Page:Proceedings of the Royal Society of London Vol 1.djvu/58

Rh efficacy of a change of figure in the lens, was derived from various instances of persons, who, after they were deprived of that part of the organ, still retained the faculty of accommodation. The result of this inquiry was, contrary to expectation, that in an eye deprived of the crystalline lens, the actual focal distance, as ascertained by the opto- meter, is totally unchangeable : for the proofs deduced in favour of this assertion, the author acknowledges himself indebted to Mr. Ware, who obligingly introduced him to several of his patients on whom the operation had been performed. Having thus, then, pointed out the inconveniences attending all the other hypotheses, and some imper- fections in the experiments adduced in their favour, and having re- moved the principal objections to the opinion of an internal change of the figure of the lens, the Doctor proceeds to describe some ex- periments which he conceives come very near to a mathematical de- monstration of the existence of such a change, and likewise in a great measure explain its origin and the manner in which it is effected. The results of these experiments are deduced from the different dis- tribution of light in the image of a lucid point on the retina, according to the different states of the eye; and inferring thence, on the ma- thematical principles above laid down, what form of the lens will ac- couut for those different impressions. It is here acknowledged that the mere action of the external coats of the lens, does not, as was stated by the author eight years ago, afford a satisfactory explanation of the phænomenon. It seems, however, manifest, that changes of figure take place in the lens, which can be produced by no external cause; and this seems to establish the muscularity of the lens, long since suggested by Dr. Pemberton, Albinus, and others. The words of the author on this subject are, "If we compare the central parts of each surface of the lens to the belly of a muscle, there is no diffi- culty in conceiving their thickness to be immediately" (or sponta- neously) "increased, and to produce an immediate elongation of the axis, and an increase of the central curvature; while the lateral parts cooperate according to their distances from the centre, and in dif- ferent individuals in somewhat different proportions." And an inti- mation is here added, that it would be worthy of inquiry, whether the state of contraction may not also immediately add to the refrac- tive powers of the lens.

In the last section we find some anatomical illustrations of the construction and capacity of the organs of various classes of animals for the functions attributed to them. The human lens is not only ascertained to be of a radiated structure, but, on close inspection, the number of radiations is found to be ten on each side. The greatest pains were taken to trace nerves into the lens, but as yet without success. The author, however, declares his conviction of their existence, and of the precipitancy of those who have absolutely denicd them ; and adduccs some observations in favour of this asser- tion. He next describes a zone, or gland, surrounding the margin of the crystalline, which he has obserred in many animals, and which, from somc phænomena of vision, he also infers in the human eye.