by William H. Bates, M. D. Д. БейтсReprinted from New York Medical Journal, May 18, 1918, vol 107, pp. 916-924.
A STUDY OF THE IMAGES REFLECTED FROM THE CORNEA, IRIS, LENS, AND SCLERA.
By W. H. Bates, M. D.,
(From the Department of Physiology of Columbia University and the New York City Aquarium.)
It is generally believed that the accommodative power of the eye is due to a change in the curvature of the lens. This view, Helmholtz says, was first advanced by Descartes (1596-1650), while the first proofs in support of the theory were presented by Young in his celebrated treatise, On the Mechanism of the Eye, published in 1801.
The theory attracted little attention at the time, but was accepted later, mainly upon the authority of Helmholtz, whose investigations into the cause of accommodation were published about the middle of the last century. Helmholtz was led to this conclusion by what appeared to him to be changes in the size of an image, or images, reflected from the front part of the crystalline lens. It appeared to him that during accommodation these reflections were smaller than when the eye was at rest; and since an image reflected from a convex surface is diminished in proportion to the convexity of that surface, he concluded the front of the lens must become more convex during accommodation. In the cornea he observed no change, and while he believed that a change took place in the back of the lens, he considered it so slight as to be negligible. Helmholtz used for his experiments: first a candle so placed that it was reflected from the cornea and the two surfaces of the lens; and then two lights—or one doubled by reflection from a mirror—so placed behind a diaphragm having two rectangular openings that the rays shone through the openings upon the cornea and lens. Of the images thrown upon the lens by means of the naked candle he says in his Handbuch der Physiologischen Optik:
"Both these images are very much fainter than the reflection from the cornea. That from the front of the lens forms an upright image of the flame somewhat larger than that reflected from the cornea but usually so faint that the form of the flame cannot be definitely distinguished."
The results obtained when a diaphragm was used with two lights were better. Two images were then formed on each of the reflecting surfaces; and it appeared to the investigator that those on the front of the lens approached each other during accommodation and separated when the eye was at rest. (See diagram, Handbuch der Physiologischen Optik, Bd. I,, p. 122.)
Helmholtz appears to have been convinced of the correctness of these observations and of the theory based upon them, and was only doubtful of the means by which the supposed change was accomplished. His explanation of the phenomenon of accommodation was soon universally accepted, and has been universally stated as a fact. It is the accepted belief of modern ophthalmology, and has been summed up by G. E. de Schweinitz in his recent textbook on the eye as follows:
"Inasmuch as the eyeball is inextensible, it cannot adapt itself for the perception of objects situated at different distances by increasing the length of its axis, but only by increasing the refractive power of its lens." (Diseases of the Eye, pp. 35 and 36.)
There have, however, been many other theories of accommodation. Arlt ascribed the phenomenon to a lengthening of the eyeball, but later abandoned the theory out of deference to the authority of Helmholtz and Cramer. In the introduction to his treatise on shortsight (Ueber die Ursachen und die Enstehung der Kurzsichtigkeit) he says:
"An hypothesis of the mechanism of accommodation (movement of the posterior wall of the eye—Locomotion der hinteren Augenwand) which later was proven to be untenable led me to the question whether, in myopia, the eyeball, as was to be expected according to that hypothesis, might be lengthened in the direction of the sagittal axis, and in the course of time it was possible to present anatomical proof that shortsight was generally associated with such a lengthening, due to a permanent bulging (Rückdrängung) of the posterior wall
... Since the introduction of the ophthalmoscope into ophthalmological practice and since the demonstration by Cramer and Helmholtz that accommodation is effected through a change in the form of the lens, not of the eyeball, many different theories as to the origin and development of shortsight in relation to the aforementioned deviations from the normal in the shape of the eyeball have been advanced and defended."
By some the muscles of the eye were believed to play a part in accommodation. Of this theory Donders says:
"Before physiologists were acquainted with the changes of the dioptric system they often attached importance to the external muscles in the production of accommodation. Now that we know that accommodation depends on a change of form in the lens this opinion seems scarcely to need refutation." (On the Anomalies of Accommodation and Refraction of the Eye, p. 22.)
