38 



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- 

 count 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 phenomenon. 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 

 Avithout success. The author, however, declares his conviction of 

 their existence, and of the precipitancy of those who have absolutely 

 denied them ; and adduces some observations in favour of this asser- 

 tion. He next describes a zone, or gland, surrounding the margin 

 of the crystalline, which he has observed in many animals, and which, 

 from some phenomena of vision, he also inters in the human eye. 



