SI GET AND THE VISUAL ORGAN. 4 6 7 



behind the lens, while that of the distant objects falls with almost 

 parallel lines. To return to the caraera-obscura, you must draw out 

 the tube with the lens, in other words, remove the latter farther from 

 the intercepting plate if nearer objects are to be impressed, and on the 

 other, push it in, if more distant ones are wanted. The same effects 

 might be produced at equally the same distance by simply substituting 

 lenses of different power. Now, the human eye has to fulfil the re- 

 quirement of projecting clearly-defined images on the retina, whether 

 they are but a few inches off, or at an immeasurable distance. The 

 eye being strictly subject to lenticular laws, either the space between 

 the lens and the retina must have the power of varying, or the lens 

 itself, by a change of form, must exercise now a stronger now a weaker 

 power of refraction. 



Fig. 4, 



The same parts as in Fig. 2, besides: I, Iris ; K, Ciliary Muscle; L, Suspensory Ligament of the 



lens (zonula). 



The conclusion has now been arrived at, that this power of accom- 

 modation depends on a varying curve of the lens. 1 



In order to effect this, a great degree of elasticity, chiefly of the 

 outer capsule, was obviously necessary, and we find this requisite com- 

 plied with by an admirably delicate structure of concentric lavers, ac- 

 cording to which its density reaches the minimum at the periphery of 

 the lens, while its aggregate power of refraction is increased, as if it 

 were composed entirely of the strong refracting substance of which 

 the centre consists. 



As the power of accommodating its focus rests on this quality of 

 the lens, it is necessarily accompanied by a loss of elasticity with in- 

 creasing age. The eye of a man of sixty, that sees distinctly at a dis- 



1 The mode of procedure is now known down to the last and most minute detail. The 

 surfaces of the lens give back extremely delicate reflections, which with the proper aids 

 may be measured in the living eye, and from the size of which the curve may be calcu- 

 lated as in convex and concave mirrors. As auxiliary of this change of form, there is a 

 peculiar agent a muscle embedded in the choroid (K, Fig. 4) which has the power 

 of contracting and expanding the suspensory ligament of the lens. 