According to other theories, accommodation is effected by a change in the curvature of the cornea; by a change in the position of the lens; by the contraction of the pupil; through the agency of the iris and so on. There have also been many hypothesis as to the means by which the supposed change in the curvature of the lens was accomplished, some of the guesses being so wild that Donders refused even to refer to them, considering they would detract from the scientific character o£ his work.
My own experiments, carried on during the last five years in the Physiological Laboratory of the College of Physicians and Surgeons, Columbia University, New York, and at the New York City Aquarium, demonstrate that the lens is not a factor in accommodation, but that the change of focus necessary for perfect vision at different distances is effected by a change in the length of the eyeball, brought about by the action of the muscles on the outside of the globe. In the earlier of these experiments, the results of which were published in the New York Medical Journal of May 8, 1915, it was found that accommodation, as measured by the objective test of simultaneous retinoscopy, occurred in all normal eyes of dogs, rabbits, and fish after the removal of the lens, and that it never occurred after one or both of the oblique muscles had been cut across and the insertion of the muscle to the fascia completely separated. It was also found that any form of refractive error could be produced in the normal eyes of these animals by manipulation of the outside muscles of the eyeball, indicating that these conditions are not due to permanent deformations in the shape of the eyeball, as generally believed.
By normal eyes is meant those in which, in addition to other conditions of a healthy structure, both oblique muscles are present and active. In some animals it was found that one oblique muscle was absent or rudimentary. This was true in the case of all cats, and accommodation could never be produced in these animals by stimulation with electricity. Even in cats, however, when the rudimentary oblique muscle was strengthened by advancement, accommodation was always produced by stimulation of the eyeball, or of the third or fourth nerves, near their origin in the brain, the fourth nerve, contrary to previous belief, being just as much a nerve of accommodation as the third.
After the results of these experiments were published it was suggested to me by Dr. Frederic S. Lee that it would be well for me to repeat the experiments of Helmholtz, making a thorough investigation of accommodation from a study of the images reflected from the front of the crystalline lens and other parts of the eyeball. This work was undertaken some four years ago. For a year or more I was unable to obtain an image from the front of the lens which was sufficiently clear or distinct to be measured. It was much blurred, and because of this lack of distinctness, it was impossible to tell whether it became smaller or larger during accommodation. With a diaphragm I got a clearer image, but it still was not sufficiently clear to be measured. To Helmholtz the indistinct image of the naked candle seemed to show an appreciable change, while the images obtained by the aid of a diaphragm showed it more clearly; but I was unable, either with a diaphragm or without it, to obtain images which I considered sufficiently distinct to be reliable. Men who had been teaching and demonstrating Helmholtz's theory repeated his experiments for my benefit, but the images which they obtained on the front of the lens did not seem to me to be any better than my own.
After a year or more of failure I began work at the Aquarium on the eyes of fishes. It was a long story of failure. Finally, I became able, with the aid of a strong light—1,000 watts—a diaphragm with a small opening, and a condenser, to obtain, after some difficulty, a clear and distinct image from the cornea of fish. This image was sufficiently distinct to be measured, and after many months a satisfactory photograph was obtained. Then the work was resumed at the Physiological Laboratory on the eyes of human beings. By means of nearly the same technic an image was obtained on the front of the lens which was sufficiently clear and distinct to be photographed. This was the first time, so far as I have been able to ascertain, that a clear image was ever photographed from the front of the lens. The work was continued, until, after almost four years of constant labor, I finally obtained satisfactory pictures, not only from the front of the lens, but also from the iris, cornea, the front of the sclera, and the side of the sclera.
TECHNIQUE BY WHICH THE IMAGES WERE OBTAINED.
THE FRONT OF THE LENS.
Strength of the light.—Experiments were made first with a candle and then with electric lights of thirty watts, fifty watts, 250 watts, and 1,000 watts.
With a candle as a source of light a clear and distinct image could be obtained on the cornea. On the posterior surface of the lens it was quite clear; but on the front of the lens it was very imperfect, undefined, and of extremely variable intensity. At times no reflection could be obtained at all, while at others the size varied within wide limits, regardless of the angle of the light to the eye of the subject, or of the eye of the observer to that of the subject. Again the size might remain very nearly the same; and yet there would be a wide variation in the appearance of the image. After studying these appearances almost daily for more than a year, no reliable observation could be made. In fact, it seemed that an infinite number of variable appearances, or images, might be obtained on the front of the lens when a candle was used as the source of illumination.
