost ingenious instrument was devised byProfessor Roscoe, in which expo sure takes place for certainregulated times, at fixed intervals, during every hour.
The registration i s effected by the following contrivance
(fig. 104)F 1G. 104.
A long strip ofpaper 15 ro lled upon F,and fastened to D .
In the latter is a clockwo rk arrangement, the escapemen t, B,being placed as shown . To move the clockwo rk, the armature ofan electro -magnet takes the place ofa pendulum,
and every time i t is attracted and re leased by the magnet ,a too th ofthe whee l is re leased, and the paper i s moveda small piece fo rward acro ss the weak spring, E, which i sseen on the top ofD. The use ofthe spring is to cause thepaper to be in contact with a
'
circular aperture ofabout i
Standard Tint. 33 I
inch in diame ter, left 1n the cover ofthe instrument, andthrough which the expo sure i s given.
-The result oftheexposure 18 thus to leave circles ofmore or less b lackness
(the blackness varying acco rding to the intensity ofthe
light and the time ofexpo sure) at intervals along the stripofpaper.
The different expo sures were given by a toothed whee l
annexed to a clo ck, which gave the necessary con tacts forfixed times to move the armature in fig. 104.
This instrument has been superseded by aform in which
the paper is sensitised in a sheet and wrapped round a
cylinder attached to clo ckwo rk . The cylinder moves roundat intervals every hour by the clockwo rk, andwhen a com
plete revo lution has taken place i t shifts laterally, so that a
fresh piece ofcyl inder i s brought under the aperture.
The fo llowingis the standard tint ofgreyadopted by
Fro . 105.
Ro scoe. He took parts ofz inc oxide and 1 part oflamp-black, and ground them tho roughly togethe r to sucha po int that no further grinding altered the tint. This he
found the mo st convenient tint for comparison and, whencarefully gummed on to paper, i t was unaltered in shade.
This mixture then gave the shade from which all his
measurements were made. all other tints being referred
to i t.
332, Aétlnoznetry.
'
T0obtain agraduated" shade he applied what
.
i s known
as the pendulum apparatus, which in . general outline con
s ists ofa pendulum swmgingin front ofsensitised paper insuch a manneras-t o give a:gradation ofexpo sure to it, anda .consequent variation in -t1nt. At each po int of.the paper
the t ime ofexposure was known, and the po int was then
found answering to "the standard tint, and the re lative valuesofthe other,
~
por.tions ofthe gradations calculated.
I t may pe rhaps be found necessary he reafter to apply acorrection to . tho se readings taken in sun light
,as it maybe
found that the different integrations ofthe spectra fo rmedby. sky, cloud, and sunlight ppoduce slightly. different effects.
“
.Ano ther method ofsecuring uniformity in measurementhas been employed by the writer. I t consists ofa -rapidlyrevo lving cylinder,
'
0r drum,B,
’
on which is attached a series
of.black andwhite s ecto rs , as in the diagram. A convenientlength for this drum has been found to be 6 inches. To
the cyl inder, B , is fixed a small pulley firmly attached to one
end, over which is passed a co rd communicating with the
wheel, A. These are ofsuch relative dimensions that thecylinder ro tates at least 15 times in a second, when A i scaused to ro tate but once. Along the top, and nearlytouching the cylinder, i s a blackened brass support, 0, witha slo t in i t
, o n each s ide ofwhich is a scale ofinches,
dividing the length of6 inches into 1 20 parts , that is, eachinch into 20parts. Monochromatic l ight is thrown verticallydownwards, on the scale , and any tint to be compared isbrought o n to the scale, andmoved till an exactly identicalshade is f0und on the ro tat ing cylinder. A series
‘
of6 readings is taken
,beginning bymoving the t int from white to
black, and next from black to white . It will nearly alwaysbe found that this is necessary, as the readings in one casewould be as much too high as in the o ther they wouldbe .
to o low. A mean of. the six gives very nearly the truth.
The accompanying diagram gives the _ results ofthe reading
ofa s trip'
ofpaper which had been exposed beneath an,
334 Actz'
nometry .
the gradation as seen by the eye . The fo llowing figure
shows the results ofmeasurements obtained by the diam
phanometer,l from plates . to which different expo sures have
been given behind the ro tating whee l. A, B, and c are the
curves from o rdinary wet-plate negatives. The o rdinates
measure the amoun t oftransparency, I be ing total trans
parency, and0 to tal Opacity the abscissae deno te the rela
tive times ofexposure , or what is approximately equ ivalent
to it, the relative intensitie s oflight acting on the negative,suppo sing Ro scoe ’s l-aw referred to above to ho ld good. It
FIG . 107.
will be. seen that the curves have very nearly, that is, withinthe l imits ofe rro r ofobservation, the same fo rm. Thus,takingthe expo sure ofA equal to °
5 and 1 , or 1 to 2,the
co rresponding transparenc1es are“
77and Taking thesame transparencies ofB
,the time s ofexpo sure are 3 4 to
"66, or ve ry nearly 1 to 2. The same will be found with Cthe do tted curves
,E and D
,show a portion ofthe negatives,
B and c , intens ified in the o rdinary manner, and the samere lation ,to expo sure still ho lds good. This is an importantpo int, as i t shows that the same relative intensities oflight
are maintained in a negative as the opacity is increased.
Spurge’
s Sensitometer. 335
The ordinates to the chain-do tted straight l ines show the
transparency that should result ifpho tography gave perfect
gradatio ns. It will be seen that the tendency in all negativesis to cause a lo ss ofgradation in the deep shadows as well
as in the l ights. This acco unts for the lo ss ofdetail that is
always seen in the extreme tints ofa pho tograph . It is alsoworthy ofremark that a thin negative seems to give a bettergradat ion than one intensified.
It must be distinctly understood that the above curvesapply to negatives only under one class of development.Unde r o thers the curves would show considerablevariations.
Amongst the mo st striking would be the fo rm they take
when near the parts in which to tal transpare’
ncy‘
is repre
sen ted A mo re detailed account ofthese will be found' in
the Photographic News for July andAugust, 1877,
F16 . x08.
An excellent instrument for making inve stigations on
the den sity ofdepo sit of negatives and for the darkeningofdifferent papers is what is known as Spurge ’s Sens1tometer.
In fig. 108 c represents the appearance ofthe top and B.that ofthe bo ttom ofthe instrument. Each ofthe variousholes shown in c has a fixed ratio ofr to 29 with the
335 Ad z'
nohzei’rx, i
f
,
néXt'
h’
o le..
