NOTES ON THE CONSTRUCTION AND DESIGN O F METALLURGICAL MICROSCOPES.

BY PROF. CECIL H. DESCH, GLASGOW.

The use of the microscope in the examination of metals, first introduced by Sorby more than 50 years ago, has become so wide- spread that a microscope is now an indispensable item in the equip- ment of a metallurgical works, whilst the recognition of its importance to engineering works and other places in which metals are employed for constructional purposes is rapidly extending. It is therefore essential to the conduct of these industries that instru- ments should be available which will allow of the rapid and con- venient examination of such metals as present themselves in the course of routine testing, whilst it is obviously desirable that elaborate and detailed investigation of specimens of special interest should be possible. It is quite true that any ordinary microscope of good construction may be used for metallographic work, provided that the higher power objectives are duly corrected for uncovered objects, but the increased convenience of a properly designed instru- ment is so great as to justify its use, even for routine work. There are now many patterns of metallurgical microscopes on the market, and the following remarks are based on an experience of some 12 or 13 types of instrument, and the examination of the details of many others. The writer has been reluctantly forced to the con- clusion that, in spite of many excellent features in some of the British microscopes, the German instruments have proved better in ase, and that their superiority is more marked, the longer the micro- scopes are used. The British designs are often good, and the work- manship, so far as the cutting of racks and screw-threads, etc., is concerned, is often quite satisfactory, but in the course of prolonged use the mechanical arrangements show defects, racks and screws becoming loose, and the accurate focussing of high power objectives becoming troublesome, to an extent which is not met with in the German microscopes. The cause of this looseness after use appears t'o be insufficient attention to the quality of t,he metal employed in construction. A rack cut in soft brass, howevelr accurate a t first, becomes loose through wear, and no conipensatjcn by means of adjusting screws can be quite satisfactory. The fact that, such screws are provided seems to be a confeseion of welakness, since the writer has used a Zeiss microscope, without such screws, for years continuously without any sign of play in the mechanical movements. Racks should be cut in hard, incorrodible metals or alloys instead of in soft br'ass, whilst the pinions might also be of much harder metal than is usually the case. It is probable that manufacturers have been too much guided by tradition in the choice of the metals to be used in the construction of scientific instruments, witness the tendency, only now disappearing, to use highly polished brass for heavy portions where cast iron would serve the purpose equally well.

175

Publ

ishe

d on

01

Janu

ary

1920

. Dow

nloa

ded

by U

nive

rsity

of

Win

dsor

on

22/1

0/20

14 1

9:16

:33.

View Article Online / Journal Homepage / Table of Contents for this issue

136 NOTES O N THE CONSTICUCTION AND DESIGN

The principal parts of the metallurgical microscope may now be considered in succession.

(a) The Sta?zd.-There is no reason why the shape of the medical or biological microscope should be slavishly copied in the construction of metallurgical instruments, whilst there are many reasons for choosing a different form, especially when there is a possibility of large specimens being examined. The tripod form of foot, so convenient in work by trans- mitted light, is awkwardly in the way when examining metals and having occasion to use the rackwork movement for raising and lowering the stage. The Jackson foot is better, and a heavy horseshoe foot still better, as heavy specimens, such as rail sections, may be laid on it for examination under low powers. This is further facilitated by making the bracket which holds the stage capable of swinging to one side, and leaving a clear space betweeo the objective and the heavy horseshoe foot, as in the old vertical Reichert microscope. Special forms of foot, as in the Beck-Rosenhain microscope, have the advmtage of great rigidity in both the vertical and horizontal positions. This stand is the most rigid of those examined. The design of Sauveur’s universal Metalloscope is also unconventional, and appears to be good, but the writer has no actual experi- ence of it. For photographic work the form adopted in the Zeiss-Martens instrument and in Watson’s horizontad micro- scope is both convenient and steady.

The inverted stand, due to Le Chatelier, has been copied by several makers, but the construction is apt to be flimsy, and the writer has found great difficulty in moving even small specimens on the stage without altering the focus, the light arms which support the optical parts being liable to whip. This could perhaps be overcome by better engineering design, and the type is certainly preferred in some works on account of the rapidity with which specimens can be inserted and examined. The optical conditions of this form are discussed below. It is probable that for the larger instruments to be used for photography the ordinary type of stand might be departed from entirely, and an arrangement modelled on the optical bench adopted, the various optical parts and specimen carriers being sup- ported in such a way as t o move freely along a heavy bar of geometrical form to preserve alignment.

(b) Coa7.se djttstment.-The rack and pinion should be geo- metrically cut in metal of sufficient hardness to withstand prolonged usage without working loose. The improvement in the methods of gear cutting in engineering practice has been so great in recent years that much would be gained by adopting the methods of marine engineering shops in the instrument maker’s workshop. I n large instruments, the length of travel might well be greater than a6 present, so as to allow of a wide range of objectives, and stops should be provided a t the ends of the rack to prevent over-

Publ

ishe

d on

01

Janu

ary

1920

. Dow

nloa

ded

by U

nive

rsity

of

Win

dsor

on

22/1

0/20

14 1

9:16

:33.

View Article Online

OF NETALLURGICAL MICROSCOPES 137

racking. This is particularly desirable in students’ micro- scopes, as it would prevent a common accident in labora- tories where inexperienced students use the instruments.