With a thirty watt lamp, a fifty watt lamp, a 250 watt lamp, and a 1,000 watt lamp there was no improvement. The light of the sun reflected from the front of the lens produced an image just as cloudy and uncertain as the reflections from other sources of illumination, and just as variable in shape and size. To sum it all up, I was convinced that the front surface of the lens was a very poor reflector of light, and that no reliable reflections could be obtained from it by the means described. But with a condenser and diaphragm, the use of which was suggested by their use to improve the illumination of a glass slide under the microscope, and a 1,000 watt lamp, satisfactory results were at last obtained, although many difficulties still remained to be overcome. The addition of a condenser and the use of a strong light proved to be a decided improvement over the method of Helmholtz (Fig. 1).
Reflections.—Complicating reflections were a perpetual source of trouble. Reflections from surrounding objects were easily prevented, but those from the sides of the globe were difficult to deal with, and it was useless to try to obtain images on the front of the lens until they had been eliminated, or reduced to a minimum, by a proper adjustment of the light. The same adjustment, however, did not always give similar results. Sometimes there would be no reflections for days; then would come a day when, with the light apparently at the same angle, they would reappear. When the light was placed below the point of fixation the best results were usually obtained by directing the long axis of the globe exactly toward the eye and then tipping the front downward so that the angle of its axis to a line drawn from the light to the eye was about ten degrees. By this adjustment reflections were often prevented entirely. The subject was able to tell when a satisfactory adjustment had been obtained by regarding the reflection of his eye in a concave mirror.
Multiple images.—With some adjustments of the light, multiple images were seen reflected from the front of the lens (Fig. 2). Sometimes these images were arranged in a horizontal line, sometimes in a vertical one and sometimes at angles of different degrees, while their distance from each other also varied. Usually there were three of them. Sometimes there were more; and sometimes only two. Occasionally they were all of the same size, but usually they varied, there being apparently no limit to their possibilities of change in this and other respects. Some of them were photographed, indicating that they were real reflections. Changes in the distance of the diaphragm from the light and from the condenser, and alterations in the size and shape of its opening, appeared to make no difference. Different adjustments of the condenser were equally without effect. Changes in the angle at which the light was adjusted sometimes lessened the number of images and sometimes increased them, until at last an angle was found at which but one image was seen. The images appear, in fact, to have been caused by reflections from the globe of the electric light.
Distinctness of the image.—Even after the light had been so adjusted as to eliminate reflections it was often difficult or impossible to get a clear and distinct image of the electric filament upon the front of the lens. One could rearrange the condenser and the diaphragm and change the axis of fixation, and still the image would be clouded or obscured and its outline distorted. The cause of the difficulty appeared to be that the light was not adjusted at the best angle for the purpose, and I was not always able to determine exactly what this was. As in the case of the reflections from the sides of the globe, it seemed to vary without a known cause. There were, however, angles of the axis of the globe which gave better images than others, although these angles could not be determined with exactness. I have labored with the light for two or three hours without finding the right angle. At other times the axis would remain unchanged for days, giving always a clear, distinct image.
It was interesting to note that there were angles of the line of the light to the eye at which a clear and distinct image could be obtained from the iris, and none whatever from the front of the lens; also that with some adjustments no image could be obtained from the cornea, although the cornea is a much better reflecting surface than any other part of the eye. When the adjustments were such that an image could be obtained from the front of the lens, however, one could always be obtained from the iris, or the front of the sclera, and sometimes from the cornea also.
Distance of the light from the observed eye.—The distance of the light from the observed eye was very important. By experiment it was found that when the lamp was adjusted at a distance of nearly sixty-five inches from the eye an image of a desirable size could be obtained on the front of the lens; that is, it almost filled the area of the moderately dilated pupil of a normal eye. When the light was brought closer, the image obtained in the pupil was too large, less clearly defined, and. less bright. With the light at a greater distance than sixty-five inches the image, although brighter and more distinct, was so small that it could not be so readily measured.