‘
Thu‘
s the amount of“
l ight a dmi tted to the plateor paper-i s halved every third ho le .
.f employing such an instrument almo st every variation
in the action oflight may be studied.
’
For estimating thedifferent optical densities ofdepo s it, the writer has adopted the
fo llowing plan, which gives very great facili ty ofmeasurement.
F IG.‘
109 .
The l ight, whatever it'
maybe, i s placed at A a lensL at distance ofits equivalent focus, in this case 9 inches.
Thenegative, N, the varying thickness ofdepo si t mwhichit is desired to measure, i s placed in front ofthis lens, andano ther lens
,L,throws the 1mage on the screen 5, in front
ofwhich is a rod, R, whose shadow is cast by the l ightcoming through
This is the o rdinary optical lantern fo rm ofapparatus . At
_
one side, at a conv-enient distance, amirror, M,is placed, with
the angle so adjusted 1n az imut h that i t reflects the lightfrom A o ver the patch illuminated by the lens 1m. Thisnaturally throws ano ther shadow ofthe rod alongside the
first shadow. The screen, 5, may be transparent o r opaque,whichever i s deemed best. Where the shadows ofthe rod
fall, a square mask is cut out to enable the two shades to beviewed witho ut distraction to the eyes by glare from adjacentparts. I t will be seen that, as the l ight and the reflectedbeam are stat ionary, the method ofvarymgdistance canno tbe adopted to vary the intensity ofthe light. To obtainthe necessary variation, revo lving sectors a re employed.
338 Actz'
fl ometry .
ofa gelatine plate expo sed behind Spurge ’s sensitometer and
deve loped with ferrous oxalate andmeasured in this instrus
ment.Putting this table ano ther way, i t means that the l ight
in passing through the various parts is dimmished by so
many 177ths, thus
No . 1 square allows is; ofthe to tal light to pass ;
96
177
The film and glass 13333,or
“
? ofthe“light nearly.
and so on through the who le scale.
The following fi gure_
shows the c’
urve‘
ofdensi ty ofthesame negative for intensity oflight
"
increasing“
in arithmeticalprogression.
F1G. xu .
Sometimes it i s convenient to fo rm a screen ofvaryingthickness
,such as Warnerke
’
s, and this may be done by
CelestialPkotogmp/zy. 339
taking a un iformly coated plate and exposingadjacent portions ofit through a square aperture to a feeble light, sayofa 10-candle gas flame 10 feet off, commencing with
10seconds’
exposure, and go ing up next to then to 16,
next to 20,next to 25, next to 32, then to 40, and so on,
doubling the exposure every third hole. The developmentshould take place with ferrous oxalate carefully neutralised,to which are added 2 or 3 drops per ounce ofa 4 per cent .so lut ion ofpo tassium bromide . The development shouldbe carried sufficiently far to render the mo st expo sed squarenearly opaque, and then i t should be optically measured as
described above. It has been found that the depo si t givenby ferrous-oxalate development i s practically black, and thati t cuts offthe visual and the pho tographic rays in exactlythe same propo rtion.
Such an instrument is excessively handywhen Spurge’s
sensi tometer is absent, and the results can be equally re
l ied upon.
CHAPTER XL.
CELESTIAL PHOTOGRAPHY.
PHYSICISTS have turned pho tography to account in their
study ofthe heavenly bodies, mo st ofwhich, in one way or
ano ther, have been made to 1mpress their image on sensitiveplate s. The student who may take a landscape with the sun
shin ing direct into the lens will soon satisfy himselfthat the
expo sure necessary to obtain a good p ho tograph ofour
luminary, when unclouded, i s very small, so sho rt, indeed,that so larisation is frequently induced, though the landscape
i tselfmay be capable ofproper development. Taken with
the o rdinary camera and lens a pho tograph ofthe sun i s prac
tically useless, s ince a lens ofsho rt fo cus is only capable of
giving a very smal l image, and one on which none ofthe
2 2
340 CelestialP/zologmp/zy.
markings which characterise hissurface can be seen, even
wi th the aid ofa magn ifier. Since the prime object ofso larpho tography is to enable the surface ofthe sun to be
stud ied, i t is evident that o ther means must be adopted ino rder that i t may be del ineated on a sufficiently large scale.
A lens or. object-glass gives an image ofthe sun ofa
Fxc . 1 12.
diame ter ofabout { 5 ofan inch to each foo t ofits focallength. It is
,therefo re, evident that in o rder to secure a
pho tograph ofit of4 inches (about 10 centimetres) diame ter, the lens employed must have about 40 feet focallength . Now
, 4 inches has been proved by experience to be
abo ut the least diameter for a so lar image in which sunspo tscan be effectually s tudied. Betore the introduction of
342 Celestial Pkelegrap/zy.
the movement ofthe clo ck . H H are cords wo rking on the
movable arm, to which B i s attached a vertical adj ustmen tcan therefo re be given to the reflected beam.
The fo llowing method can be employed for obtaining aso lar image with the lenses ofvery long fo cus by the aid of
the sidero stat . The lens with its tube i s placed in a po sitionsuch that the direction ofthe i r axes cuts approximately thecentre ofthe mirro r. Since the mirro r i s supposed to be aperfect plane , i t is manife st that an image ofthe sun shouldbe fo rmed at the principal focus ofthe lens, as perfect as ifthe axis ofthe lens i tselfwere po inted to the luminary. I t
i s needle ss to describe the camera,which, in fact, instead of
being attached to the tube, may remain detached so long asthe plane ofthe sensitive plate i s kept accurately perpendicular to the ax is ofthe lens, and so long as all light, exceptthat admi tted through the lens, be excluded. This is
,
perhaps, be tter than rigidly attaching it to the body ofthetube, as i t gives facili tie s for expo sing the plate very close to
the principal fo cus. I t has been considered mo st impo rtantthat such a po sition for the expo sure should be obtained .
