Skiascopy : and its practical application to the study of refraction

Front matter

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UNITED STATES OF AMERICA.

CHAPTER I.— History, Name, Difficulties and Study History

Name

Difficulties

How to Study the Test

CHAPTER II.— General Optical Principles. The reversal of movement Real Movement of light on retina, Plane Mirror Real Movement of light on retina, Concave Mirror Apparent Movement of light in pupil Rapidity of Movement of light on the retina Magnification of the retina Form of the light area ....

Brilliancy of light in the pupil Finding the point of reversal .

CHAPTER III— Conditions of Accuracy. The Source of light

Focusing of light on the retina Position of greatest accuracy .

Irregularities of the Media and Surfaces Distance of Surgeon from Patient .

CHAPTER IV.— Regular Astigmatism. Two points of reversal

The Band-like appearance

Changes in the Light area at different distances Direction of movement of the band ....

CHAPTER V. — Aberration and Irregular Astigmatism. Appearance of Irregular Astigmatism . Symmetrical Aberration

The Visual Zone

Appearance of Positive Aberration .....

46 47 53

56 58 59 60

Appearances of Negative Aberration ..... 63

Appearance of Conical Cornea 65

Scissors-like' Movement . . . , , . . . .68

CHAPTER VI.— Practical Application with Plane Mirror. Position and Arrangement of Light . . , . • 71

Hyperopia . . . . , .72

Emmetropia 76

Measurement of Accommodation 86

CHAPTER VII. — Practical Application with Concave Mirror. The Source of Light 89

Myopia 91

Measurement of Accommodation 99

CHAPTER VIII —General Considerations. Apparatus 100

Mydriatics 106

Relative advantages of plane and concave mirrors . . . 107

INDEX in

PREFACE

THIS little book was written to bring about the more general adoption of Skiascopy as an essential part of the examination for ametropia. It is not supposed that any ophthalmologist is quite ignorant of the test ; but many do not know its full practical value, or how best to apply it.

The demonstrations and descriptions here given assume a general knowledge of the eye and of physiological optics. And the writer, having observed that students of this subject do not generally think in the terms of algebraic formulas, but more readily grasp the graphic or geometric presentation of a fact, has governed himself accordingly.

The claims of this subject to careful consideration are :

First. — Skiascopy, is an objective test, independent of the patient's intelligence or visual acuteness, and more largely than any other, independent of the patient's cooperation.

Second. — It is by far the most accurate objective test. The limits of its accuracy depend on details of its execution, and the skill and patience of the observer ; but, it does not require any rare natural qualifications, to carry it, for many eyes, to the extreme limits of accuracy for subjective tests.

Third. — It requires but little more time than the use of the refraction ophthalmoscope or the ophthalmometer, which are able to give very inferior information. It saves time in making a complete diagnosis.

Fourth. — It requires no costly, complex or cumbersome apparatus.

Fifth. — It lays before the surgeon the refraction in each particular part of the pupil as it is revealed by no other test, and opens up the principal avenue for farther advance in the scientific study of the refraction of the eye.

Of the use of the data obtained by means of skiascopy, it is not the purpose of the present monograph to speak. These data include the refraction of the visual zone, corresponding to the refraction of the eye obtained by other methods ; an accurate knowledge as to the location and limits of that zone ; and the refraction outside of it ; the latter having in some cases important bearings on the practical adjustment and use of lenses.

Denver, Col., March, 1895.

Chapter I. History, Name, Difficulties, And Method Of Study- Ing The Test

History. — From the earliest use of the ophthalmoscope, by the direct method, it has been recognized that the seeing of an erect image of the fundus at some little distance from the eye indicated hyperopia, and the seeing of an inverted image indicated myopia. So long ago as 1862, Bowman {The Royal London Opthalmic Hospital Reports, Vol. II, p. 157), called attention to the rotation of the mirror as a means of bringing out appearances characteristic of irregular astigmatism and conical cornea, and Donders, in his work on Accommodation and Refraction of the Eye, published in 1864, included (p. 490) the following note :

" My friend Bowman recently informs me that c he has been sometimes led to the discovery of regular astigmatism of the cornea, and the direction of the chief meridians, by using the mirror of the ophthalmoscope much in the same way as for slight degrees of conical cornea. The observation is more easy if the optic disc is in the line ol sight and the pupil large. The mirror is to be held at two feet distance, and its inclination rapidly varied, so as to throw the light on the eye at small angles to the perpendicular, and from opposite sides in succession, in successive meridians. The area of the pupil then exhibits a somewhat linear shadow in some meridians rather than in others.' "

The use of the ophthalmoscope above referred to, for the detection of irregular astigmatism, became widely popular. It was generally adopted as the most satisfactory test for this kind of defect. But the observation that the same method was capable of revealing regular astigmatism and the direction of its principal meridians, does not seem to have attracted the same attention.