The diaphragm.—The diaphragm was usually a piece of cardboard from two to six inches square, with a small opening in the centre. The smaller the opening the more distinct the image, but it was also less bright than when the opening was larger. When the opening was too large, or when the diaphragm was not used at all, the image obtained was very cloudy and indistinct. An opening one eighth to a quarter of an inch in diameter was found to be the most satisfactory. If it were made smaller, so little light was thrown upon the front of the lens that no distinct reflection was obtained. The shape of the opening seemed to be immaterial, as good results were obtained whether it was round, triangular, or square, regular or irregular. The distance of the diaphragm from the light and from the eye was very important. By varying this, one could increase or diminish the size of the image, its brightness or its distinctness. The closer it was placed to the eye, within certain limits, the smaller, more distinct, but less bright the image. Usually it was placed about forty-eight inches from the light. When brought closer than this, with a small opening, an image could be obtained on the front of the lens without the aid of the condenser; but it was not sufficiently clear or distinct. It should be emphasized that changes in the size of the opening, or in the distance of the diaphragm from the light, would alter very, materially the size of the image reflected from the lens.
The adjustment of the opening in the diaphragm in its relation to the light was best made by the subject, who regarded the light with the condenser removed, using a blue glass screen to mitigate its intensity. When the subject obtained an adjustment of the opening which enabled him to see the light clearly, the diaphragm was moved to his right until the light was just at the edge, or beyond the edge of the opening. This adjustment of the diaphragm in its relation to the light and the left eye of the subject, yielded better results after the condenser was adjusted than when the light could be seen by the subject through the opening with the condenser removed.
The concave mirror.—The mirror was three and a half inches in diameter, with a focus of nine inches, though a smaller mirror might be used or a plane one, but the latter would not be so satisfactory, because the image would not be seen so clearly as when magnified in a concave glass. The mirror was supported at the end of a horizontal bar, with its plane at right angles to the line of fixation, and its centre at the same height from the table as the eye of the subject. The horizontal bar moved back and forth in the opening of an arm supported by a stand, and an adjustment was used whereby the arm could be raised or lowered, and turned at different angles on a horizontal plane. The horizontal bar was placed in the axis of vision, and when the mirror was properly adjusted, it could be moved toward or away from the eye, without altering the angle of fixation when the subject regarded the reflection of the image upon the front of the lens. The mirror was a great convenience in adjusting the diaphragm, the condenser, and the light; because the image was seen therein by the subject more clearly than by the observer, and the former could, therefore, determine the accuracy of the adjustments better than any one else. When the light was placed on a level with the eye it was necessary, in order that the subject might see past the condenser and observe the reflection of his own eye in the mirror, to place the latter in such a way that the axis of vision was at least ten degrees to one side of the line of the light. When the light was lowered ten degrees or more below the axis of vision, the mirror was placed directly over the line from the eye to the light, in order to enable the subject to see his own eye in the mirror over the top of the condenser. When the mirror was adjusted as close to the line of sight as it was possible to place it, clear and distinct images were seen by the subject reflected from various parts of his eye. Photographs were taken with the axis of vision not less than ten degrees from the line of the light to the eye. It should be understood that the images were photographed from the eye itself, not from the reflection in the concave mirror.
The condenser.—The condenser was a convex 11 D.S., about an inch and a half in diameter. This strength was found to be the most satisfactory in obtaining clear and distinct images. A stronger lens produced a brighter and smaller image; it had to be brought closer to the eye; and its adjustment required more careful manipulation, this being the greatest objection to its use. With a weaker condenser, +6.00 D.S., the image was too large for the size of the pupil. The condenser was supported by a stand, with an adjustment by which it could be raised or lowered, rotated either on its vertical or its horizontal axis, and moved nearer to or farther from the eye as desired. In nearly all cases the best results were obtained when the condenser was supported vertically, and was held nearly at an angle of ninety degrees to the line from the light to the eye. When tipped on its vertical or its horizontal axis five degrees, or even less, toward the light, or away from it, a clear and distinct image could not be obtained. Without a diaphragm the image focussed by the condenser on the lens was cloudy; but with a diaphragm, a clear and distinct image was obtained with the condenser at about three inches from the eye. With a diaphragm, and the condenser at more than four inches from the eye, a faint and unsatisfactory image was sometimes produced. This was the cause of much trouble until the fact that there were two points at which an image could be obtained was discovered. Without the diaphragm, one of these points, the more distant one, was eliminated. It was therefore found to be an advantage to focus the condenser with the diaphragm removed, and then, after replacing the latter, to continue the adjustments. The conditions under which the fainter image was produced, with the condenser at a greater distance from the eye, were not discovered. The fact is mentioned, however, for the benefit of any one who may desire to repeat these observations.