The reason ofthis will be evident when it i s remembered
that the only means ofgiving the exposure is by causing an
opaque screen,in which a sli t i s cut , to pass acro ss the beam
ofl ight. Were such a screen passed in front ofthe lens,
o r at any part of the telescope o ther than the principal
fo cus, the impression ofthe image might continue during
the enti re exposure . When the expo sure, however, takes
place at the principal fo cus ofthe lens , during each portion
ofthe exposure a definite po rtion ofthe image alone isimpressed. To secure good detail in the representation of
the sun ’
s surface such a method ofimpressing the image i snecessary, s ince, however excellent may be the workmanship ofan instrument, there i s always some small tremo r
in the movements, and consequently a risk ofan imperfect ion in the image . There i s much to be said in favour ofthis method of solar photography, and something to be
Pkoto-lzelz’
ografilz. 343
said against i t, and i t seems a po int which has yet to bedecided as to whether this o r the plan next to be describedis like ly to give the mo s t accurate results .A le ss unwie ldy instrument which was first adopted for
solar pho tography was one designed byDe laRue, andknownas the Pho to -he liograph . The accompanying figure shows
the late st pattern, and is taken from one oftho se which waslate ly employed by the expeditions for observing the transitofVenus . At a is a lens ofabo ut 4 feetfocus, having a ce llOn which is cut a ve ry fine screw, so fine and accurate,indeed, that the lens can be caused to advance or recedefrom B by the fi
lo—
ath part Ofan inch by turning the cellthro ugh a portion ofa turn. Aboutfi s the principal fo cusofthe lens, at which po int are placed cro ss wires o r a ruledgrating the fo cus ofwhich can be accuratelyobtained byaslow-mo tion screw turned by the handle, H. This moves
an inner tube in which the diaphragm ho lding the wires isinserted. Immediate ly in front ofj §and running in a pair
Ofgrooves, i s the expo sing screen, in which there i s an ad
justable Opening or sl it. Atgi s a spiral spring, whi ch tendsto keep the sl it below the po int where the image i s fo rmed,whilst at e i s a l ittle pulley, over which runs a threadattached to the top ofthe exposing diaphragm,
and terminating by a loop. The prel iminaries to expo sure are to
draw the diaphragm up to e by the thread, and then to place
the loop over a pin (no t shown in the figure) this brings the
sli t above the place where the image is fo rmed. The expo
sure is given by cutting the thread the spring, g, pulls the
diaphragm towards it, and the sl it traverses the image . The
duration ofexpo sure can be regulated between 91—oth and
“fifth part ofa second,’ a margin sufficiently wide to sui t the
sun as seen through almo st any condition Ofthe atmo sphere .
Below fis placed a magnifying lens, which take s thefo rm known as ‘
the rapid rectilinear. ’ Its function i s the
same as that ofan eye-
piece in a tele scope, and by altering
the distance between i ts Optical centre and the fo cus ofthe
346 Celestial Paotegrapky.
It -seems that in the earl iest days Of the discovery of
pho tography by Daguerre impressions of the so lar imagewere made
,and it would require a somewhat long list to
record the names Oftho se who have successfully adaptedthe art to astronomical purpo ses. Fo r the registration Of
the phenomena connected with the to tal eclipses ofthe sunthe same diffi culties as to the names ofthe wo rkers wouldarise . The fi rst recorded endeavour to employ pho tographyfor this wo rk dates back to 1 851 , when Berkowsky Obtaineda daguerreo type ofthe so lar prominences during the to taleclipse . From that date nearly every to tal so lar eclipse, theObservation Ofwhich was po ssible to
_
European observers,has been studied by its aid, and has tended to the so lutionofsome ofthe problems which arose concerning the so larphysics . In 1860 the first regularly planned attack on the
problem by means ofpho tography was made by De la Rueand Secchi
,and in subsequent eclipses it has been con
unued.
As regards photographing the corona the general opinionseems to be that i t is better to employ an o rdinary pho tographic lens ofa focal length ofsome 1 80 cent imetres withthe camera moun ted equatorially, than to employ the o rdinary te lescopic objective. The co ronal light during the
eclipse i s faint , andin o rder to get full effect it was necessarythat the ratio ofthe aperture to the focal length should beas great as po ssible. Since the advent ofthe new sensitiveprocesses it i s now po ssible to use an enlarged image, suchas given by the pho toheliograph, though up to the presentthis instrument has no t been employed with pe rfect succe ssduring eclipse s . In the future
,however, mu ch maybe hoped
for by its employment.Lunar pho tography has o ccupied the attent ion Ofvario us
phys ici sts from time to t ime, andwhen Daguerre’s proce ss
was firs t enunciated, Arago proposed that the lunar surfaceshould be studied by means ofthe pho tographically produced images. In 1840, Dr. Draper succeeded in impress
De la Rae’
s Lunar l logmpks.
inga daguerreo type plate with a lunar image, by the aid of
a 5- inch telescope. The earliest lunar pho tographs, how
ever, shown in England were due to Professo r Bond, oftheUni ted States. These he exhibited at the Great Exhibi tion
of1851 . Dancer, the Optician,OfManchester, was, perhaps,
the fi rst Engl ishman who secured lunar images, bu t theywere ofsmall siz e . Afte r these might be mentioned manyname s, but it i s unnecessary to .refer to any befo re that
ofCrookes, who took the next step in the matter. The
instrument that Crookes employed was an 8 - inch refrac to r,be longing to the Liverpo o l Observato ry, which had a focallength ofabout 1 23 feet. The diameter ofthe moon was
therefore about 5 c entimetres . Crookes affixed at smal lcame ra to the telescope and fo cussed the actin ic rays bytrial
,there being found a great deviation between the i r fo cus
and that Ofthe visual rays. The mo tion ’
ofthe moon no t
be ing capable ofbeing fo llowed in the telescope by means
of the o rdinary equato rial arrangement driven by clo ck-1
wo rk, the necessary accuracy was Obtained by mechan ically
fo l lowing it by means ofthe slow-mo tion screws attached
to the declination and right ascension circle s . The crosswires in the finder were kept on one po int ofthe image Of
the lunar surface, a highly magn ifying powe r being u sed in
the eyepiece . Crookes found that with different te lescopes
the necessary expo sure varied between 4 seconds and 6
minutes .In 1852, De la Rue began expe rimenting in lunar
pho tography. He employed a reflecto r ofsome 10 feet
foc al length, and about 13 inches diameter. An abstract Ofa paper read befo re the British Association appeared inthe Briti sh Journal OfPho tography. ’ In it i s given a ve rycomplete account ofthe me thods he adopted.
