604 LOUISE L. SLOAN A N D D A R L E N E J. B R O W N

N E W F O C U S A B L E S T A N D M A G N I F I E R S *

LOUISE L . SLOAN, PH.D. Balthnore, Maryland

INTRODUCTION From the Wilmer Ophthalmological Institute of Stand magnifiers differ from hand and * e Johns Hopkins Medical School and Hospital.

1 „ Λ 1 „ „ j - • „ f t , . , 4 - „ This research was supported by grant B-810 from the head-borne readmg aids m that a support is j^^^j^^^, ^^^ . ^^^ Neurolfgical Diseases and provided to maintain the lens at the desired Blindness, Public Health Service, Bethesda, Mary-distance from the reading page. In the focus- 'a"d- (Until a demand which warrants commercial

, , . 1 -c ii · J - I u manufacture can be demonstrated, these magnifiers able stand magmfier this distance can be a e being made in a local shop. W e will be glad to changed to compensate for uncorrected supply them at cost to other Low-Vision clinics.)

REFERENCES 1. Sloan, L. L., and Habel, Α . : Reading aids for the partially blind: New methods of rating and pre­

scribing optical aids. Am. J. Ophth., 42:863-872 (Dec.) 1956. 2. : New methods of rating and prescribing magnifiers for the partially blind. J. Optical Soc.

Am., 47:719-726 (Aug.) 1957. 3. Sloan, L. L., and Jablonski, M . : Reading aids for the partially blind: Classification and measurement

of more than 200 devices. A M A Arch. Ophth., 62:465-484 (Sept.) 1959. 4. Stimson, R. L.: Optical aids for low acuity. Braille Institute of America, Los Angeles, Cal., 1957. 5. Pameijer, J. K. : An investigation of optical corrections for enabling patients with low visual acuity

to read. E. Kulver, Amsterdam, 1959. 6. Sloan, L. L., and Brown, D.: Reading cards for selection of optical aids for the paritally sighted.

Am. J. Ophth., 55 :1187-1199 (June) 1963. 7. D'Esposito, M. : Clinical consideration of a system of increasing utilization in subnormal vision. Arch.

Ottal., 63:211-226 (May/June) 1959. 8. Feinbloom, W . : Outline of the technique of examination of the partially blind patient with the

clear image lens. Bull. Opto. Soc. City of N. Y., 1953. 9. Fonda, G.: Report of five hundred patients examined for low vision. A M A Arch. Ophth., 56:171-175,

1956. 10. : Characteristics and low-vision corrections in albinism. A M A Arch. Ophth., 68:754-761

(Dec.) 1962. 11. : Subnormal vision correction for aphakia. Am. J. Ophth., 55:247-255 (Feb.) 1963. 12. Freeman, E.: Optometrie rehabilitation of the partially blind: A report on 175 cases. Am. J. Optom.,

31:230-239 (May) 1954. 13. Gettes, B. C.: Optical aids for low vision. Sight Saving Rev., 28:81-83 (Summer) 1958. 14. Gunstensen, Ε. : Visual aids for the partially sighted. Brit. J. Ophth., 44:672-678 (Nov.) 1960. 15. I.H.B. Optical Aids Service Survey: First 500 Cases. The Industrial Home for the Blind,

Brooklyn, N . Y , 1957. 16. Kaplan, H . : Refractive techniques for the decreased vision problems of the aged. Am. J. Optom.,

36:511-519 (Oct) 1959. 17. Levi, G. Α., and King, J. H., Jr.: Evaluation of certain optical devices in the correction of sub­

normal vision. Am. J. Ophth., 40:29-34, 1955. 18. Milder, B.: Advantages of the optical aids clinic. Sight Saving Rev., 30:78-84 (Summer) 1960. 19. Moffatt, McG.: Visual aids for the partially sighted. X V I I I Cone. Ophth., Bélgica Acta, 2:1567,

1958. 20. Schwartz, R. E.: Special report: Low Vision Center, Maryland Workshop for the Blind. Optom.

Weekly, Dec. 29, 1960, pp. 2685-2687. 21. Volk, D. ; Conoid ophthalmic lenses in legal blindness. Am. J. Ophth., 56:195-203, 1963. 22. Waddell, M. C.: Progress in subnormal vision aids. Virginia M. Monthly, 86:595-597 (Oct.) 1959. 23. Weiss, S.: Optical aids for the partially sighted. Am. J. Ophth., 55:255-261 (Feb.) 1963. 24. Willets, G. S.: A survey of patients using spectacle magnifiers. Brit. J. Ophth., 44:547-550 (Sept.)