Apparatus for supporting the head.—The most difficult part of the technique had to do with the apparatus for holding the head of the subject perfectly steady while the pictures were being taken. A rod of metal, firmly supported horizontally and covered with a sheet of paper, was grasped by the teeth, and served to hold that part of the head steady. A second horizontal rod pressed against the forehead, and it was sometimes an advantage to have a vertical rod pressing on the right temple. The subject was seated in a comfortable position.
A thirty candle power lamp—simply an ordinary electric globe—is sufficient to form a very large image on the cornea. It can be placed within an inch or two of the eye, as the heat is not great enough to interfere with the experiment. The closer it is, the larger the image. A blue glass screen can be used, if desired, to lessen the discomfort of the light, as the photographs of the image and the time of exposure will be the same whether the light is blue or white. The white light, however, is easier to focus than the blue. For absolute accuracy the light should be immovable, but for demonstration this is not essential. The subject can hold the bulb in his hand; and can demonstrate that the image varies according to whether the eye is at rest, accommodating normally for near vision, or straining to see at a near or a distant point. The clearness of the image may vary according to whether the light is adjusted vertically, horizontally, or it an angle. When the left eye is used by the subject—and in all the experiments it was found to be the more convenient one for the purpose—the source of light is placed to the left of that eye, and as much as possible in front of it at an angle of about forty-five degrees. For demonstration it is not necessary that the eye of the subject should be immovable. He can look into a plane mirror, or into a concave one, which enlarges the image, using the image itself as the point of fixation, and the distance at which the eye focus can be altered by changing the distance of the mirror from the eye. The mirror should be fastened to a rod which moves in a groove backward and forward, and the angle of the rod must be so adjusted that the angle of fixation does not change when the mirror approaches the eye, or is withdrawn from it. The eye should be able to see the reflection by looking straight ahead, and the closer the reflection is to the edge of the mirror on the camera side the closer the camera can be brought to the line of fixation (Fig. 3).
Usually, not always, the retinoscope indicates that the eye is at rest—emmetropic—at the farthest distance of the mirror from the eye at which the subject is able to see the details of the reflection clearly. The greatest amount of accommodation is obtained at the nearest point at which the filament of the electric light can be seen distinctly. At this point the filament is distinctly smaller than when the eye is at rest. When the mirror is moved so far away that the image is no longer seen clearly, and the eye strains to see it more distinctly, the retinoscope indicates myopic refraction and the image again becomes smaller than when the eye is at rest. When the mirror is brought so close to the eye that the image appears indistinct and the eye again strains to see it more distinctly, the retinoscope indicates less myopic refraction and the image becomes larger. If the strain to see it is great enough, the eye becomes hypermetropic, and the image appears larger than when the eye is at rest. All these changes in the size and shape of the image can be correctly observed by the subject.
The angle of the camera to the optic axis is not very important. Better pictures can be obtained, however, when the camera is directed as nearly as possible on a line with the optic axis. Satisfactory pictures are obtained when the angle is thirty, forty or even sixty degrees; but after passing beyond sixty the results are not at all good. Generally it is not possible to get an angle smaller than ten degrees. While the photographs are being taken a screen should be placed between the light and the mirror to prevent the formation of a double image on the cornea.
THE SIDE OF THE SCLERA.