In the first part ofthe paper De la Rue po ints out thatifthe image ofa bright star i s allowed to traverse a pho tographic plate, the result i s no t, as one would expect, a
straight line, but one which is broken up and disturbed, and
348 r
i
. Celeslial Ph logmplzy.
wh1ch consi sts ofan immense number Ofpo ints crowdedtogether in some parts, and scattered in o thers . Thesedisturbance s being due to our atmo sphere, it fo llows that ifthe te lescope be made to fo llow the mo tion ofa heavenlybody, an expo sure o ther than instan taneous must, to a greaterOr less extent, render every po int ofi t a confused disc, andthat
,therefo re, a pho tographic image will neve r be so
pe rfect as the Optical image given by the same te lescopeuntil instantane i ty be secured.
No twithstanding, however, the di sadvan tages underwhich a pho tographer labours , I have Obtained picture s ofceles tial objects showing details which o ccupy a Space lessthan two seconds in each dimension. I might
,I think, say
even one second. . Now 1 second film ofan inch on the
co llodion plate, a second on the lunar surface, at the moon’s
mean distance, be ing about 1 mile. The lunar picture inthe focus ofmy telescope i s abo ut 1 1
111 inch diameter, but
this van es ofcourse with the distance ofour satelli te fromthe earth:
De la Rue then stated that he considered a magnifyingarrangement attached to the telescope as impracticable to
secure go o d pictures, owing to the increase ofexpo sure thatwould be nece ssary, and the consequent defects due to
atmo sphe ric disturbances. He considered that the enlargement o ught to take place after the negative i s taken.
He then de scribes the adjustment ofthe mo tion Ofhistelescope to the lunar mo tion,
which he effected by altering
the length ofa conical pendulum or friction go verno r, whichal tered the time Ofits ro tation (or double beat). He propo sed to effect the same alteration by ano ther plan,
\
which
he subsequently adopted.
De la Rue at first Obtained his lunar pictures in his1 3
- inch reflecto r, by placing the sens itised plate at the side
ofthe tube Oppo site the diagonal reflecto r, the l ight being
thus twice reflected. Subsequently he obtained pictures
directly at the fo cus ofthe mirror, which did no t give him
350 CelestialP/zotegrapAy.
rapid bromide emulsion plates developed by the alkal inemethod wil l furnish pictures which are equal to those pro
duced by. the wet method as described above, and certainlygive a great decrease in expo sure.
Mr. Rutherfurd at a later date having tried an r ig-inchrefractor ofthe ordinary type, and also a 13
-inch reflecto r,
finally constructed a refracting telescope in which correc
t ion was made only for. the chemical rays, andwith this ih
strument he has produced some ofthe finest pictures ofthe
moon which have ever been taken. With the great Mel
bourne reflector, however, pho tographs which are ,nearly
perfection have been obtained, and there seems even yet tobe a balance ofOpinion in favour Ofthe reflector as againstthe refracto r for this kind ofwork. Undoubtedly, whereabso lute co incidence offoci ofall rays can be secured, all
o ther conditions being the same , the best pho tographsought to be Obtained. In lunar pho tography an unfavourable condition ofthe atmo sphere i s undoubtedly the greatestdifficul ty to be encountered. In a climate like England theair i s rarely steady enough for the purpose. In countrie swhich are mo re favourably situated as regards hygrometricconditions the difficulty is much reduced. In 1 874 5,whilst the writer was in Egypt, Sir A. Campbell and himselfhad umopportunity Oftaking some lunar pho tographswith a refracting telescope of7-inch aperture belongingto Mr. W. Spo ttiswoode. On the nights that the experi
ments we re made . really exce llent negatives were obtained,which bo re enlarging to 1 2- inch diame ter. The apparatu semployed was extemporised
, and therefo re ofa rather rudedescript ion, but quite sufficiently true to give an idea oftheexcel lent pictures that might be taken in such a climatewith the appliances usually adopted for such wo rk .
The pho tography ofthe plane ts has not as yet yieldedanyth ing ofgreat value . Mr. Common and o thers
,howeve r,
have Obtained excellent pictures, though necessarily smal l,ofJupiter, Saturn and some o thers . NO doubt before long
P/zotegrap/zs ofNebula ; 351,
there will be a stride taken in this branch ofpho tography
which will render its un iversal adoption by astronomers a.
necessity.
In the earlier editions ofthis wo rk it was stated, Pho tographing the s tars i s mo re a feat ofpho tography than of
practical u tility in the present state ofour knowledge,
though at some future time it may be po ssible to map theheavens mo re thoroughly by its aid than has at present beendone .
’The prophecy which was then made is be ing amply
fulfilled. The Bro thers Henri Ofthe Paris Observato ry inFrance, Mr. Common and o thers in England
,and Mr. Gi ll
at the Cape ofGood Hope have not only shown that stellarpho tography is some thing mo re than a feat that i t is themost ready means ofmapping the heavens, and ofasce r
taining the star magni tudes in a way which was befo reimpo ssible . There i s a divergence ofopinion as to whe thera reflecto r or a refracto r is the better instrument to employ
for this purpose our own opinion tends to the use ofthe
fo rmer. By means ofpho tography stars which“
the eye
canno t see in the telescope have been made to impress theirimages on a sensitive plate , since, with pho tography, time isa function which . i s as important as the brightnes s ofastar.Mr. Isaac Roberts has used pho tographs Ofthe midnight
sky to discover the po sition ofmino r planets . During the
exposure Ofa plate the telescope accurately fo llows themo tion ofthe stars. The apparent mo tion ofthe mino rplanets differs sl ightly from that ofthe stars, hence, whilethe latter leave small po ints on the plate on development,the former leave a linear track. This Observer, too , haspho tographed nebulae with which astronomers were un
acquainted,showing that our knowledge in this direction
may be increased by the use ofthe sensitive plate .
Regarding the pho tography ofnebulae there i s a classicseries ofpho tographs by Mr. Common ofthe great nebulaeinOrion, a series which is not only interesting but impo rtant,
352. Pkotograp/cy wit/t zlic M icroscope.
as i t will be a standard : by which to compare o thers takenin future years.
Comets have been pho tographed andevenmeteo rites, sothat in the scientific study ofthe heavens it canno t bedoubted that pho tography will play an even more important
part in the future than the eye has done in the past.
CHAPTER XLI.
PHOTOGRAPHY WITH THE MICROSCOPE.