In 1872, Couper, in his paper before the Fourth International Ophthalmological Congress (see Trans, page 109), alluded to Bowman's observations, and said : " The greater dispersion in one meridian than in the opposite, gives rise to the linear shadows. Only the fact of astigmatism is thus established." He then went on to describe a method of using the ophthalmoscope as an optometer in astigmatism, which is rather a modification of the ordinary use ot the ophthalmoscope than a variety of skiascopy, since it depends on the recognition or non-recognition of the retinal vessels in different meridians, when the ophthalmoscope mirror is held at a considerable distance from the eye, and takes no account of the movement of a light area on the retina. It had already been pointed out (Donder's Accom. and Ref. oftlie Eye, page 106), that the distance of the inverted image before the eye indicated the degree of myopia.

In 1873, Cuignet, of Lille, published (Rec. d'Ophtalmol) 1873, pp. 14 and 316) an account of the test, as he had used it, as one capable of revealing not only the presence of hyperopia or myopia as well as astigmatism, but also as giving a practical method of measuring the amount of these errors of refraction. He seems not to have appreciated fully the optical principles involved in the test, and his account of it attracted no attention. However, in 1878, his pupil, Mengin, introduced the practice of the method at Galezowski's clinic in Paris. There it was taken up,

HISTORY. y

and Parent demonstrated its true optical basis and urged its advantages in a series of articles published in the Recucil oV Ophlaimologie in 1 880-81, pp. 65 and 229.

L,ytton Forbes, in the Royal London Ophthalmic Hospital Reports for 1880, (p. 62), published a paper on the test, giving a minute account of the various forms assumed by the light and shadow in the pupil, but without full explanation of their optical significance. In 1881, A. Stanford Morton included a full description of the test in his little work on the Refraction of the Eye. In 1882, Charnley gave the fullest demonstration of its optical basis in the Royal London Ophthalmic Hospital Reports, X, 3, p. 344. And Juler called attention to it in the Ophtlialmic Review Vol. I, p. 327. The method described and advocated by Parent and those who followed him, had been that with the concave mirror. Cuignet had used the plane mirror, and, in 1882, Chibret pointed out (Annates d' Oculisti que, Vol. xxxviii, p. 238) the advantages of the plane mirror in determining the presence and degree of myopia in the examination of large numbers of recruits. In 1883, Story (Ophthalmic Reuieiv y Vol. II, page 228) advocated the use of the plane mirror,, but in the same manner as the concave, except that the ob~ server should place himself at a distance of four metres. from the patient, a distance which renders the test of little value for a considerable proportion of cases.

In 1885, was published (American Journal of the Medical Sciences, April, 1885) the writer's account of the test with the plane mirror, as applicable to all varieties of ametropia, the determination being made by measuring the variable distance of the surgeon from the patient. Since that time the test has been widely recognized, but even yet is far from being universally adopted and depended upon as it deserves to be. The literature of the subject has since grown 1 quite extensive. But it must be noted that a considerable proportion of the accounts of the test bear evidence that their author's acquaintance with it has been theoretical rather than practical, and the mass of 'them contribute nothing to the common fund of professional knowledge. The writer's contributions as to the retinal illumination (Ophthalmic Review, Feb., 1890) and the relative positions of the source of light and the observer (Archives of Ophthalmology, July, 1893), with the suggestions as to the special pieces of apparatus to facilitate the test, to be mentioned in Chapter VIII, complete the evolution of this method of diagnosis as now practiced, and here described.

Name of the test. — Neither Bowman nor the others, who early employed the test for the detection of irregular astigmatism and conical cornea, proposed for it any special name.

Cuignet, who brought it forward as a distinct method for the diagnosis of the refraction of the eye, seems to have thought at first that the play of light and shade in the pupil depended entirely on the curvature of the cornea, and described it under the name keratoscopie. Considering the real causes of the movement of light and shade in the pupil and the purposes for which it is employed, this name seems especially inappropriate.

Parent, realizing this inappropriateness, proposed retinoscopie, in allusion to the fact that it was the movement of light and shade on the pigment layer of the retina that commonly gave rise to the phenomena studied. Yet this name was obviously open to criticism, in that the condition of the retina itself was not at all the matter in consideration, and that the same play of light and shade could be watched on the head of the optic nerve, or, where the retinal pigment was wanting, upon the choroid or sclera.

