THE power of the eye to change its focus for vision at different distances has puzzled the scientific mind ever since Kepler (see note 1) 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, (see note 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, (see note 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 (see note 4). “He adduced reasons,” says Donders, “which, properly understood, should be taken as positive proofs” (see note 5) . At the time, however, they attracted little attention.
- No. 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 Langenbeck (see note 6) to seek light on the problem by the aid of what are known as the images of Purkinje (see note 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 (see note 8), acting on this suggestion, examined the images as magnified from ten to twenty times, and thus convinced himself that the one reflected from the front of the lens became considerably smaller during accommodation.
Subsequently Helmholtz, working independently, made a similar observation, but by a somewhat different method. Like Donders, he found the image obtained by the ordinary methods on the front of the lens very unsatisfactory, and in his “Handbook of Physiological Optics” he describes it as being “usually so blurred that the form of the flame cannot be definitely distinguished (see note 9). So he placed two lights, or one doubled by reflection from a mirror, behind a screen in which were two small rectangular openings, the whole being so arranged that the lights shining through the openings of the screen formed two: images on each of the reflecting surfaces. During accommodations, it seemed to him that the two images on the front of the lens became smaller and approached each other, while on the return of the eye to a state of rest they grew larger again and separated This change, he said, could be seen “easily and distinctly” (see note 10). The observations of Helmholtz regarding the behavior of the lens in accommodation, published about the middle of the last century, were soon accepted as facts, and have ever since been stated as such in every text-book dealing with the subject.
“We may say,” writes Landolt, “that the discovery of the part played by the crystalline lens in the act of accommodation is one of the finest achievements of medical physiology, and the theory of its working is certainly one of the most firmly established; for not only have “savans” furnished lucid and mathematical proofs of its correctness, but all other theories which have been advanced as explaining accommodation have been easily and entirely overthrown…. The fact that the eye is accommodated for near vision by an increase in the curvature of its crystalline lens, is, then, incontestably proved” (see note 11).
- R is supposed to be the resting state of the lens, in which it is adjusted for distant vision. In A the suspensory ligament is supposed to have been relaxed through the contraction of the ciliary muscle, permitting the lens to bulge forward by virtue of its own elasticity.
“The question was decided,” says Tscherning, “by the observation of the changes of the images of Purkinje during accommodation, which prove that accommodation is effected by an increase of curvature of the anterior surface of the crystalline lens” (see note 12).
- English physician and man of science who was the first to present a serious argument in support of the view that accommodation is brought about by the agency of the lens.
“The greatest thinkers,” says Cohn, “have mastered a host of difficulties in discovering this arrangement, and it is only in very recent times that its processes have been clearly and perfectly set forth in the works of Sanson, Helmholtz, Brucke, Hensen and Volckers” (see note 13).
Huxley refers to the observations of Helmholtz as the “facts of adjustment with which all explanations of that process must accord” (see note 14), and Donders calls his theory the “true principle of accommodation” (see note 15).
Yet in examining the evidence for the theory we can only wonder at the scientific credulity which could base such an important department of medical practice as the treatment of the eye upon such a mass of contradictions. Helmholtz, while apparently convinced of the correctness of his observations indicating a change of form in the lens during accommodation, felt himself unable to speak with certainty of the means by which the supposed change was effected (see note 18), and strangely enough the question is still being debated. Finding, as he states, “absolutely nothing but the ciliary muscle to which accommodation could be attributed”(see note 19), Helmholtz concluded that the changes which he thought he had observed in the curvature of the lens must be effected by the action of this muscle; but he was unable to offer any satisfactory theory of the way it operated to produce these results, and he explicitly stated that the one he suggested possessed only the character of probability. Some of his disciples, “more loyal than the king,” as Tscherning has pointed out, “have proclaimed as certain what he himself with much reserve explained as probable (see note 20), but there has been no such unanimity of acceptance in this case as in that of the observations regarding the behavior of the images reflected from the lens. No one except the present writer, so far as I am aware, has ventured to question that the ciliary muscle is the agent of accommodation; but as to the mode of its operation there is generally felt to be much need for more light. Since the lens is not a factor in accommodation, it is not strange that no one was able to find out how it changed its curvature. It “is” strange, however, that these difficulties have not in any way disturbed the universal belief that the lens does change.