To obtain an image from the side of the sclera, a plane mirror was used in addition to the concave one and other apparatus previously mentioned. It was about three inches in diameter, was supported on a stand at about the height of the eye, and was held vertical to the surface of the table, with one edge resting against the left temple of the subject and the opposite edge tipped about thirty degrees from the plane of the temple toward the nose. The concave mirror was so placed that the horizontal bar which supported it made an angle of about eighty, degrees with the line from the eye to the right. When the two mirrors were properly adjusted, the image of the filament was reflected from the plane mirror into the concave mirror, where it was seen by the subject an inch or more above the centre. The concave mirror was so adjusted that when it was moved nearer to, or farther away from the eye, the angle of fixation did not change. The condenser was slightly, perhaps half an inch, farther from the eye than from the centre of the plane mirror, and was almost in contact with the edge of the mirror on the side nearest the light. Numerous very small reflections from the neighborhood of the sclera were a source of failure which was not easily overcome. Sometimes these reflections were very numerous when the image was reflected from the side of the sclera, and absent when it was reflected from the part nearer, the cornea. They were finally eliminated by adjustments of the light. Another difficulty was the dropping of the upper eyelid. This occurred when the point of fixation was lower than the eye, and was corrected when the eye looked more nearly straight or slightly above the horizon. To accomplish this the concave mirror was lowered part of an inch. The camera was placed where the object glass was seen by the subject in the space between the two mirrors. The axis of the camera made an angle with the line from the light to the eye of about fifteen degrees. The adjustments of the light, diaphragm, condenser, chin rest, head rest, two mirrors and the camera required a great deal of care. The subject was placed in a comfortable position to avoid the slightest strain, and during the few seconds of exposure of the plate the breath was held, because the act o£ breathing was sufficient to produce a movement of the eye. In order to illuminate the general surface of the eye during the time the plate was exposed two thirty candle power lamps were placed on the table (Fig. 4).
THE POSTERIOR SURFACE OF THE LENS.
In order to see the image reflected from the posterior surface of the lens a telescope was employed, the telescope of the ophthalmometer being utilized, for convenience, after the removal of the prism, which produced a double image. A thirty candlepower lamp was placed as close as possible to the tube just below the distal end and secured immovably. The head of the subject was also held immovable by a head rest. A plane mirror, two inches by one inch, had a letter of diamond type pasted on it below the centre and near the left edge, as regarded by the subject. This mirror was supported by the subject in contact with the right half of the objective glass, with the letter of diamond type in the line of the horizontal axis of the tube. Although one half of the end of the tube was covered by the mirror, no difficulty was experienced in obtaining a good view of the image reflected from the posterior surface of the lens. Twenty feet behind and above the head of the subject was hung a Snellen test card, and by tipping the mirror slightly he was able to read, reflected in it, without changing the line of fixation, a letter of the twenty line. When the subject regarded the small letter on the mirror at five inches simultaneous retinoscopy indicated the focus of the eye to be -8.00 D.S. When the letter on the Snellen test card was regarded without change in the position of the mirror, simultaneous retinoscopy indicated that the eye was at rest. At times the letter on the mirror was recognized by the subject when the accommodation was less or more than -8.00 D.S. When this happened, the fact was revealed by the retinoscope. During these changes of focus the observer was unable to note any change in the size or form of the image reflected from the posterior surface of the lens. Several persons have repeated this experiment and confirmed the original observations. Potential sources of error in the experiment were the possibility that the subject might not accommodate accurately, and possible movements of the eye and head. The first was eliminated by the use of the retinoscope, the second by an arrangement of the letter on the mirror and the letter reflected from the Snellen test card in such a way that either could be seen without altering the line of fixation; and the third by the head rest. The experiment was the first of the series described which was successful, the image being obtained without difficulty about three years ago.
IRIS AND FRONT OF THE SCLERA.
Images on the iris and the front of the sclera were obtained by the same technique as was used for the front of the lens (Fig. 1). It was interesting to find that when the angle of the line of light to the eve and the line of fixation was as small as possible, about ten degrees, an image could be obtained on the iris without obtaining one on the cornea or lens. The camera was placed as close as possible to the line of fixation, its axis forming an angle of ten degrees with the line of fixation. The light was placed ten degrees below the horizon, and the line of fixation was directed to the concave mirror just above the upper edge of the condenser.
