the vision, and absolutely unavailable for the study of the refraction of the lower animals, of infants, and of adult human beings under the conditions of life.
The test card and trial lenses can be used only under certain favorable conditions, but the retinoscope can be used anywhere. It is a little easier to use it in a; dim light than in a bright one, but it may be used in any light, even with the strong light of the sun shining directly into the eye. It may also be used under many other unfavorable conditions.
It takes a considerable time, varying from minutes to hours, to measure the refraction with the Snellen test card and trial lenses. With the retinoscope, however, it can be determined in a fraction of a second. By the former method it would be impossible, for instance, to get any information about the refraction of a baseball player at the moment he swings for the ball, at the moment he strikes it, and at the moment after he strikes it. But with the retinoscope it is quite easy to determine whether his vision is normal, or whether he is myopic, hypermetropic, or astigmatic, when he does these things; and if any errors of refraction are noted, one can guess their degree pretty accurately by the rapidity of the movement of the shadow.
With the Snellen test card and trial lenses conclusions must be drawn from the patient's statements as to what he sees; but the patient often becomes so worried and confused during the examination that he does not know what he sees, or whether different glasses make his sight better or worse; and, moreover, visual acuity is not reliable evidence of the state of the refraction. One patient with two diopters of myopia may see twice as much as another with the same error of refraction. The evidence of the test card is, in fact, entirely subjective; that of the retinoscope is entirely objective, depending in no way upon the statements of the patient.
In short, while the testing of the refraction by means of the Snellen test card and trial lenses requires considerable time, and can be done only under certain artificial conditions, with results that are not always reliable, the retinoscope can be used under all sorts of normal and abnormal conditions on the eyes both of human beings and the lower animals; and the results, when it is used properly, can always be depended upon. This means that it must not be brought nearer to the eye than six feet; otherwise the subject will be made nervous, the refraction, for reasons which will be explained later, will be changed, and no reliable observations will be possible. In the case of animals it is often necessary to use it at a much greater distance.
For thirty years I have been using the retinoscope to study the refraction of the eye. With it I have examined the eyes of tens of thousands of school children, hundreds of infants and thousands of animals, including cats, dogs, rabbits, horses, cows, birds, turtles, reptiles and fish. I have used it when the subjects were at rest and when they were in motion - also when I myself was in motion; when they were asleep and when they were awake or even under ether and chloroform. I have used it in the daytime and at night, when the subjects were comfortable and when they were excited; when they were trying to see and when they were not; when they were lying and when they were telling the truth; when the eyelids were partly closed, shutting off part of the area of the pupil, when the pupil was dilated, and also when it was contracted to a pin-point; when the eye was oscillating from side to side, from above downward and in other directions. In this way I discovered many facts which had not previously been known, and which I was quite unable to reconcile with the orthodox teachings on the subject. This led me to undertake the series of experiments already alluded to. The results were in entire harmony with my previous observations, and left me no choice but to reject the entire body of orthodox teaching about accommodation and errors of refraction. But before describing these experiments I must crave the readers patience while I present a resume of the evidence upon which the accepted views of accommodation are based. This evidence, it seems to me, is as strong an argument as any I could offer against the doctrine that the lens is the agent of accommodation, while an understanding of the subject is necessary to an understanding of my experiments.
1. Herman Snellen (1835-1908). Celebrated Dutch ophthalmologist, professor of ophthalmology in the University of Utrecht and director of the Netherlandic Eye Hospital. The present standards of visual acuity were proposed by him, and his test types became the model for those now in use.
CHAPTER III - EVIDENCE FOR THE ACCEPTED THEORY OF ACCOMMODATION
THE power of the eye to change its focus for vision at different distances has puzzled the scientific mind ever since Kepler1 tried to explain it by supposing a change in the position of the crystalline lens. Later on every imaginable hypothesis was advanced to account for it. The idea of Kepler had many supporters. So also had the idea that the change of focus was effected by a lengthening of the eyeball. Some believed that the contractive power of the pupil was sufficient to account for the phenomenon, until the fact was established, by the operation for the removal of the iris, that the eye accommodated perfectly without this part of the visual mechanism. Some, dissatisfied with all these theories, discarded them all, and boldly asserted that no change of focus took place,2 a view which was conclusively disproven when the invention of the ophthalmoscope made it possible to see the interior of the eye.
The idea that the change of focus might be brought about by a change in the form of the lens appears to have been first advanced, according to Landolt,3 by the Jesuit, Scheiner (1619). Later it was put forward by Descartes (1637). But the first definite evidence in support of the theory was presented by Dr. Thomas Young in a paper read before the Royal Society in 1800.4 "He adduced reasons," says Donders, "which, properly understood, should be taken as positive proofs."5 At the time, however, they attracted little attention.
Fig. 9. Diagrams of the Images of PurkinjeNo. 1. - Images of a candle: a, on the cornea; b, on the front of the lens- c, on the back of the lens.No. 2. - Images of lights shining through rectangular openings in a screen while the eye is at rest (R) and during accommodation (A): a, on the cornea; b, on the front of the lens; c, on the back of the lens (after Helmholtz).Note that in No. 2, A, the central images are smaller and have approached each other, a change which, if it actually took place would indicate an increase of curvature in the front of the lens during accommodation.
About half a century later it occurred to Maximilian Langenbeck6 to seek light on the problem by the aid of what are known as the images of Purkinje.7 If a small bright light, usually a candle, is held in front of and a little to one side of the eye, three images are seen: one bright and upright; another large, but less bright, and also upright; and a third small, bright and inverted. The first comes from the cornea, the transparent covering of the iris and pupil, and the other two from the lens, the upright one from the front and the inverted one from the back. The corneal reflection was known to the ancients, although its origin was not discovered till later; but the two reflections from the lens were first observed in 1823 by Purkinje; whence the trio of images is now associated with his name. Langenbeck examined these images with the naked eye, and reached the conclusion that during accommodation the middle one became 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 that the front of the lens became more convex when the eye adjusted itself for near vision. Donders repeated the experiments of Langenbeck, but was unable to make any satisfactory observations. He predicted, however, that if the images were examined with a magnifier they would "show with certainty" whether the form of the lens changed during accommodation. Cramer,8 acting on this suggestion, examined the images as magnified from ten to twenty times, and thus convinced