182 THE EYE. 



The structure of the crystalline lens is very complex. It con- 

 sists essentially of fibres united side by side to each other (Fig. 8), 

 and arranged together in very numerous laminae, which are so 

 placed upon one another, that when hardened in spirits the lens 

 splits into three portions in the form of sectors, each of which is 

 composed of superimposed concentric laminae. The lens in- 

 creases in density, and consequently in power of refraction from 

 without inwards, the central part, usually called the nucleus, being 

 the most dense. The individual fibres dovetail laterally into each 

 other, something after the fashion of those of the cod (see Fig. 9). 

 According to Dr. Brewster and Dr. Gordon, the refractive indices 

 of the outer coat, the middle, and the central parts, are 1*3767, 

 1-3786, and I '3999 respectively. This increase of density serves 

 to correct the aberration by increasing the convergence of the 

 central rays more than that of the extreme parts of the pencil.* 

 The lens is believed to be kept slightly compressed and flattened 

 out by the tension of its elastic capsule. The ciliary muscle (;/), 

 by its action on the hyaloid membrane, reduces this tension, and 

 the movements which ensue are thought to be those which take 

 place when the eye is accommodated for near vision. 



The normal condition of the eye is for sight of distant objects 

 (Fig. 10). The accommodation is extremely rapid from near 

 objects to distant ones, but comparatively slow from distant to 

 near; the alteration, depending on the recoil of the elastic 

 tissues, being swift as compared with the slow construction of 

 the involuntary muscular fibres of the ciliary muscle. If it be 

 so arranged that the recoil should take place in the dark, the 

 resulting shock is sufficient to produce the sensation of a flash of 

 light, like that caused by a blow. 



In spite of what a late writer has said with regard to the 

 incompleteness of the eye, we must look upon it as an example 

 of consummate wisdom and perfect adaptation to the ends it is 

 designed to serve. That there is a great deal more to be learned 

 by us in its structure, and especially in that of the eyes of 

 other creatures, we must all feel to be the case. The inferior 

 animals are probably able to appreciate rays of light which 



* Lloyd on " Light and Vision." 



