388 BURNETT: Incidental Phenomena of the Shiadow-Test.
SOME INCIDENTAL PHENOMENA OF THE SHADOW-TEST. BY SWAN M. BURNETT, M.D., WASHINGTON, D. C.
As with all other methods of determining refraction, the shadow-test has been undervalued by some and overestimated, perhaps, by others; but I think that no one who has used it long, with a knowledge of its underlying principles, can have failed to recognize its great advantage in certain classes of cases and its assistance as a control method in all. Infallibility, or an accuracy closer, in the majority of instances, than o.5 D. should not be claimed for it. One obstacle to its more general use, as I have stated elsewhere * is the want of. a handy method of employing it, -a fault which I have endeavored to overcome by using a disk whose periphery is set with a series of lenses that can be brought in rapid succession before the eye to be examined.t Though I have found the method generally reliable and use it, by routine, in every case of refraction which I examine, there are some incidental phenomena which I have observed in a number of cases which seem to be of sufficient importance to have attention directed to them more pointedly than has been done hitherto. When the refracting media are uniform in curve and transparency the illuminated area with a shadowy edge which moves across the pupillary space is of an even reddish color and in its passage from side to side, through the rotation of the mirror, is unchanged except sometimes when the faint shadow of a large retinal vessel obtrudes itself. I have found, however, that this illuminated area is by no means constantly uniform. In quite a percentage of cases there is within this space a very *Skiascopy: with a description of
an apparatus for its ready employment. Knapp's
Archives, No. 2, I890. t I think it should be stated, in this connection, that the instrument constructed and advertised by Queen & Co. of Philadelphia, as "Wurdemann's Skiascope" differs in no essential particular from that exhibited by me at the meeting of this Society in Washington in i888. Dr. Wurdemann, I am informed, had some made in Vienna, with his name attached, in 1890.
BURNETT: Incidental Phenomena of the Shadow-Test. 389
appreciable play of shadows, more or less crescentic, which change their position and shape with each change in the position of the mirror, in the same manner exactly as we have it in conical cornea, only, of course, not so pronounced. These appearances must come necessarily from some obstacle to the regular refraction of the rays that proceeds from the illuminated retinal area in their passage outward to form the aerial image of this area at the far-point of the eye. The phenomenon is more evident when the plane mirror and a weak illumination are used. Exactly where the defect lies I have not been, as yet, able to determine. The appearance and movements of the shadows are so like those of conical cornea that one's first thought is naturally towards some change in the corneal curvature. But every case in which the phenomenon was at all pronounced was examined carefully by the ophthalmometer of Javal, the measurements being taken at every five degrees along the principal meridians from the line of the visual axis to the periphery of the cornea in the manner described in my paper on the " General Form of the Cornea, etc.," and in only a single instance did the change in the corneal curve deviate from that of cornee which can be considered normal, sufficiently to account for the phenomena. Neither did the corneal reflex of Placido's disk show any marked change in the size or form of the rings. I have excluded from this consideration all cases where even the slightest trace of a corneal or lenticular opacity was discernible by ordinary oblique or direct illumination. In some cases, however, there was a history of a previous inflammation of the eye, and it is possible that an iniammation of the cornea might have healed with a normal surface curve but with some permanent change in the density of the tissue beneath, which would not reveal itself by the usual methods of examination as a clearly-defined opacity. My next thought was that the irregularity might be due to a conicity of the lens or, at least, an irregular curve, on one or both of its surfaces. Of the existence of such irregularity I have not been able to satisfy myself by the Sanson-Purkinje experiment. The method is not, however, I think, sufficiently
390 BURNETT: Incidental Phenomena of the Shadow-Test. refined to be of any use in determining small irregularities of curves. Then there comes in another possible cause, the variations in the density of the lens tissue. Pronounced opacities in the lens, such as we find in commencing cataract and particularly when the nucleus is first or most affected, do unquestionably give rise to these shadows, but the instances where I have found them most pronounced have been in young people in whom there was no reason to suspect any pathological change in the lens. Another feature which has shown itself in many of these cases is one which tends to set a limit to the reliability of the test. In a number of cases where I have observed this phenomenon I have found a want of correspondence between the lens which gave a reversal of the shadow movement and the glass which gave the best visual acuteness. Of this the following case is an example. Miss E. F., aged i 8, has vision in either eye of A,+, and so far as she remembers it has never been any better. The cornea examined at the visual axis gave'an astigmatism of 0.5 according to the rule. Skiascopy gave a general M. of I.S D. As minus glasses of any strength failed to improve vision she was put under a mydriatic. The shadow-test still gave, with the dilated pupils, a reversal of movement only with -i. D. at two meters examining distance; her best vision, however, was obtained with +0.5 go9 L. ioo° R., when it was barely k. The crescentic unstable shadow within the illuminated area was very marked, and in the direct ophthalmoscopic examination there was observed that changing in the distinctness of the retinal vessel along its course or with a shifting of the mirror which is so characteristic of conical cornea. An examination with Placido's disk did not reveal any noticeable change in the form or course of the rings. The cornea was measured in its principal meridian at intervals of 5° from the. visual axis to -200 on each side. The measurements of the left cornea expressed in dioptries of refraction are given in the following table:
BURNETT: Incidental Phenomena of the Shadow-Test. 391 In.