1960. 25. MacDonald, D.: Low vision aids. Modern Med., Sept. 1, 1959, p. 257. 26. Tillett, C. W . : Visual aids in office practice. Am. J. Ophth., 46:186-194 (Aug.) 1958. 27. Gordon, D. M., and Ritter, C : Magnification. A M A Arch. Ophth., 54:704-716 (Nov.) 1955, 28. Bier, N.: Correction of Subnormal Vision. London, Butterworths, 1960.

N E W FOCUSABLE S T A N D M A G N I F I E R S 60S

spherical errors of refraction. It was shown in a previous study* that when high mag­nification is required, a stand magnifier is usually the only type of reading aid accept­able to the elderly patient. Because of inade­quacies in the commercially available devices, they were supplemented in this study by a series of six inexpensive magnifiers con­structed in our own shop. The purpose of this report is to describe the special features of these reading aids.

LENS SYSTEM

The magnifiers have doublet lens systems providing equivalent powers of 18, 23, 29, 36, 44, and 53 diopters. ( A 13-diopter unit, originally included, has been discontinued because it was seldom chosen in preference to other types of reading aid giving the same magnification.) Except in the two highest powers, the doublet consists of plano-convex lenses with the convex surfaces in contact. In powers of 29 diopters and less this simple and inexpensive type of compound lens proved to be quite satisfactory from the point of view of aberrations. The 36-diopter unit shows defects in the peripheral field which are obvious to the observer with nor­mal vision. When, however, the user's vision is so low that he needs a lens of this power to read ordinary print, these defects are al­most never objectionable and are often not even perceptible. In powers higher than 36 diopters, the aberrations are quite marked in this simple system composed of two plano­convex lenses. For the 44- and 53-diopter units, therefore, a compound lens composed of two plastic Igard lenses is used. This lens system was adopted after comparison with several other double and single aspheric lens magnifiers. It was preferred by a majority of our patients because of greater clarity and a wider usable field of vision.

PROVISIONS FOR FOCUSING

Many focusable stand magnifiers have a focusing adjustment that is much too fine

for easy use by the patient with low vision. With small adjustments he notices no change and does not know in which direc­tion to continue. The coarse thread used in our magnifiers produces an obvious change in sharpness of the letters to assist the pa­tient in finding the best adjustment.

Each magnifier has an index mark show­ing the adjustment which locates the reading page in the principal focal plane of the lens system. A t this setting the emergent light is of zero vergence, and the emmetrope there­fore obtains a clearly focused retinal image with fully relaxed accommodation. If he prefers to accommodate he will adjust the magnifier to give a shorter object-to-lens distance.

The user with an uncorrected myopia will also require an object-to-lens distance shorter than the principal focal length. The uncorrected hyperope will require a longer distance. All o f the magnifiers provide for a decrease in distance sufficient to compen­sate for at least 10 diopters of myopia or to permit 10 diopters of accommodation. Pro­visions for adjusting the magnifier to permit some degree of accommodation are of im­portance in the case of the nonpresbyope who may find it difficult not to accommodate because of the close location of the reading material. The amount of uncorrected hy­peropia which can be compensated for by increasing the length o f the supporting tube is about three diopters in the 18D. magnifier, six diopters in the 23D. unit and more than nine diopters in the other four.

AUXILIARY SOURCE OF LIGHT

Because the supporting tubes are of clear plastic these magnifiers can be used without an attached source of light. For the occa­sional patient who requires more light than is provided by the general room illumina­tion, an auxiliary attachment consisting of a diffusing plastic ring and a small flashlight is provided. The six magnifiers and the illuminator are shown in Figure 1.

606 LOUISE L. SLOAN

EXTENT OF USABLE FIELD Stand magnifiers of high power are nor­

mally used with the eye close to the lens in order to obtain as wide a field of view as possible. T o permit the normal eye to in­spect or measure \-ery small objects, they employ lenses of small diameter providing images free from distortions, color fringes and other aberrations. Consequently, they have fields of view which are unnecessarily limited when the same magnifier is used to read ordinary print. Continuous text can be read more rapidly with a magnifier which provides a wide field of view even when there are noticeable defects in peripheral areas of the field. Our series of magnifiers were designed to give fields of the greatest linear extent compatible with the focal length of the magnifier.

Figure 2 gives data on the usable fields of view in comparison with those provided by several commercially available focusable stand magnifiers. The extent of field is measured as described in a previous paper^ with a line of typed lower-case letters of such size that 14 letters cover an extent o f 30 mm. A field which includes 21 letters corre­sponds in width to the average newspaper column. In Figure 2, extent of field is shown on the ordinate, focal length of the magnifier on the abscissa. Focal lengths rather than dioptric powers are plotted because, when

other factors such as lens diameter are con­stant, there is a linear relation between focal length and diameter of field.

