VISION. 507 



the middle circle is very thin and flexible, 

 admitting of the ready separation or approxi- 

 mating of the other portions, so as to elongate 

 or contract the axis of the eye ; just as a tele- 

 scope can be drawn out or shortened, in order 

 to adapt it to the distance of the object to be 

 viewed. The whole eye-ball is surrounded by 

 strong muscles which are capable of effecting 

 these requisite changes of distance between the 

 cornea and the retina. The Dolphin, which, lives 

 more constantly in the water, has an eye still 

 more nearly approaching in its structure to that 

 of fishes; the crystalline lens being nearly 

 spherical, and the globe of the eye furnished 

 with strong and numerous muscles. In birds 

 which frequently plunge their heads under 

 water the crystalline lens is more convex than 

 in other tribes ; and the same is true also of 

 aquatic reptiles. 



