178 SENSORIAL FUNCTION'S, 



scope than when he would examine an object ; and look- 

 ed through both in the manner of a telescope, at the 

 steeple of a church, which was 299 feet high, and 750 

 feet distant, and could plainly see through every little 

 lens, the whole steeple inverted, though not larger than the 

 point of a fine needle ; and then directing this curious 

 optical instrument toward a house, he saw not only the 

 front, but also the doors and windows, and could plain- 

 ly discern, through each, whether they were open or 

 clc 



547. Eyes of Fishes. In the fishes, the cornea is near- 

 ly Hat, as is the case with all aquatic animals. This is an 

 adaptation to the element in which they live, for since 

 there is little difference between the density of the water 

 and the cornea, there would be but little refractive power 

 in this part, were its convexity ever so great. The refrac- 

 tion is therefore chiefly performed by the crystalline lens, 

 which has great power in this respect, its form being 

 spherical, and its texture of great density, properties de- 

 signed to bring the rays to a focus at a very short distance, 

 the whole eye being flat instead of oblong through the 

 axis, as in land animals. 



548. This structure is shown by Fig. 100, which rep- 

 resents the eye of the perch ; c, being pig. 109. 



the flat cornea ; /, the spherical lens ; v, 

 the vitreous humor ; r, the retina ; o, the 

 optic nerve ; s, the sclerotic coat, and k, 

 a part called the choroid gland, shaped 

 like a horse-shoe, but the use of which 

 is entirely unknown. The eyes of fish- 

 es, being continually washed by the ele- 

 ment in which they live, require no glands, to secrete a 

 dust fluid for moistening them, or any eyelids to prevent 

 the dust from falling into them. 



549. Remarkable Structure of the Lens of a Codfsh. 

 Sir David Brewster has recently made an analysis of the 

 structure of the crystalline lens of the codfish, to which he 

 was led by noticing some remarkable optical appearances 

 presented by thin layers of this substance. 