HISTORY. 11

Chibret, to bring out the point that it was the movement of a shadow that was the subject of investigation, proposed the name of fantoscopie retinienne, and, Mr. Priestley Smith, probably anglicizing this term and dropping the allusion to the retina, called it the shadow-test. This name though a compound word, where a simple one should do, became extremely popular, and its appropriateness led Chibret to call to his aid, the linguistic skill of M. Egger, who rendered it in the term skiascopia, which in its French form skiascopie, or its English form, skiascopy, has been most widely accepted as the proper term to designate the test.

Umbrascopy, proposed by Hartridge, is indefensible on linguistic grounds, and the same is true of pupilloscopie proposed by Landolt, and for which he afterwards offered the equivalent, koroscopie. Dioptroscopie was advocated by Galezowski (Atlas d' 'Ophthalmoscopic) and is appropriate, though equally applicable to other methods of measuring refraction.

Retinophotoscopie and retinoskiascopie have have also been recently suggested by Parent, but there seems to be no sufficient reason for retaining in the name any allusion to the retina. Fundus-reflex-test suggested by Oliver is also unnecessarily long for a name.

The suggestion has sometimes been made to apply one of these names to one form, and another name to another form of the test. But such a use of them is not warranted by their original suggestion or by custom, nor is there any sufficient reason for the employment of separate names to differentiate various forms of the test. In all its different forms, the test is essentially the same ; the difference being merely as to the apparatus and mechanical detail.

Difficulties of the Test. — That skiascopy, though a valuable method of examination, is one difficult to master.

becomes more and more evident as one continues to work with it. The theoretical basis is perfectly simple ; the fundamental phenomena readily observed ; and, with a few days practice, the merest tyro may be able by it to estimate the refraction in favorable eyes with an accuracy not to be attained by any other objective method. But long after the stage of such acquirement has been passed, the surgeon will again and again encounter cases that still prove difficult and puzzling. Nothing but a thorough understanding of the optical principles involved, and patient study of the eyes which prove most puzzling, under carefully arranged favorable conditions, will enable one to master the test.

The importance of careful arrangement of the relative positions of the light and of the observer, and the adaptations of the mirror have not heretofore been sufficiently insisted upon. What these adaptations and arrangements are will appear under their proper headings in chapters III and IV. It is here only necessary to emphasize their importance. For instance : All descriptions of the shadow-test allude to the characteristic band-like appearance of the light in astigmatism. Now, as a matter of fact, even in the highest degrees of astigmatism, such an appearance cannot be perceived, except with certain lenses, or at certain distances in front of the eye ; and it is a distinctive and exact indication only when the light, the mirror, and the patient's and the observer's eyes are brought into a certain relation.

It would be as rational to attempt to measure refraction with an ophthalmoscope devoid of any lens series, or to test the acuteness of vision in a darkened room, as to expect definite and satisfactory results from skiascopy, applied without careful attention to details that have usually not been referred to in descriptions of the test.

DIFFICULTIES. 13

The fact that this test shows, as does no other, the actual refraction of the eye for each particular portion of the pupil, increases enormously the wealth of phenomena it offers for study, adding to its scientific and practical value, but also making it more difficult by rendering it necessary to discriminate between the particular portions of the movement of light and shade which are of practical importance, and others which are not.

How to Study the Test. — The study of skiascopy is something quite different from its practical application. To start from a few bare rules as to the placing of glasses, and the movements of the mirror, and the light in the pupil, and attempt to learn the test by using it will never give a mastery of it. It is better to make a careful study of it before attempting to employ it as a method of ascertaining the refraction.

Such a study is chiefly a use of the test, but from a standpoint entirely different from that of its application in practice. To study the test, one should as far as possible, start with known conditions of refraction, with lenses of known strength, with the eye at a known distance, and should observe the character of the movements of light and shadow in the pupil, which belong to these known conditions. He should work from known refraction to the pupillary appearances that belong to it. While in using the test for the measurement of ametropia, he has to deduce from observed pupillary appearances the state of refraction causing them.

The student may, from time to time, test his progress towards proficiency by attempts to measure refraction by skiascopy, but familiarity with the appearances indicative of known conditions of refraction is chiefly to be sought.

The appearances upon which the attention is fixed in skiascopy are those of the red reflex in the pupil. The first step is to learn just what this appearance is and some of the variations of which it is capable. Let the beginner with his eye at the sight-hole of the skiascopic mirror throw into the observed eye, from a distance of 20 or 30 inches, the light from a lamp flame as in the ordinary ophthalmoscopic examination. Looking into the observed eye with the light properly directed, he will see the brilliant point of light, the reflection from the surface of the cornea of the lamp flame he is using ; and he may also see reflections of his own face or of other objects from the surface of the cornea. These are to be disregarded. The real object of study, the phenomena upon which attention is to be fixed, is the general red glow perceived within the pupil, the fundus reflex.

