31. Multiple Images Upon the Front of the LensThis picture illustrates one of the difficulties that had to be overcome in photographing images reflected from various parts of the eyeball. Unless the light was adjusted at precisely the right angle the filament was multiplied by reflection from the sides of the globe. Usually the image was doubled, sometimes it was tripled, as shown in the picture, and sometimes it was quadrupled. Often days of labor were required to eliminate these reflections, and for reasons that were not definitely determined the same adjustment did not always give the same results Sometimes all would go well for days, and then, without any apparent reason, the multiple images would return.
The images photographed from the cornea and from the front and side of the sclera showed, however, a series of four well-marked changes, according to whether the vision was normal or accompanied by a strain. During accommodation the images from the cornea were smaller than when the eye was at rest, indicating elongation of the eyeball and a consequent increase in the convexity of the cornea. But when an unsuccessful effort was made to see at the near-point, the image became larger, indicating that the cornea had become less convex, a condition which one would expect when the optic axis was shortened, as in hypermetropia. When a strain was made to see at a distance the image was smaller than when the eye was at rest, again indicating elongation of the eyeball and increased convexity of the cornea.
The images photographed from the front of the sclera showed the same series of changes as the corneal images, but those obtained from the side of the sclera were found to have changed in exactly the opposite manner, being larger where the former were smaller and vice versa, a difference which one would naturally expect from the fact that when the front of the sclera becomes more convex the sides must become flatter.
Fig. 32. Reflection of the Electric Filament From the IrisThis picture is shown to illustrate the fact that it is possible to get a reflection from any reflecting surface of the eyeball without reflections from the other parts, although these may be exposed. This is done by changing the angle of the light to the eye. In No. 1 observations of the eye at the time the picture was taken demonstrated that the image was from the iris, not from the cornea, and the fact is also apparent in the picture. (Compare the image with the corneal reflection in Fig. 28.) In No. 2, where the image overlaps the margin of the pupil, the fact that the reflection is from the iris is manifest from the circumstance that only part of the filament is seen. If it were from the cornea, the whole of it would be reflected. Note in this picture that there is no reflection from the lens. The images on the iris did not change their size or shape during accommodation, demonstrating again that the lens, upon which the iris rests, does not change its shape when the eye adjusts itself for near vision.
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 that this part of the sclera had become less convex or flatter, because of elongation of the eyeball. 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 most pronounced of the changes were noted in the images reflected from the front of the sclera. Those on the side of the sclera were less marked, and, owing to the difficulty of photographing a white image on a white background, could not always be readily seen on the photographs. They were always plainly apparent, however, to the observer, and still more so to the subject, who regarded them in a concave mirror. The alterations in the size of the corneal image were so slight that they did not show at all in the photographs, except when the image was large, a fact which explains why the ophthalmometer, with its small image, has been thought to show that the cornea did not change during accommodation. They were always apparent, however, to the subject and observer.
Fig. 33. Demonstrating That the Back of the Lens Does Not Change During Accommodation.The filament of an electric light (L) is shining into the eye of the subject (S), and the reflection on the back of the lens can be seen by the observer (O) in the telescope (T). The subject holds in her hand, at a distance of four inches, a mirror on which is pasted a small letter, and in which is reflected a Snellen test card hung above and behind her head at a distance of twenty feet. The retinoscope reveals that when she looks at the reflection of the test card and reads the bottom line the eye is at rest, and that when she looks at the letter pasted on the mirror it accommodates. The image on the lens does not change during these changes of focus. The telescope is the telescope of the ophthalmometer, the prisms having been removed. As there is no dispute about the behavior of the back of the lens during accommodation this image was not photographed.
The corneal image was one of the easiest of the series to produce and the experiment is one which almost anyone can repeat, the only apparatus required being a fifty-candlepower lamp - an ordinary electric globe - and a concave mirror fastened to a rod which moves back and forth in a groove so that the distance of the mirror from the eye can be altered at will. A plane mirror might also be used; but the concave glass is better, because it magnifies the image. The mirror should be so arranged that it reflects the image of the electric filament on the cornea, and so that the eye of the subject can see this reflection by looking straight ahead. The image in the mirror is used as the point of fixation, and the distance at which the eye focuses is altered by altering the distance of the mirror from the eye. The light 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, and according to whether it is adjusted vertically, horizontally, or at an angle, the clearness of the reflection may vary. A blue glass screen can be used, if desired, to lessen the discomfort of the light. If the left eye is used by the subject - and in all the experiments it was found to be the more convenient for the purpose - the source of light should be placed to the left of that eye and as much as possible to the front of it, at an angle of about forty-five degrees. For absolute accuracy the light and the head of the subject should be held immovable, but for demonstration this is not essential. Simply holding the bulb in his hand the subject can demonstrate that the image changes according to whether the eye is at rest, accommodating normally for near vision, or straining to see at a near or a distant point.
In the original report were described possible sources of error and the means taken to eliminate them.
1. Bates: The Cause of Myopia, N. Y. Med. Jour., March 16, 1912.
CHAPTER VI - THE TRUTH ABOUT ACCOMMODATION AS DEMONSTRATED BY CLINICAL OBSERVATIONS
THE testimony of the experiments described in the preceding chapters to the effect that the lens is not a factor in accommodation is confirmed by numerous observations on the eyes of adults and children, with normal vision, errors of refraction, or amblyopia, and on the eyes of adults after the removal of the lens for cataract.
It has already been pointed out that the instillation of atropine into the eye is supposed to prevent accommodation by paralyzing the muscle credited with controlling the shape of the lens. That it has this effect is stated in every text-book on the subject,1 and the drug is daily used in the fitting of glasses for the purpose of eliminating the supposed influence of the lens upon refractive states.
In about nine cases out of ten the conditions resulting from the instillation of atropine into the eye fit the theory upon which its use is based; but in the tenth case they do not, and every ophthalmologist of any experience has noted some of these tenth cases. Many of them are reported in the literature, and many of them have come under my own observation. According to the theory, atropine ought to bring out latent hypermetropia