(c) Elirie ddjzistnzc~~t.-This does not’ call for much remark, as there are several good forms in use. The speed is some- times made tooo great for comfortable focussing of high powers. The side arrangement of small milled heads is perhaps the most convenient.

(d) Bodjy Tube.-This should be of the short Continental form, and preferably of wide diameter. The latter condition is essential in instruments to be used for photographic work, and should always be adopted, but i t has also great advan- tages for visual observation, and can be introduced without interfering with the general design.

(e ) T h e Stage.-A plain stage of fairly large size is suitable for most ordinary work. It should be provided with a rack- work focussing movement, but a fine adjustment is un- necessary. A central hole, sufficiently large to allow an objective to pass through it, allows of the examination of heavy specimens resting on the foot, unless the support of the stage he arranged to swing aside entirely, as mentioned above. Levelling stages are a nuisance, and should never be used. The specimen should always be levelled before placing on the stage, either by means of plasticine and one of the usual mounting devices, or by means of Dr. Rosenhain’s auto-collimating instrument. Mechanical movements to the stage are essential for high power work, and rotation is also a very great convenience, but when both are provided the rotation should be con- centric. A rotating plate which is carried by the traversing movements is useless. When a microscope is intended to be used in the horizontal position, it is desirable to provide the mechanical movements with clamping screws, as other- wise a heavy specimen may cause a gradual downward slip during the exposure of a photograph, pulling down the rackwork by its own weight,. This has often been noticed when photographing a t high magnifications. The rotating circle should have a clamping screw. The Zetiss-Martens stand has a very convenient rotating and traversing stage, but the range of movement is too limited.

The examination of fractures, large crystals in ingot sections, and other things requiring very low powers and great distances, is troublesome when an ordinary microscope is used, and i t is often preferable t o employ a camera with a landscape or copying lens instead of a microscope. The telephoto attachment of the Davidson microscope gives good results in this kind of work, and the arrangement in the recent pattern, by which the object is carried on a separate st.and, movable along a base board, is con- venient. On the other hand, the writer does not approve

Publ

ishe

d on

01

Janu

ary

1920

. Dow

nloa

ded

by U

nive

rsity

of

Win

dsor

on

22/1

0/20

14 1

9:16

:33.

View Article Online

138 NOTES ON THE CONSTRUCTION AND DESIGN

of the " super-microscope " arrangement, by which the iinage formed by one objective is magnified by a second objective.

( f ) Y ' h e l 'ertical I l luu, iiiutor*.-Whilst the prism form has the advantage for visual work of causing much less loss of light than the transparent plate, it is unsuitable for high powers, on account of the fact that it only uses one-half of the aperture of the objective, and is consequently liable to produce false images of fine structures. The 'same objection applies to silvered half-discs or other similar devices. The Beck or transparent illuminator is the only suitable form for photographic work a t any but low mag- nifications. The mistake is very commonly made of fitting a small cover glass, which only imperfectly covers the back lens of the objective, into such illuminators. A plate of larger size should be used. Moreover, cover glasses are not accurately flat, and have no advantage except cheapness and thinness. A large, optically worked plate is used in the Conrady-Watson illuminator and in the Jackson and Blount microscope. The writer has found the thin, square plates used for counting blood corpuscles very suitable, being sufficiently flat and so thin as not to produce doubling of the image. The plate should be capable of a t least partial rotation, and should have a sufficiently large milled head to allow of delicate adjustment. Vertical illuminators often leave much to be desired in regard to mechanical construction.

The inverted or Le Chatelier type of microscope calls for a different form of illuminator. As usually constructed, the numerous reflections required tend to injure the defini- tion of the image and to cause loss of light. To a great extent this might be obviated by better optical workman- ship, the prism being made in one piece with accurately ground faces, as in the modern range finder. The possi- bilities of new optical arrangements for illumination are not exhausted.

(g) The Objectives.-It is now generally agreed that short, mounts are to be preferred for metallograpk-ic objec- tives. A high numerical aperture is necessary for the highest powers. Apochromats are usually recommended for the medium and high powers, but such objectives are commonly deficient in flatness of field, a very desirable quality in metallographic work, and it may be questioced whether good achromats, giving flat fields, are not t u be preferred for photographic purposes. It is usual to insert a colour screen when making such photographs, and now that screens which transmit so narrow a band of the spec- trum that they may be regarded as practically mono- chromatic are obtainable, i t seems of less importance that the colour correction of the objectives should be perfect. Oil immersion objectives are, of course, necessary for the highest magnifications.

Publ

ishe

d on

01

Janu

ary

1920

. Dow

nloa

ded

by U

nive

rsity

of

Win

dsor

on

22/1

0/20

14 1

9:16

:33.

View Article Online

OF METALLURGICAL MICROSCOPES 139

(h) Eye-pieces.-These giv2 the least trouble of all the parts of the microscope), the quality being usually satisfactory . Projection eye-pieces are to be preferred for photographic work.

These few notes are presented by way of suggestions for dis- cussion. Each worker will have formed some opinion on the points mentioned, and a coniparison of such opinions may be of assistance to manufacturers in determining the design of their future instru- ments. There is a large demand for metallographic microscopes a t present, whilst the supply is very limited, and the time seems appro- priate for a consideration of the question whether improvements might not be made in the light of experience.

Publ

ishe

d on

01

Janu

ary

1920

. Dow

nloa

ded

by U

nive

rsity

of

Win

dsor

on

22/1

0/20

14 1

9:16

:33.

View Article Online