222 EYE-BALL OF FKESH-WATER FISH. 



more powerful agent in the refraction of the rays 

 of light, and approaches in some of its characters 

 to the finest glass. By it the rays of light which 

 pass through the transparent window (cornea) in 

 the front of the eye are immediately received, 

 and are as suddenly concentrated on the smooth 

 expanse of the optic nerve, to be transferred by 

 the latter to the vital apparatus. As is usual in 

 optical instruments, there is a thin partition 

 pierced with a circular aperture (the pupil), inter- 

 posed between the transparent membrane of the 

 front of the eye and the lens. This membrane is 

 intended to regulate the admission of the lumin- 

 ous rays. In some fishes there are two apertures, 

 or pupils, in place of one in this partition, a 

 curious modification, while in some others, as in 

 the common skate, a broad curtain with a van- 

 dyked border, is suspended immediately behind 

 the pupil at its upper part, and serves to veil the 

 interior of the eye against the vertical rays which 

 would otherwise interfere with vision. The enu- 

 meration of all the modifications in the construc- 

 tion of the eye of the fish with which the naturalist 

 is acquainted, would form of itself an interesting 

 chapter; but the subject is too comprehensive to 

 be more than glanced at in this brief sketch. In 

 most of the fresh-water fishes the eye-ball plays 

 in a bony niche, which is specially destined for 

 its use; while in other fishes in which the bony 