In two of the six magnifiers the field of view is slightly less than would be expected on the basis of a simple linear relationship. In the case of the 36D. unit, this is because the lenses are of smaller diameter than those of the other units in order to reduce aberra­tions. In the case of the weakest ( 1 8 D . ) unit, the field of view is limited by the di­ameter of the supporting plastic tube and could therefore easily be increased by cut­ting away a portion of the tube. Since its field of 24 letters exceeds that of the usual newspaper column, this was not considered to be necessary.

The lower graph of Figure 2 also gives data for representative commercially avail­able magnifiers of low cost. It is obvious that these provide an unnecessarily restricted field when they are used to read ordinary print. The upper graph of Figure 2 gives data on two types of commercially available magnifiers designed specifically as reading aids for patients with markedly subnormal vision. These relatively expensive aids are of high optical quality. The fields of view are essentially the same as those of our mag­nifiers of the same dioptric power. One of the two types was intended by the manu­facturer (Keeler) to be worn in a spectacle

Fig. 1 (Sloan). Focusable stand magnifiers and auxiliary lighting attachment. The equivalent powers are 18, 23, 29, 36, 44 and 53 diopters.

N E W FOCUSABLE S T A N D MAGNIFIERS 607

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Fig. 2 (Sloan). Relationship between equivalent focal length and extent of field for a group of focusable stand magnifiers.

frame. Some low-vision clinics''^ have found that they can also be used as stand mag­nifiers. Both tyi)es are shown in Figure 3 for comparison with one of our magnifiers.

Although the fields of view of the Keeler illuminated "spectacle" magnifiers are not quite as great as those of their illuminated "hand" magnifiers, the lesser size and weight of the former probably outweigh this slight

disadvantage. In the two highest powers of the "hand" magnifier the illuminating attach­ment is inadequate because the lens housing covers the aperture intended to admit light to the reading page.

Because the lens supports are opaque neither the "spectacle" nor the "hand" mag­nifier can be used without the attached light source. The manufacturer will, however,

608 LOUISE L. SLOAN

Fig. 3 (Sloan). One of the Sloan magnifiers, fo-cusable stand type with illuminator, is shown for comparison with Keeler illuminated "hand" and "spectacle" magnifiers.

supply on special order clear plastic lens mounts for those who do not wish to use the lighting attachment.

SUMMARY

The following features are desirable in

stand magnifiers of high power to be used as reading aids by the patient with subnor­mal vision:

1. They should be focusable over a range sufficient to permit compensation for several diopters o f uncorrected hyperopia or my­opia.

2. The focusing adjustment should be coarse enough to make it easy for the patient to locate the optimal setting.

3. The magnifiers should provide as wide a field of view as is compatible with the focal length.

4. A n optional source of illumination should be provided.

Described are a series of six inexpensive magnifiers which range in power from 18 to 53 diopters and which meet the require­ments just listed.

The Johns Hopkins Hospital (5).

REFERENCES

1. Sloan, L. L., and Brown, D. J.: Relative merits of headborne, hand and stand magnifiers. Am. J. Ophth., in press.

2. Sloan, L. L., and Jablonski, M . : Reading aids for the partially blind: classification and measure­ment of more than 200 devices. A M A Arch. Ophth., 62:46S-484, Sept., 1959.

3. Tillett, C. W . : Visual aids in office practice. Am. J. Ophth., 46:186-194 (Aug.) 1958.

D A R A P R I M : A F O L I C A C I D A N T A G O N I S T *

THE DETECTION OF EARLY TOXICITY

J. P . M c G o w A N , M . D . , A . LUPOVITCH, M . D . , AND R . Y . KATASE, M . D . Baltimore, Maryland

The use of Daraprim (pyrimethamine) in the treatment of acute ocular toxoplasmosis has been associated with various hematologic complications.^ Animal and human toxicity studies have shown this drug to have anti-folic acid properties that result in a syndrome comparable to folic acid or vitamin Bjo de­ficiency.^"^ In the human this is associated

•From the U. S. Public Health Service Hos­pital.

with a characteristic hematologic spectrum: ( a ) hypersegmentation of neutrophils, ( b ) macrocytic anemia, ( c ) neutropenia and ( d ) thrombocytopenia.* The changes in the pe­ripheral blood are associated with megalo­blastic erythropoiesis and giant, atypical myeloid precursors in the bone marrow.

By presenting the following two cases, we wish to suggest that, if the early and subtle effects of this drug are sought for, more severe complications may be anticipated.