PHOTOGRAPHY from a very early period of its existencehas been utilised for securing accurate drawings in mono
chrome ofwhat the eye can_
see in the microscope. Thisbranch '
Of the art is excessive ly fascinating, and can be
wo rked in any leisure moments , e ither by day o r, night,
when the enlargement is limi ted to say 50 diame ters ;but in o rder to secure image s ofgreater dimensions i t isalways advisable to employ sun-light The apparatusrequired is no t very extensive. An o rdinary micro scopewithsay ’
f-inch and 1 - inch object ives and .an A eyepiece i sall that is necessary as far as the instrument itselfis ‘
con
cerned. Ifthe Objects to be pho tographed are mounted on
a sl ide,and not merely placed in a ce ll for examination,
any o rdinary camera may be attached to the micro scope, asthe tube can then be brought into a ho ri z ontal po s ition. It
has Often been recommended to employ a camera as muchas 6 feet in length, in o rder to secure great increase in the
siz e Ofthe object, but in the writer’s experience i t is unwise
to go beyond 18 inches, a length j ust sufficient to enable
the operato r to grasp the slow-mo t ion fo cussing screw, whils this eye can be directed to the fo cussing-screen. When thelonger camera is employed, the operation offocuss ing has to
354 Photograp/zy wit/c tlce M icroscope.
should also the camera ; and care should be taken that theaxis ofthe tube fi xed in the latter should be in the exactcontinuation ofthe axis ofthe lens. This can only be effectedby very careful arrangements . As a rule it will be foundthat when the body of the micro scope i s in a ho riz ontalpo sition the friction on the axis on which it turns issufficien t to cause it to remain in the po sition in whichit i s placed ; ifnot, Obvious precautions must be taken to
prevent anymo vement be tween the time offocussing and
exposing the plate . Supposing that sunlight is to be em
ployed for the purpose Ofilluminating the object, the nextOperation i s to throw the image Ofthe sun by a con
denser on“
the object, in such a manner that the axis Of
the condenser and that ofthe Obj ective may be in a linewith o ne ano ther. This may readily be ascertained byno ticing the illumination when no Object intervenesbetween the rays emerging from the condenser. I t isadvisable, first ofall, however, to place the helio stat (the
one de scribed at p. 268 answers the purpo se) in po si tion.
This can be done with sufficient accuracy by rough Obser
vation with the eye, and no ting that the centre ofthe mirrori s about the same he ight, and in the same horiz ontalline as the tube ofthe micro s cope. The condenser isthen brought into the reflected rays, and an 1mage Ofthe
'
sun brought to a focus on the object. In some cases the heatrays have to be cut off
, o therwise inj ury to it ensues. A
glass cel l wi th parallel s ides containmgwel l -bo iled distilledwater i s found to subdue the heat sufficiently when placedin the
‘
path ofthe beam. The fo cussing is now proceededwith, and is best performed by removing the o rdinary
"
ground glass,and substituting for i t a plate ofordinary
patent plate,viewing the image by a focussing glass, as
described in pho to -spectro scopy, page 266. The po rtion
ofthe Object to be photographed. shouldbe brought into
the centre ofthe field, and when nearly in po sition the'
slide should be clipped on to the stage by a couple ofwire
M onoc/cromatic‘
n /zt. 55
springs,and the adj ustment effected in the usual manner.
The abso lute focuss ing should next be taken in hand. A
rough approximation i s first Obtained by the rack and
pinion mo tion, and the final fo cus Obtainedfiby the slow
mo tion screw, which is attached to every good micro scope .
When viewing the image through the focussingglass it willbe found that in no po sition is the Object quite free fromco lour. In One focus it will appear sharply defined , thoughsurro unded by a redband, whilst the definit ion wil l appear
equally good when in a different fo cus, when surrounded bya blue halo . These colours are due to a want ofachromatism ih the Objective, and theformer position should bechosen to Obtain a sharp pho tograph for since the blue raysare as a rule the most active in causing the pho tographicimage to be formed, it i s evident, ifthe latter focus, whichis mo st accurate for the red rays, be chosen, the resul tingpicture will be blurred.
M onoc/zromaticLiglzt.— The fact that co loured fringes aresure to bo rder the image shows at once that the Obje ctivei s not properly corrected, and there would evidently be an
advantage were it possible to wo rk with monochromatic l ight.This can be accomplished in the fo llowing manner. In fig.
I 14 R R are rays coming from a he l io stat or o ther source of
light andan image ofthe source (or in the case Ofan art ificiallight
,an image ofthe condenser) is formed on the sl it s, of
the "co ll imator c by the lens The parallel rays producedby the lens L2 pass through the two prisms P1 and P2, andarefo cussed -by a lens L3 on to the screen D
, in which is a sl i t 82,fo rming a spectrum on D. The rays which pass through 52are . again co llected by a lens L4, and an 1mage ofthe surfaceofthe second prism focussed on the Object on the stage of
themicro scope. This givesmono chromatic l ight, andacco rdingto the kind Ofplate used for pho tographing the object
,
so is the slit Sgtmoved into that part ofthe spectrum to
which . ir i s most sensitive. L 2, may all be ofabo ut 9inches focal length, and L
4considerably shorter. There isA A Z
356 P/zotograp/zy wit/z t/ze“
M icroscope.
no need to have these lenses'
achromatic ordinary spectaclelenses may be made to answer.
F IG. 1 14.
The photographic del ineation ‘
ofopaque‘
Objects is muchmo re difficult to accomplish than
.
that Oftho se which can beexamined by transmitted light. The difficulties will befound to increase rapidly ifany endeavour be made to use
a higher power than a finch. The same arrangemen t as ~
that indicated may be employed, causmg, however, the raysto fall on the object. This give s a very feeble illumination,and with great magn ifying power the difficulty offocussingis excessive. When once a . fo cus is obtamed all difficultyvanishes, and by the use ofdry plat es any amount ofexposure may be given without any de terioration:ofthe image.
The annexed figure shows an arrangement by which ami cro scope maybe employedin its o rdina
‘
ry vertical po sition.
The instrument was in the Loan Co llection of'
Scientific ’
Instruments at South Kensington, and i s ofGerman make.
The form ofthe cameramight advantageouslybe altered to a
t
335-8 P/zotograp/zy wit/z t/ze .Microscope.
very great enlargement i s not required, now that plates
are Obtained which are so sensitive as tho se prepared by
the ge latine process ; and no doubt the majo rity ofm icroscopic objec ts are at present photographed with one ofthe sesource s ofil lumination.