Although precautions were taken to prevent any movement of the head of the subject during the time the pictures were being taken, or while the images were being studied by the observer, and the subject even refrained from breathing for the five or ten seconds during which the plate was exposed, photographs usually showed, in addition to changes of size, manifest changes in the location of the images and changes in the exposed parts of the eyeball. This is what would be expected as the result of an elongation of the eyeball during the production of myopic or hypermetropic refraction. In many of the photographs it seemed that the diameter of the iris was increased or diminished. In some cases a larger or a smaller area of sclera was exposed. A protrusion or a recession of the eyeball often occurred. However, it should be emphasized that in spite of changes in the location of the image, before and after changes of refraction, the changes in its size were always what one would expect under the circumstances.
Lens.—Images reflected from the front (Fig. 5) and back of the lens showed no change in size during accommodation.
Front of the sclera.—Images reflected from the front of the sclera (Fig. 6) always showed marked changes when the refraction was changed, no matter whether the line of fixation was ten or ninety degrees from the light. When an effort was made to see, unsuccessfully, at a distance, simultaneous retinoscopy always indicated myopic refraction and the image always became smaller than when the eye was at rest, indicating that the front of the sclera had become more convex. The change was greater than those occurring under similar conditions with images reflected from other parts of the eye. During accommodation of 3.00 D., 6.00.D., or 8.00 D., measured by the retinoscope, the image became relatively much smaller than did images reflected from other parts of the eye when a similar change of refraction took place. Similarly, when hypermetropic retraction of 2.00 D., or more, was produced by an unsuccessful effort to see near, the image became relatively much larger than images reflected from other parts of the eye when the same degree of hypermetropic refraction was produced. The most marked changes in the shape of the eyeball obtained during these experiments were manifested by the front of the sclera, the changes in the size of the images reflected from the side of the sclera, the cornea, and the. iris being so slight that sometimes they were scarcely apparent in the photographs, although they were always plainly apparent to the subject when magnified in the concave mirror, and could also be seen by the observer without the mirror.
The side of free sclera.—The changes observed in the images reflected from the side of the sclera (Fig. 7) were exactly the opposite of those noted on the front of the sclera, being larger where the former were smaller and vice versa. When an effort was made to see at a distance the image reflected from the side of the sclera was larger than the image obtained when the eye was at rest, indicating a flattening of the side of the sclera, a condition which one would expect when the eyeball was elongated. The image obtained during normal accommodation was also larger than when the eye was at rest, indicating again a flattening of the side of the sclera. The image obtained, however, when an effort was made to see near, was much smaller than any of the other images, indicating that the sclera had become more convex at the side, a condition which one would expect when the eyeball was shortened, as in hypermetropia. The changes of the images on the side of the sclera were not so marked as those on the front of the sclera, but the alterations in size were always sufficient to be readily recognized by the subject in the concave mirror, and by the observer without the mirror. They could be observed when the angle of the line of fixation to the line of the light to the eye was sixty degrees, or even less. The photographs usually showed changes, but to a less marked degree because, owing to the difficulty of photographing a white image on a white background, they were imperfect.
The cornea.—When the images reflected from the cornea were small no change in size was observed under varying conditions of refraction. When the images were large (Fig. 8) a series of slight changes similar to those noted on the front of the sclera could be observed. The change in the curvature of the cornea during accommodation is so slight that the ophthalmometer, with its small image, fails to show it, and has therefore been supposed to demonstrate that the cornea did not change during accommodation. The method described accomplishes what the ophthalmometer has failed to do.
The iris.—Images reflected from the iris were more readily obtained than those from the cornea or lens, and slight variations in size were always apparent to the observer and subject when hypermetropic or myopic refraction was produced, but these, however, were not always evident in the photographs.
These studies of the images reflected from the various parts of the eyeball demonstrate:
The accommodation of the eye is effected by an elongation of the eyeball.
The lens is not a factor in accommodation.
Myopia is produced by a strain to see distant objects.
Hypermetropia is produced by a strain to see near objects.
They have, therefore, confirmed my previous conclusions regarding the mechanism of accommodation, based on experiments on the eyes of animals, and also my earlier conclusions as to the cause of myopia and hypermetropia, based on observations with the retinoscope and published in the New York Medical Journal of March 16, 1912 [link].
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