When the lens has been removed for cataract the patient usually appears to lose his power of accommodation, and not only has to wear a glass to replace the lost part, but has to put on a stronger glass for reading. A minority of these cases, however, after they become accustomed to the new condition, become able to see at the near-point without any change in their glasses. The existence of these two classes of cases has been a great stumbling block to ophthalmology. The first and more numerous appeared to support the theory of the agency of the lens in accommodation; but the second was hard to explain away, and constituted at one time, as Dr. Thomas Young observed, the “grand objection” to this idea. A number of these cases of apparent change of focus in the lensless eye having been reported to the Royal Society by competent observers, Dr. Young, before bringing forward his theory of accommodation, took the trouble to examine some of them, and considered himself justified in concluding that an error of observation had been made. While convinced, however, that in such eyes the “actual focal distance is totally unchangeable,” he characterized his own evidence in support of this view as only “tolerably satisfactory.” At a later period Donders made some investigations from which he concluded that “in aphakia (see note 21) not the slightest trace of accommodative power remains”(see note 22). Helmholtz expressed similar views, and von Graefe, although he observed a “slight residuum” of accommodative power in lensless eyes, did not consider it sufficient to discredit the theory of Cramer and Helmholtz. It might be due, he said, to the accommodative action of the iris, and possibly also to a lengthening of the visual axis through the action of the external muscles (see note 23).
For nearly three-quarters of a century the opinions of these masters have echoed through ophthalmological literature. Yet it is to-day a perfectly well-known and undisputed fact that many persons, after the removal of the lens for cataract, are able to see perfectly at different distances without any change in their glasses. Every ophthalmologist of any experience has seen cases of this kind, and many of them have been reported in the literature.
In 1872, Professor Forster of Breslau, reported (see note 24) a series of twenty-two cases of apparent accommodation in eyes from which the lens had been removed for cataract. The subjects ranged in age from eleven to seventyfour years, and the younger ones had more accommodative power than the elder. A year later Woinow of Moscow (see note 25) reported eleven cases, the subjects being from twelve to sixty years of age. In 1869 and 1870, respectively, Loring reported (see note 26) to the New York Ophthalmological Society and the American Ophthalmological Society the case of a young woman of eighteen who, without any change in her glasses, read the twenty line on the Snellen test card at twenty feet and also read diamond type at from five inches to twenty. On October 8, 1894, a patient of Dr. A. E. Davis who appeared to accommodate perfectly without a lens consented to go before the New York Ophthalmological Society. “The members,” Dr. Davis reports (see note 27), “were divided in their opinion as to how the patient was able to accommodate for the nearpoint with his distance glasses on”; but the fact that he could see at this point without any change in his glasses was not to be disputed.
The patient was a chef, forty-two years old, and on January 27, 1894, Dr. Davis had removed a black cataract from his right eye, supplying him at the same time with the usual outfit of glasses, one to replace the lens, for distant vision, and a stronger one for reading. In October he returned, not because his eye was not doing well, but because he was afraid he might be “straining” it. He had discarded his reading glasses after a few weeks, and had since been using only his distance glasses. Dr. Davis doubted the truth of his statements, never having seen such a case before, but found them, upon investigation, to be quite correct. With his lensless eye and a convex glass of eleven and a half diopters, the patient read the ten line on the test card at twenty feet, and with the same glass, and without any change in its position, he read fine print at from fourteen to eighteen inches Dr. Davis then presented the case to the Ophthalmological Society but, as has been stated, he obtained no light from that source. Four months later, February 4, 1895, the patient still read 20/10 at the distance and his range at the near-point had increased so that he read diamond type at from eight to twenty-two and a half inches. Dr. Davis subjected him to numerous tests, and though unable to find any explanation for his strange performances, he made some interesting observations. The results of the tests by which Donders satisfied himself that the lensless eye possessed no accommodative power were quite different from those reported by the Dutch authority, and Dr. Davis therefore concluded that these tests were “wholly inadequate to decide the question at issue.” During accommodation the ophthalmometer (see note 28) showed that the corneal curvature was changed and that the cornea moved forward a little. Under scopolamine, a drug sometimes used instead of atropine to paralyze the ciliary muscle (1/10 per cent solution every five minutes for thirty-five minutes, followed by a wait of half an hour), these changes took place as before; they also took place when the lids were held up. With the possible influence of lid pressure and of the ciliary muscle eliminated, therefore, Dr. Davis felt himself bound to conclude that the changes “must have been produced by the action of the external muscles.” Under scopolamine, also, the man’s accommodation was only slightly affected, the range at the nearpoint being reduced only two and a half inches.
The ophthalmometer further showed the patient to have absolutely no astigmatism. It had showed the same thing about three months after the operation, but three and a half weeks after it he had four and a half diopters.