The use ofpho tography with the microscope-has no t as
yet been fully developed, but there can be no doubt that asmore workers enter into this field the greater will be the
advances made.
Names.
A luminiumAntimonyA rsenicBariumBismuthBoronBromine
‘
CadmiumCaesiumCalciumCarbonC eriumChlorineChromiumCobaltCopperDidymiumErbiumFluorineG luciniumGoldHydrogenIndium .
Iodine
IridiumIronLanthanumLead
LithiumMagnesiumManganeseMercury
A PPEN D I X.
LIST OF ELEMENTS.
Symbols.Al
Sb
A s
Ba
MolybdenumNickelNiobiumNitrogenOsmiumOxygenPalladium
360 Appena’za’.
L IST OF ELEMENTS— c ontinued.
Symbols.
COMPARISON OF THE METRICAL WITH THECOMMON MEASU RES.
FROM‘
DR.
‘
WARREN DE LA'
RU E’S TABLES.
MEASURES OF LENGTH .
In English Inches I II e t
MillimetreCentimetre
,
Decime treMetreDecametreHectometreKilometreMyriome tre
I Inch 2°
539954 Centimetres.1 Foo t 3
'
Q479449 Decimetres.
000328090032809003 280899
39‘
37079 3‘
2808992
393‘
70790 32 8089920
39 37‘07900 3280899200
39370‘
79000 32808 992000
393707‘
90000 32808 9 920000
1 Yard Metre.
1 Mile L‘6093 I49 'Kilometre .
I ND EX.
ABE
BERRATION ,Spherical, 209
Abso rbents, use of, for chlorine,&c . , 22 , 25
Absorption oflight and consequent work,10
Spectra ofdyes and glasses, 323Ace tic acid in the deve loper, 67
-69necessity of, in the calo type sensi
rising so lu tion , 141
Achromatism, 207Acid, hypochlorous,fo rmation of, byaction
oflight on silver chlo ride , 22hypobromous , fo rmation of, by ac tion
Oflight on silver bromide , 26pyrogallic , employment of, in the alka
lme dev loper, 92
gallic , employment of, in the alkalinedeveloper, 95
Ac tinome ters, Bunsen and Roscoe '
s, 326,
328
Draper’s, 326, 327Ac tinome try, 326
Ac tion ofsolvents employed in collodion,4s
Albert’s process, 203Albumen, use of, fo r paper positives, 1 45beer pro c ess, 1 10plates, defec ts in, 1 12
substratum,100
Albuminising paper, 1 54Alcohol in the developer , 69Alkali, effec t of, on glass , 57
on grease , 57Alkaline deve lopers , strengths of. 97
fo rmulaefo r, 108development
, 9 1— 100
comparison of the , with the acid
me thod, 98effect of, in sho rtening expo sure , 99
Alkalinity, efi'
ec t of, on collodion , 52
A-nilme process, 186
Antimoniure 'ted hydrogen and sulphur,ac tion oflight on, 34
Aperture ofa lens, 221double t lens, method offinding
the correct, 224
CHI.
Apparatus, 225Archer
’
s employment ofcollodion, 5Asce rtaining the adj ustments ofthe darkslide
,232
Astigmatism,2 1 2
Auto type process, 179
BACKING for pos1t1ves on glass , 9 1
to revent blurring , 88Bath, sensitising, fo r negatives , 58
dry plates , 102
positives, 89Bath holder, travellmg, 79Be echey
'
5 process, 1 16
Bitumen ofJudaza, ac tion oflight on, 2 , 33Blurring, o r irradiation , 86
ALCIU M chloride in the toning bath,Catlo ty
l
pe
8
roces‘
s,1 37
brus fo r use 1n the , 1 39
paper to be used 1n the , 1 38sensitising the pape r in the , 1 39
Camera, 225carte-de visite 227exposing sens1tive paper inthe , 23 1foc ussmg the pic ture in the , 255for the microscope , 352front , 225, 226Meagher’
s , 265
pantasc opic , 228revolutio n ofthe , ro und the optical
centre ofthe lens, 228reversing back, 227ect roseopic, 265
Car 11 print , development ofthe , 1 83 , 184.process , single transfe r, 1 85tissue , manufacture of
,179
se nsitising, 181
elestial pho tography, 339Changing box (Hare 5) fo r dry pro cess, 230Charles
'
5 c laims to Wedgwo od pro cess, 2Chlo ride m an emulsion , 1 14Chlo rine and hydrogen, ac tion oflight on
a mix ture of, 33
CHR
Chromic acidand alcohol , 32Chromium salts, ac tion oflight on, 3 1
printing with, 174Cleanmgglass plates, 55, 77Coating the plate with c ollodion, 78Co llod1o -chloride process, 162
emulsion , washed, 163fuming, with ammonia, 164fixing, 164
Collodion , .12
ac tion ofsolvents employednin, 49and india rubbe r as a support, 35coating the plate w1th, 78considered as a vehicle for sensitive
salts, 37discussion as to z
u
odzsers in, 51
effect ofalkalis _1n
, 52
for albumen bee r pracess, 1 10fo r dry plates , 101for “
po sitives, 101
formula for plain , 50formu lmfo r bromised. 1 19
—,formula3 for negatwe 53 , 54limpid and visc ous, 55re ticulation m , 55testing plain, 54
Colloidal bodies, addition of, to developers , 67
Colour anddensity ofthe deposit, 82Copper bromide
O
intensifying solution,theory ofthe 7sulphate 1n e
7vel0pers, 67
Corona, photographing the , 346
Cyanide , po tassium, as a fixing agent, 74
DAGUERRE’
Sdiscoveries, 3Daguerrean image
,deve lopment of
the , 40intensifying the’
, 41‘
fixing, 4 1
Daguerreo type 38
e tching by Grove ’
5 method, 42
plate , manipulations in the preparationofthe , 39Daguerreotyp es, reproduction of, by electro typy, 41
Dallmeyer, his po rtrait lens, 21 6Dark-room, 239Dark-tent , 236Defec ts in gum gallic plates, 109
silver prints, 162Defects in negativeBlack specks, 85Blurring, 86Dark lines, 1 37F0 ,
’
85, 1 36
Fri ling, ’ 1 36Markings like wateredsilk, 86Pin holes, 85, 1 37Scum on the film, 85Spo ts, 85, 137Transparent markings, 86Want ofsharpness 1n the 1mage , 86