It will be seen from these measurements that the corneal curve is much more regular than the average as exhibited in the table in my paper on the "General form of the human cornea." * Mr. O. R., i8 years old, has R. vision = with +2 I00°. Skiascopy on repeated examinations gives invariably + 1.75 i8o and +4.90. The ophthalmometer reveals 1.25 D. of astigmatism according to the rule at the visual axis. The general corneal curve is not abnormal. The unstable shadows within the illuminated area were very marked. He said he had had granular lids when he was younger, but there are no corneal scars visible by the usual methods of examina-
tion. Mrs. S. R., 33 years old, V.= - and no glass improves. Skiascopy gives -I .75; corneal curve regular and not abnormal. Shadows quite pronounced. Miss E. M., 23, V. =- . No glass improves. Internal shadows very pronounced. The shadowy edge in skiascopy was so ill defined that nothing definite could be learned from the movements. Corneal curve normal. Mrs. H. B. W., 30, under homatropine R. V.=152 with 0.75 c + i. i65. Ophthalmometer atvisual axis4o D.at meridian 30°, 39.5 D. at 120°. She had inflammation of the eyes in childhood, but there are no corneal scars visible. The corneal curve is normally regular, except at the upper inner portion, where it becomes slightly irregular. The internal shadows are very marked. In the other, L. eye, the shadows are less marked and the corneal curve is normal in both principal meridians. V.
=-6 See page 319.
392 BURNETT: Incidental Phenomena of the Shadow-Test. In some cases in which the internal shadows were clearly discernible the visual acuteness was normal, but in none have I seen it exceed the normal of J. In no case, however, in which it was at all pronounced have I seen the acuteness of vision come up to the standard. It would seem, therefore, from this, that the shadow-test is to be relied upon implicitly only when there is perfect regularity in the refraction, and, as a converse corollary to this, the method is one of great value in detecting such irregularities when they are not otherwise determinable. DISCUSSION.
DR. GARDNER, New York.-The shadow-test is a hobby with me and I agree with Dr. Burnett in his remarks. It is a most valuable aid in astigmatism and with a moderate degree of practice you can easily ascertain an astigmatism of .5 D. I have found even .25 D. You can find out not only the degree but also the axis. The great objection to the method is its inconvenience. I have tried to suggest some method to facilitate the examination. The difficulty has been to introduce the cylinder. Mr. Meyrowitz has worked out an apparatus. The cylinders are introduced in a parallelogram in front of the mirror, the minus lenses with the axis horizontal and the plus lenses with the axis vertical. The whole apparatus turns and it is easy to determine the axis. In this way the test can be readily applied. I find this test more accurate than any other. I also use the astigmometer of Javal. DR. GUSTAVUS HAY, Boston. -I think that Dr. Burnett stated that in one case he found with the Javal instrument that there was astigmatism of .5 D. according to the rule, but the glass given was a plus cylinder with axis horizontal. DR. BURNETT. -The ophthalmometer does not give the lenticular astigmatism. DR. HAY. - I know that when the Javal instrument gives a certain measure of astigmatism according to the rule, we have to subtract. The amount to be subtracted is often .75 D. or I. D. What shall we do if the instrument shows only .5 D. ? Cases may occur where after two careful examinations there is found .25 D. or .5 D. according to the rule, and yet the correction is made with plus cylinders axis horizontal. Javal alludes to this in one of his communications. One remark in regard to a recent criticism of the Javal instrument. Tscheming* *
Memoires d' Ophthalmometrie, Paris, I89I, page 594.