Seeking further light upon the subject Dr. Davis now subjected to similar tests a case which had previously been reported by Webster in the “Archives of Pediatrics” (see note 29). The patient had been brought to -Dr. Webster at the age of ten with double congenital cataract. The left lens had been absorbed as the result of successive needlings, leaving only an opaque membrane, the lens capsule, while the right, which had not been interfered with, was sufficiently transparent around the edge to admit of useful vision. Dr. Webster made an opening in the membrane filling the pupil of the left eye, after which the vision of this eye, with a glass to replace the lens, was about equal to the vision of the right eye without a glass. For this reason Dr. Webster did not think it necessary to give the patient distance glasses, and supplied him with reading glasses only – plane glass for the-right eye and convex 16D for the left. On March 14, 1893, he returned and stated that he had been wearing his reading glasses all the time. With this glass it was found that he could read the twenty line of the test card at twenty feet, and read diamond type easily at fourteen inches. Subsequently the right lens was removed, after which no accommodation was observed in this eye. Two years later, March 16, 1895, he was seen by Dr. Davis, who found that the left eye now had an accommodative range of from ten to eighteen inches. In this case no change was observed in the cornea. The results of the Donders tests were similar to those of the earlier case, and under scopolamine the eye accommodated as before, but not quite so easily. No accommodation was observed in the right eye.
These and similar cases have been the cause of great embarrassment to those who feel called upon to reconcile them with the accepted theories. With the retinoscope the lensless eye can be seen to accommodate; but the theory of Helmholtz has dominated the ophthalmological mind so strongly that even the evidence of objective tests was not believed. The apparent act of accommodation was said not to be real, and many theories, very curious and unscientific, have been advanced to account for it. Davis is of the opinion that “the slight change in the curvature of the cornea, and its slight advancement observed in some cases, may, in those cases, account for some of the accommodative power present, but it is such a small factor that it may be eliminated entirely, since in some of the most marked cases of accommodation in aphakial eyes no such changes have been observed.”
The voluntary production of astigmatism is another stumbling block to the supporters of the accepted theories, as it involves a change in the shape of the cornea, and such a change is not compatible with the idea of an inextensible (see note 30) eyeball. It seems to have given them less trouble, however, than the accommodation of the lensless eye, because fewer of these cases have been observed and still fewer have been allowed to get into the literature. Some interesting facts regarding one have fortunately been given by Davis, who investigated it in connection with the corneal changes noted in the lensless eye. The case was that of a house surgeon at the Manhattan Eye and Ear Hospital, Dr. C. H. Johnson. Ordinarily this gentleman had half a diopter of astigmatism in each eye; but he could, at will, increase this to two diopters in the right eye and one and a half in the left. He did this many times, in the presence of a number of members of the hospital staff, and also did it when the upper lids were held up, showing that the pressure of the lids had nothing to do with the phenomenon. Later he went to Louisville, and here Dr. J. M. Ray, at the suggestion of Dr. Davis, tested his ability to produce astigmatism under the influence of scopolamine (four instillations, 1/5 per cent solution). While the eyes were under the influence of the drug the astigmatism still seemed to increase, according to the evidence of the ophthalmometer, to one and a half diopters in the right eye and one in the left. From these facts, the influence of the lids and of the ciliary muscle having been eliminated, Dr. Davis concluded that the change in the cornea was “brought about mainly by the external muscles.” What explanation others offer for such phenomena I do not know.
2. Donders: On the Anomalies of Accommodation and Refraction of the Eye. English translation by Moore, 1864, p. 10. Frans Cornelis Donders (1818-1889) was professor of physiology and ophthalmology at the University of Utrecht, and is ranked as one of the greatest ophthalmologists of all time.
12. Physiologic Optics, authorized translation by Weiland, 1904, p. 163. Marius Hans Erik Tscherning (1854 – ) is a Danish ophthalmologist who for twenty-five years was co-director and director of the ophthalmological laboratory of the Sorbonne. Later he became professor of ophthalmology in the University of Copenhagen.
13. The Hygiene of the Eye in Schools, English translation edited by Turnbull, 1886, p. 23. Hermann Cohn (1838-1906) was professor of ophthalmology in the University of Breslau, and is known chiefly for his contributions to ocular hygiene.
20. Physiologic Optics, p. 166. Herman- Ludwig Ferdinand von Helmholtz (1821-1894) whose observations regarding the behavior of images reflected from the front of the lens are supposed to have demonstrated that the curvature of this body changes during accommodation.
23. Archiv. f. Ophth., 1855, vol. ii, part 1, p. 187 et seq. Albrecht von Graefe (1828-1870) was professor of ophthalmology in the University of Berlin, and is ranked with Donders and Arlt as one of the greatest ophthalmologists of the nineteenth century.
30. Inasmuch as the eye 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. – De Schweinitz: Diseases of the Eye, eighth edition, 1916, pp. 35-36.