EDG
O
I
O
NG, o r substratum fordry plates
Emulsion,experiments with bromide, 1 1 3
fo rming an, 1 16, 1 25, 1 26gelatino -citro -chloride , 164processes , 1 1
- 1 28, 165, 289Emulsions, was ed, 1 18
Defec ts in negativesWeak images, 85Yellow o r brown stain, 1 37
De la Ru e’
s lunar pho tographs, 348Density and colou r
_
ofa deposit , 82Deposition ofsitver by the develop e r, 64De tergents, formula for, 57Develope r, explanation of. the term,
2 3
ferrous oxalate , 109 , 339Developers absorb ox gen, 64
alkaline , formulaestrength of, 97
copper sulphate in, 67ferrous salts m , 99fo rmulae for dry plate , 109, 1 1 1for negative pic tu res, fo rmulae fo r, 67for positive pic tures,fo rmula: fo r, 90nitric acid 111, 96restraining ac tion ofsoluble bro rmde 1195theory ofstrong andweak, 65v iscous, 66
Development, alkaline , 9 1 —100agents employedm , 99effec t of, in shortening exposu re, 99
methods of,e xplained, 18
ofalbumen beer process, 1 1 1_ofdry plates, 9 1
-100
ofgelatino bromide plates, 13 1ofgum gallic plates, 109of1ron prints, 167ofthe calo type picture , 140ofthe carbon print , 1 83ofthe pho tographic image, 63ofwe t plates, 80strength ofsolu tions for dry plates, 97
Diamond’
s,Dr. Hugh, connec tion with th
collodion proc ess, 5Diaphragms, uses of, ‘ 209 , 2 19 , 220Dichromates and o rganic matter, 3 1Diffractio n gratings
, pho tographywith, 27spectrum,
276Dippers, 79Disc ofconfusion, 222
Dispersion oflight byy prisms, 287Distortion caused by Single lenses 2 1 1
Draining rack, 84box, 84
Dry-plate or alkaline development, 9 1-10
—processes With the bath, 100- 1 12Drying cupboard, 1 29Dryin
'g the plate , 104washed emulsion
,1 21
p ,lates 1 23Dyedplates, 320
366 Index.
INT‘
Intensifying the 1mage , 69solut1ons, fo rmulae fo r, 71
-73stains due to , 83
Intensity given by varnish, 76Iodine and sodium hyposulphite , 152j odz
'
ser s to be used in collodion, discussionas to the , 51
I odz’
s z'
ngsolutions for the calotype process,1 38
Iron, printing with salts of, 166Irradiation, 87
OHNSON, his improvements in thecarbon process, 177
ATENT image , explanation of theterm,
1 8
Le Gray, his waxed paper proc ess, 142Lens, aperture ofa, 221
Dallmeyer’
s po rtrait, 2 16intensityofa,
22 1
Petzval s po rtrait, 216rapid rect1linear, 2 17tr1plet, 2 18
wide angle rec tilinear, 218Lenses, forms of, 2 1 1 , 2 12
landscape , 2 17, 234on the choice of, 234
Light, action of, on various compounds, 2 1antimoniure tted hydrogen and
sulphur, 34chlo rine andhydrogen, 33chromium salts, 31
ferric oxalate , 327ferric salts, 29o rganic bodies, 33
salts ofsilver, 27silver bromide andsub-bromide ,
261 93 1 941 284chlo rlde
'
,2 1
,287
iodide , 23, 277u ranic salts, 30vanadic salts
, 30
apparent dest ruc tion ofthe action of, onthe pho tographic image , 302- 3 15art1ficial, 357kind and prop01
' tion of, passing throughglass ofvarious colours, 322passingthrough coloured papers, 324reflec t1on of, from po lished me tal sur
‘
faces, 275suitable for the dark-room, 322
-326
Lithographic press, 192 .
Lunar photography, 347
1\AGN IFYING lenses, use of, forfocussing, 266
Manipulations in gelatine -bromide process,1 25
— 1 30
in the preparation of photographictransfers, 190—195
EBUL/E. pho tographs of, 351.
Negat1ve , intensifying the , 69 , 81Negat1ves , over-exposed and under-ex
posed, 8 1—
.
defec ts in, 85Niepce (Nicephore de) , his process, 2N1tr1c ac 1d, action ofanhydrous, on o rganimatter, 43
‘
PHO
Manipulations in wet plate photography , 7Mercuric chloride intensifier, 70, 72, 1 35
theory ofthe , 70Metals and alloys, light reflec ted frorpolished surfaces of, 275M e thods ofdevelopment explained, 1 8
in alcohol, 75Me trical measures, equivalents of, ic ommon measures, 360Microscope , came ra fo r the, 352pho tography with the , 352
Microscopic examination ofa developeimage , 66Monochromatic li htfor themicrosc ope , 35Moon, pho tograpgs ofthe, 347
ON the picture , 239Op tical centre ofa lens, 21 2
Orthochromatic pho tography, 3 15-322plates, 320
Oscillation, efl'
ect ofwell-timed applicatioofforce on an, 17upon another oscillation, effect ofone.
14Oxygen absorbers asdevelopers, 64
pAPER, album1n1s1ng, 154
as a support and substratum, 35for callo type pro cess, 1 38, 142fo r pho to-hthographic transfers, 1 90negatives, 1 37plain salted, 155resinised, 1 55sensitive , exposing 1n the camera,
2 3 1
Papers, c oloured, amount and quality ,
01light passing through, 324Papyro tint process, 196Papyro ty
pe process, 195
Pho to—co l o type processes, 201process by Albert, 203
Pho to-engraving in half-tone, 199and reliefpro cesses, 196Pho togenic drawings, 4Pho to-heliograph, 343, 346Pho to -lithograph1e transfers, 1 90Pho to -lithography in half-tone , 1 95Pho to-spec troscopic arrangements, 261Photo -spec troscopy, 259
absorption ofSpectrum rays by differenttransparent solids, .274camera for, 265diffrac tion spectrum, 276
I ndex.