13URNETT: Incidental Phenootetta of the Shadow-test. 393 compares the refraction of the cornea, with the air in front and the aqueous behind, to that of a glass lens in the air with a} focal distance equal to the anterior focal distance of the cornea. This seems allowable for certain purposes. I understand the critic to think that we must look upon the light as passing from the air into the aqueous and that we must consider the power of the cornea as equal to that of a lens with a focal distance equal to the posterior focal distance of the cornea. Although this may be an allowable method, I think that the other is also allowable and perhaps has certain advantages. It is a question of measurinig the astigmatism of the cornea. We may start with a parallel pencil in the aqueous and consider how it will be refracted in passing through the astigmatic cornea into the air. It would seem allowable in this way to say that the cornea is equivalent for our purpose to a glass lens in air whose focal distance is equal to the anterior focal distance of the cornea. In this way the astigmatic pencil to be corrected is in the air and we can conceive our corrective glass cylinder to be directly applied to it. But in the other way, the astigmatic pencil to be corrected, or made homocentric, would be in the aqueous, and we could not apply directly to it the corrective glass cylinder. As to the relation between refraction and the images formed by reflection. The ophthalmometer makes use of images formed by reflection from the anterior surface of the cornea, but involves also the consideration of refraction of light in passing through the cornea. The size of the image formed by reflection is in relation with the refractive power of the cornea; varies with the refractive power. The instrument makes use of the difference in size of the image to get the difference in the corresponding refractive power of the cornea. DR. SWAN M. BURNETT, Washington. - I should like to say that while I have not given Dr. Weiland's paper careful perusal, I think that he misunderstands what Javal has done. Three or four years ago I read a paper before the society in which I gave the refraction of the human cornea in five hundred and seventysix eyes, transmuting the measurements into dioptres of refraction. In doing that I found that it was necessary to change somewhat the value of Javal's calculations because he had taken at first an index' of refraction which is not correct. If you transmute these measurements into dioptres you have to take into consideration the refractive index of the aqueous and cornea. DR. WILLIAM THOMSON, Philadelphia. -We all know that Javal's instrument at best only affords a convenient method of quickly determining the variation between the curves in differ-
394 BURNETT: Incidental Phenomena of the Shadow-Test.. ent meridians of the cornea, but is inferior for accuracy to the ophthalmometer of Helmholtz. I got the first of Javal's instruments that came to this country. While its value is unquestioned, it cannot be said that it determines once for all the degree of astigmatism that we may correct. It is not unusual to find the two principal meridians of the cornea similar in curve and yet to find astigmatism when the whole dioptric apparatus of the eye is tested. Another point of interest is that patients can see as well as they do with such imperfect cornea. With the shadow test it is often evident that the refraction at the center of the pupil differs very much from that of portions a short distance from the periphery. In such cases the best correction must be carefully selected by every method at our command. On the cars, coming to this meeting, I encountered a gentleman with such an eye that I had corrected in I876, and who was still constantly wearing his glasses, and carrying in his pocket the formula of them to provide against possible accident to them.
DR. EDWARD JACKSCrN, Philadelphia. - It seems to me that Dr. Burnett's irregular shadows in connection with the shadowtest, and the variations of curvature of the cornea as shown by the ophthalmometer, open up the most important path of further advance in the field of. the refraction of the eye. To understand how we see with such different refractive conditions in the same eye, when only one portion of the pupil can throw a perfect image on the retina, we have to remember that the retina is quite capable of appreciating an image formed upon it although there may be other diffused light falling upon it at the same time. Under the most favorable circumstances a large quantity of unfocused light reaches the retina. The visual acuteness depends upon the perfection of the image formed on the retina to some extent irrespective of the amount of unfocused light. The shadows resembling conical cornea were referred to by me in a paper in I888 on symmetrical aberration of the eye. The peculiar movements of conical cornea are due to different rates of movement of the light. If a portion of the cornea has its point of reversal near the observer's eye, the light moves rapidly. If its point of reversal is further off, the light moves more slowly. It is this difference that accounts for the movements resembling those seen in conical cornea, which can be brought out in a large number of eyes. As you reach the point of reversal you will get some such movement. The fact that in the majority of eyes this variation is the opposite of that which would be caused by the variation which we find in
