OPTICAL APPARATUS OF FISH. 221 



of the eye-ball strengthened by a thin cup of 

 gristle, and, in some instances, 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 example, the 

 density of water is so much greater than that of 

 air, that it is employed in the construction of 

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

 breathing terrestrial animals, as a means of di- 

 recting 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 per- 

 fectly 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 than a 

 common magnifying glass, and it is soft in its 

 texture. That of the fish, however, is a much 