PHO
Photo-spectroscopy, gratings for, 276Herschel's experiments in, 260, 261materials for prisms, and lenses 274Picture , printing the, 156
on the , 239Plahe ts, pho tography ofthe , 351Plate , coating the , with collodion, 78drying the, 104, 1 29glass, cleaning the , 57, 77Plate-cleaning solution, fo rmula for, 57Plates, drying cupboardfor, 1 29dyed, 320
fo rmula for making sensitive to yellowand gre en, 3 17for platino type process, 171 , 174me thod ofstowing, 1 30orthochromatic , 320Platino type proc ess, 171
developing the print, 173me thod ofsiz ing the paper, 171paper for, 174sensitising solu tions, 172
Points, the nearest, ofa landscape infocus,223Poitevin, his improvements on Ponton’
s
process, 6
process, 169Ponton (Mungo ), his dichromate process, 5Po sitive
_
and negative , example ofa, picture , 4
pic tures and their support, 35by the we t process, 89
produc tion of, from a ositive , 168
Po tassium cyanide as a xing agent, 74andnitric acid, dangerous effects of, 57
Powder process, 187Preservative , definition of, 103
applying the , 103Printmg, manipulations in s1lver, 153frames, 1 56sensitising ba'ths for silver, 155the pic ture , 156Prints, silver , fixing, 159washing silver, 1 60Prismatic dispersion oflight, 206-208Prisms, method ofplacing, at the angle ofminimum deviation,
263Pure water for washing dry plates, the
necessity of, 102Pyroxiline , manufacture of, 45
efl'
ects of high temperatures in the
manufacture of, 45effec t ofdiluted acids in the manufacture of, 44fo rmula for preparation of, 45Warne rke , his formula fo r preparationof, 48
RACK fo r dryingplates, 1 29Reade , h1s discovery ofthe use ofgallic acid, 5
Red end ofthe spectrum, photographs ofthe, 267
357.
SOD
Reflectors, coincidence ofvisual andchem1cal rays, 350Refraction, laws of, 205Registration oftints, 33 1 , 332
ap aratus for, 332Reliefbloc s by photography, 199Resinised pape r, 1 55Re ticulation 1n collodion, 55Ritter, his researches, 1_Room, the dark, 239Roscoe , his actinometer, 326Rouch, his dark tent, 236Rules to be observed in choosing a landscape subj ect, 254Rutherford, his lunar photographs, 350
ALMON andGarnier's process, 168Scheele, his researches, 1
Sensit1sed paper, preservation of, 1 56Sensitising bath, 58-62, 89, 102, 155, 169,
172, 1 81for dry plates, 102formulae fo r, 61keeping in o rde r the, 62over-iodised, 62
the paper fo r the calo type process, 1 39Sensitive compounds, theory of; 1 1Sensitiveness of salts ofsilver preparedfrom double decomposit on ofvarious
‘
me tallic salts, 52
Sensitome ter, Spurge ’
s, 353Warnerke
’
s, 1 3 1
Siderostat, 269 , 341
Silver acetate in the sens1t1s1ugbath, 62bromide , ac tion oflight on, 26, 93
sensitiveness ofto Spectrum,284, i
292—302
chlo ride , action oflight on, 2 1experiments with, 27sensitiveness of, to spectrum,
287,299
-302
Silver haloids, influence ofspectrum 011,277iodide,action oflight on 23dissolved by silver nitrate , 60sensitiveness of
,when exposed to the
spec trum, 277, 292- 302nitrate , imprac ticability ofusing, twithgelatine vehicle , 99
purity of, 60silver iodide dissolvedby, 60
organic salts of, ac tion oflight on, 27experiments with, 27
printing, 143manipulations in, 1 53sensitising baths fo r, 155
salts of, effect ofcertain dyes on, 315,2 1
Slide , adj ustment ofthe dark, 232dry plate (double back), 229
Sodium ace tat e in. the toning bath, 150hypo sulphite and iodine , 152
as a fixing agent, 74
SOD WOO
tes'
ting’
for, 161 Testing plain collodion, 54
tetrathionate , its formation, 1 52 Toningbaths, 1 53 , 165Solar pho tography, 339 Toning prints, theory of, 147So lve nts , action ofthe, employedfor collo Silver prints, 158dion, 49
Transfer, double , ofcarbon prints, 1 623,1
Spectra, absorption, ofdyes and glasses, Transfers, photo-lithographic , 1 90
3 23
photographic , 277- 302Spec trum,
influence of, on the ha101d salts RANIC salts, action oflight on , 30
ofSilver, 277—302 print ing W ith, 165fo cussing the, 265, 266luminosity of
, 3 1 8
photography, experiments showing the
radiations causingreversal ofthe image ,3 10
rays, absorption of, by various trans
parent solids, 274sensitiveness ofdifferent parts of, to
various dyes, 3 17simple means offorminga, 8
Spurge’
s sensitometer, 353Stains due to intensifying, 83Starch as siz e fo r paper, 146Stellar pho tography, 351Stripping films, 142Substitute fo r ground glass, 23 1Substratum o r edging fo r dry plates, 100Sulphite of soda solu tion
,fixing Silve r
prints with, 1 53Sulphur and antimoniuretted hydrogen ,action oflight on , 34
Sulphuric acid, ac tion of, o n o rganicmatter, 42
Support and substratum, 35
Swan, his double transfer carbon process,175
Swing back, uses ofthe , 2 10, 257the natural position of, 258
ABLE ofelements andthe ir combiningwe ights, 359
Table ofmetricala nd c ommon weights andmeasures compared, 360
Talbo t , his discoveries, 5, 199Tent
,dark
, Rouch'
s, 236Testing for sodium hyposulphite , 161
PRINTED BY
5101 1 15110005 AND co . , NEW-STREET SQ UARE
ANADIC salts, action 0 light cm, 3
Varnish, fo rmu la for, 76intensity given by, 76
Varnishing the fi lm , 75, 84Vegara
’
s negative tissue , 142
Vehic le , collodion as a, 38
Vehic les for ho ldingsensitive salts in 1522
6-
3defec ts ofgelatine and albumen as,, 1c
Verticalposition ofthe swing-back, 257Viscidity in developers, 66
WARNERKE’
S negative tissue , 1142ro ller slide , 23 1
sensitometer,1 3 1
\Vashingan emulsion, 12 1- 1 27
dry plates,‘
102trough
,160
Water, distilling, 58
impurities in , 59wash for dry plates 102
Waves oflight and heat , 8co-e xistence ofSho rte r with longer , 16
Waxed paper, L e Gray’
s proce ss , 142
Waxing “
composition for Z inc plates used
the au to type process, 1 83
pape r negatives , 141Wedgwood, his process , 2We t plate pho tography, manipulation
1
7Willis’s aniline process, 1 86platinotype process, 171Woodburytype , 188