BURNETT: Incidental Phenomena of the Shadow-Test. 395 the curve of the cornea with the ophthalmometer, establishes conclusively that the aberration of the lens is greater than that of the cornea. In cases of irregular cornea the proper thing to do is to find out what portion- it may be a small portionof the cornea will give a perfect image with any lens that we can give. In the case reported by Dr. Burnett, I cannot help thinking that it must be accounted for on the supposition that there was some portion of that pupil that would have given on the shadow-test a refraction that was indicated by the acceptance of the convex lens although in the greater part of the pupil the refraction was. more myopic. In analyzing the pupillary area in that way it is important to -use a small point of light and at the distance where it is most perfectly focused on the retina. With a small definite light area, you can get the different movements in the different parts of the pupil. With a large light area, they will all run together, so that the small parts of the pupil which have a different refraction may escape our notice. DR. GuSTAVUs HAY, Boston. -With reference to the images on the cornea and refraction. In those tests where we get an image on the cornea, a certain image corresponds to a certain dioptre. The gentleman to whose criticism I have referred says that there is a certain relation between magnitude and corneal refraction. DR. HENRY D. NOYEs, New York. -One practical deduction from this discussion is that the use of atropia for determining the refractive error is by no means so necessary as has been inculcated. I have used atropia more as a therapeutic measure than as an aid in the determination of the refractive error, pure and simple. The exposure of a large area of the cornea may lead to error in the prescription of glasses. It strikes me also that some of the difficulties in the shadowtest are dependent upon the lens, - to lack of accurate curvature of the lens or to lack of homogeneousness of the lens. I think that if some of these cases were submitted to the experiment of viewing a small luminous point to determine how much irregular refraction appears, some light might be thrown upon the discrepancies described. DR. B. ALEXANDER RANDALL, Philadelphia. - Since the appearance of the Javal ophthalmometer, I have always regretted that I did not have one except when I was using the instrument or listening to its friends discussing it. The instrument undoubtedly has its value, but how closely it comes to measuring accurately the corneal curve, and how far we can
396 ButnETT: Incidental Phenomena of the Shadow-Test. from that deduce the corneal refraction, and from that guess what is the ocular refraction, are points on which we still need much light. I hope soon to have one of these instruments and to be able to discuss its demerits with the same friendliness which those who use it do. An important point brought up by Dr. Burnett is in regard to these unusual appearances in retinoscopy. While recognizing the importance of what has been said, there is another element which enters into the matter even in the method advocated by Dr. Jackson of small sharply defined images, that is retinal asymmetry. I believe that with the retinoscopic mirror at three or four metres we can determine not only .5 D. or .25 D. but that in easy cases I constantly determine o. i D. I think in that we have the explanation of these irregular shadows, which are frequently due to the asymmetry of the surface of the retina exposed to illumination by the retinoscope. It is that which contributes in no small amount to these irregularities. DR. W. F. MITTENDORF, New York.-In regard to the use of atropia, I have found it so misleading in low degrees of astigmatism that I have given it up. This is the position I took some years ago in a paper before the New York State Medical Society.
DR. EDWARD JACKSON, Philadelphia. - I cannot agree with Dr. Noyes in his deduction with reference to the slight value of mydriatics. In" nine cases out of ten the dilatation of the pupil does not interfere with the subjective tests. The only part of the comuea that gives an image that the visual centers will consider at all and will be influenced by is the central part in nine cases out of ten. The other part will be entirely disregarded. My feeling in regard tp the use of mydriatics is that if you have any accurate measurement to make, what you want to do is to get and hold the thing still that you want to measure. If it is a measure of weight, distance, or refraction it is the same. Unless you use a mydriatic to get rid of the variations of accommodation, you do not get that fixity, and the results are vitiated by an element of uncertainty. You may be right or you may be wrong. When a patient comes to the ophthalmic surgeon to have the refraction ascertained, I think he wants certainty, and should have the refraction ascertained as it is, not merely guessed at. DR. SAMUEL D. RISLEY, Philadelphia. -I am glad that Dr. Jackson has made these last remarks. I think that mydriatics must be used if we are to determine the refraction error with accuracy.
OLIVER: Correction of Error in Plastic fntis.
DR. W. S. DENNETT, New York. -If you want to find out what the eye shows with a mydriatic, of course you must use a mydriatic. If 'you want to find out with what glass the patient should read, you can nearly always find that out without a mydriatic. DR. SWAN M. BURNETT, Washington. -In reference to what Dr. Jackson has said, I would state that I was familiar with his paper in regard to the refraction of the cornea at the center and at the periphery. I examined all these corneae in this way. I examined as far out on the periphery as I was able, but it made no appreciable difference. Neither did it make any difference in this particular case. In this case the cornea was unusually regular. It was examined half a dozen times and under a mydriatic. It was the most pronounced case that I had and it set me to working the subject up.
THE CLINICAL VALUE OF REPEATED CAREFUL CORRECTION OF MANIFEST REFRACTIVE ERROR IN PLASTIC IRITIS. BY CHARLES A. OLIVER, M.D., PHILADELPHIA, PA.
Since seeing the paper of Dr. John Green "On a Transient Myopia occurring in Connection with Iritis," read before this society in 1887, which was followed a year later by Dr. Mittendorf's communication on " Symptomatic Myopia," to be found in the Transactions of the same society, the writer has been so impressed with the importance of the subject, that he has taken advantage of his abundant clinical opportunities to make routine and periodic study of the refractive condition of every case possible of inflammation of the iris that has fallen under his care, hoping thereby to furnish some data upon this vexed question. Fortunately, personally, the subject has, in his hands, gone far beyond the theoretical points then at issue, and has resolved itself into a most valuable clinical procedure in his everyday treatment of this disease. To him it has been an attempt, at least, to formulate some definite guide and to obtain some cer-