OF FISH IN GENERAL. 17 



Utility of the lens. 



eyes of these creatures are proportionally in 

 greater quantity, or much larger than those of 

 animals living in air; the eye of the cod being 

 nearly of the same weight and depth, and its 

 axis of the same length as the eye of the ox. 

 The primary use of the almost completely sphe- 

 rical figure of the crystalline lens in fish, or great 

 convexity, especially of the anterior part of their 

 lens, which this gentleman found to project in 

 the cod about seven-fortieths of an inch beyond 

 the iris, is to take a large field of the objects 

 around them, which was particularly necessary, 

 as the motion of their neck is inconsiderable. 



Four chief circumstances occur in order to 

 enable these aquatic animals to collect into a 

 focus on the retina, with the same length of the 

 axis of the eye as in the quadruped, the rays of 

 lights coming from the dense medium of the 

 water; viz. 



1st, Their crystalline lens is more convex, or 

 composed of portions of smaller spheres than in 

 land animals. 



2d!y, Their crystalline lens is, in correspond- 

 ing parts, much more dense than in animals 

 which live in air. 



3dly, The lens in fish possesses power of re- 

 fracting light far beyond what has been calcu- 

 lated by authors, who have proceeded on the 

 supposition that powers were proportioned nearly 

 to its specific gravity. 



And, 4thly, The vitreous humour offish being 



VOL. iv. NO. '31, c 



