VISION. 349 



however convex, would be inconsiderable; and the chief 

 agent for performing the requisite refraction of the rays is 

 the crystalline lens. We, accordingly, in general, find the 

 cornea nearly flat, and the globe of the eye approaching in 

 shape to a hemisphere; while the lens itself is nearly sphe- 

 rical, and of great density. The circumstances are shown 

 in the section of the eye of the Perch, Fig. 430.* The 

 flatness of the cornea leaves scarcely any space for aqueous 

 humour, and but little for the motions of the iris. 



The surface of the eye in fishes, being continually washed 

 by the* water in which it is immersed, requires no provision 

 430 of a secreted fluid for that purpose; and 



there are consequently neither lacrymal 

 apparatus, nor proper eye-lids; the integu- 

 ments supplying only a thin transparent 

 membrane, which passes over and protects 

 the cornea, serving the office of a conjunc- 

 tiva. The eye retains its form by the support it receives 

 from the sclerotic coat, which is of extraordinary thickness 

 and density. In the Shark and the Skate the eye is sup- 

 ported from the bottom of the orbit, by a cartilaginous pe- 

 dicle, which enables it to turn as on a pivot, or lever. 



Sir David Brewster has recently made an interesting ana- 

 lysis of the structure of the crystalline lens of the Cod, to 

 which he was led by noticing some remarkable optical ap- 

 pearances presented by thin layers of this substance when 

 transmitting polarized light. He found that the hard cen- 

 tral portion is composed of a succession of concentric, and 

 perfectly transparent, spheroidal laminae, the surfaces of 

 which, though apparently smooth, have the same kind of 

 iridescence as mother-of-pearl, and arising from the same 

 cause; namely, the occurrence of regularly arranged lines, 



* In this figure, as in the others, c is the cornea; L, the lens; v, the vitreous 

 humour; R, the retina; o, the optic nerve; and s, the sclerotica. There is 

 also found in the eyes of most fishes an organ, lodged in the space K, termed 

 the Choroid gland, which envelops the optic nerve, is shaped like a horse- 

 shoe, is of a deep red colour, and highly vascular; its use is quite unknown. 