If the mirror be rotated about an axis lying in the plane of the mirror, the area of light thrown by it upon the face will move in the direction towards which the mirror is turned. As the test becomes familiar, the direction of this movement will be known without any conscious effort to discover it. With the concave mirror at a greater or lesser distance than its focus or with the plane mirror at all distances, except at the point of reversal which it is the object of the test to determine, the rotation of the mirror also causes a movement of the red reflex in the pupil. As the reflex disappears from the pupil, it is followed by an area of shadow, and, as it returns to the pupil, the shadow passes out before it. The movement of the light area really goes on when no shadow is visible in the pupil, but only when light and shade are both seen can the movement be recognized. We know the movement of light in the pupil by the movement of the boundary between light and shade.

STUDY OF THE TEST. 15

Having learned what it is that he has to watch in the pupil, the student should make himself familiar with the various appearances of the fundus reflex by viewing it from different distances, with different lenses before the eye, with different mirrors, and later, if he chooses, in a number of different eyes ; and all this without concerning himself as to the state of their refraction, or the especial significance of what he does. That is, he should learn to some extent what are the variations in the pupillary reflex, a few of which are illustrated on the following pages, before attempting to appreciate their significance.

Without a good understanding too of the simple optical principles underlying the test, it must remain a blind routine and rule of thumb work, and can never be of the highest utility. To aid in such an understanding of them, one may take a strong (15 D. to 20 D.) convex lens and a piece of card-board with a dot on it. The lens can represent the dioptric media of the eye, the card-board the retina, and the dot the light area upon the retina. The card-board should be held back of the lens a little farther than its focal distance, and the dot looked at through the lens from various distances. Nearer the lens an erect image of the dot (blurred of course), and, farther away, an inverted image will be seen, and between the two the phenomena of reversal. The movement of light on the retina may be imitated by a slight movement of the card in different directions.

The apparent enlargement of the dot, as the point of reversal is approached, and the diminution of its apparent size as the point of reversal is departed from, its diffusion and indistinctness near the point of reversal, and its concentration and greater definiteness away from the point of reversal, are to be observed. Such a combination of dot and lens will also beautifully exhibit the phenomena of aberration [See Chap. V] with its central and peripheral areas of differing movement, the one an erect and the other an in verted image. The difficulty of keeping the dot in view when the point of reversal is approached, will illustrate how small a portion of the retina is visible from the point of reversal when the test is applied to the eye. By holding in combination with the spherical lens a cylindrical lens of 5 D., the distortions of the fundus reflex produced by astigmatism, and the band-like appearances it causes at certain distances, should also be studied.

This is not all to be done at a single trial, but the lens and card should be kept at hand where they can be used to parallel and elucidate the different conditions as they arise in studying the pupillary reflex.

The study of the appearances in the eye may thus be carried on : Take an eye, the refraction of which is known, and from a distance that will give an erect movement, throw the light into the eye, and, by the rotation of the mirror, produce and study the erect movement. Then with a lens which it is known will give an inverted movement, the inverted movement is to be similarly studied. Finally the lens, or position of the observer is to be so varied as to bring the point of reversal to the eye, and the appearance of the pupil from this point is also to be studied. In these studies, and, indeed, throughout the whole course, the student will find it easier to master and understand first the appearances with the plane mirror.

If it is possible to get an eye free from astigmatism or aberration of any notable degree, these earlier studies of the appearances will be much simplified. After the above have become familiar, the phenomena of astigmatism may be studied by placing before the same eye, a cylindrical lens of known strength. The point of reversal with such

STUDY OF THE TEST. 17

a lens will give an observer the appearances presented by the pupil at this distance and at the other distances the other appearances presented in astigmatism can be obtained.

For example, suppose the eye at the student's disposal is hyperopic i D. Let him first place before it the convex 2 D lens. This will bring the point of reversal one metre from the eye. With the plane mirror, let him first study the erect movement at one-half metre ; then study the inverted movement at a distance of two metres ; then observe the eye from the point of reversal at one metre, and then vary his distance so as to study it from intermediate points.

When he takes up the study of astigmatism, he should place before such an eye, a convex cylindrical lens of 2D in addition to the spherical. Then from the distance of one-third of a metre he will be able to observe the band of light at right angles to the axis of the lens, from a distance of one metre the band of light running in the direction of the axis of the lens, and from other distances the other appearances indicative of astigmatism.

