290 OPTICAL APPARATUS OF FISH. 



of bone. There is another remarkable difference 

 between the eye of terrestrial animals and fishes 

 a difference which has reference to the nature of 

 the medium in which the creature lives. For ex- 

 ample, the density of water is so much greater 

 than that of air, that it is employed in -the con- 

 struction of the eye-ball of man, and the rest of 

 the air-breathing terrestrial animals, as a means of 

 directing the rays of light towards the bottom 

 of the eye, upon which the spotless curtain is 

 spread which receives the picture of external 

 nature. But in fishes it is evident that water, as 

 a medium of refraction of the rays of light, would 

 be perfectly useless, since the medium is aqueous, 

 through which all the rays reach the transparent 

 window of the organ of vision. The chambers of 

 the eye of the fish, which in other animals contain 

 water, are consequently those which are most 

 easily spared ; and it is the reduction of capacity 

 in them that gives rise to the flatness of the front 

 of the eye. The proper refracting apparatus of 

 the eye-ball of the fish is a transparent globe of 

 considerable density and size, termed the lens. In 

 terrestrial animals the lens is small, it is scarcely 

 more convex that a common magnifying glass, and 

 it is soft in its texture. That of the fish, how- 

 ever, is a much more powerful agent in the refrac- 

 tion of the rays of light, and approaches in some 

 of its characters to the finest glass. By it the 