Familiarity with the many appearances due to aberration and irregular astigmatism is only to be obtained by study of eyes presenting those defects. But, as the great majority of eyes present them in notable degree, material for such a study is not difficult to obtain. Careful observations of the corresponding appearances, with the lens and card-board already referred to, will enable the beginner promptly to recognize the appearances of aberration. And, when once he has found an eye that presents them, let him observe them with different lenses, and [with the plane mirror] from varying distances.

A considerable part of the study of skiascopy and especially of the appearances of positive aberration can be carried on with the aid of an artificial, schematic, or model eye. That of Frost is one of the best, although any, even the rudest, will answer. In the studies on the human eye, it is better to study one eye long and repeatedly, or at most to confine the earlier observations to a few eyes than to attempt to employ a large number. Each additional eye will introduce variations in the appearances presented, which will at first be only^puzzling and retard, rather than assist, the mastery of the test.

Chapter II

GENERAL OPTICAL PRINCIPLES. MYOPIA, EMMETROPIA, HYPEROPIA.

Skiascopy is a method of measuring myopia, either the myopia originally present in the eye or that produced by a lens of known strength for the purpose of measurement. In myopia, we have the retina situated back of the principal focus of the dioptric media, so that the rays of a certaindivergence, that is coming from a point a certain finite distance in front of the eye, are brought to a focus upon the retina. Conversely, the rays coming from a point on the retina and passing out through the crystalline lens and cornea, are brought to a focus at the same distance in front of the eye. The point for which the eye is focused, and the point on the retina, on which the focused rays are received, have the relation of conjugate foci to the refractive surfaces of the eye.

The Reversal of Movement. — The amount of myopia is known when we know the distance of the point in front of the eye, which has this relation of a focus conjugate to the retina. Skiascopy furnishes a method of determining the position of this point. Closer to the eye, than this point for which it is focused, the observer may see an erect image of the fundus. Farther from the eye than this point, he can perceive an inverted image. Skiascopy is a means of determining when the image seen is erect and when it is inverted, or when it passes from the erect to the inverted.

This may be understood from a study of figure I. Let M represent a myopic eye, A and B being two points of the retina from which rays emerge to reach the observer's eye ; and C and D the points at which these rays coming from the retina are focused, the rays coming from A being focused at C and those from B at D.

The apparent position of a point is determined by the direction of a ray coming from that point and passing through the nodal point of the observer's eye. Suppose the observer's eye is placed at N, closer than the point for which the observed eye is focused. The apparent position of the point A is determined by a ray which passes through the upper part of the pupil and is turned down. It appears

Fig. i.

in the direction of a. The apparent position of the point B will be located by the ray coming through the lower part of the pupil and turned up. It will be seen in the direction of b. Thus, from this position N, the point A, which is really above appears above, and the point B, which is really below appears below. The observer sees an erect image.

When, however, the observer places his eye at N', at a greater distance than that for which the eye is focused, the ray which reaches his nodal point from A, will be one that comes through the lower part of the pupil and is turned up ; so that A will appear to be located in the direction of a' in the lower part of the pupil. From this

REVERSAL OF MOVEMENT. 21

position he will judge the location of B by the ray which comes through the upper part of the pupil and is turned down, so that B will appear to be located in the direction of b' in the upper part of the pupil. That is, the point A, which is really above, will appear to be below, and the point B, which is really below will appear to be above. The image observed is inverted.

The Point of Reversal. — It is evident that this change in the relation of the rays that brings about the change in the apparent position of A and B occurs at the distance of the points C and D, at which, the rays coming from the retina are focused. Here it is that these rays intersect and take their new relation which gives the reversal of the apparent position of the points of the retina from which they come.

It is, therefore, convenient in connection with skiascopy to designate this point as the point of reversal. This name indicates the significance of this point with reference to this test. Of course, it is really the same point as the far point of the myopic eye — the point for which the eye is focused — the conjugate focus of the retina — these latter names indicating the relations of the same point in other matters.

It is only when the rays leave the eye, convergent only when the eye is myopic, that they ever come to a focus in front of it. If the eye be emmetropic or hyperopic, the rays emerging parallel or divergent remain so at all distances. Hence, in emmetropia and hyperopia, there can be no point of reversal. From whatever distance the eye is viewed, the image perceived is erect.

In myopia, the distance of the point of reversal from the eye depends on the degree of convergence of the rays as they leave the cornea — depends on the amount of myopia. The distance of the point of reversal from the eye being the distance from the eye to its far point is the focal distance of the lens required to correct the myopia. So that to ascertain the amount of myopia, we have only to determine the point of reversal and then measure its distance from the eye.