182 SENSORIAL FUNCTIONS. 



scope than when he would examine an object; and 

 looked 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 curi- 

 ous optical instrument towards a house, he saw, not 

 only the front, but also the doors and windows, and 

 could plainly discern through each, whether they were 

 open or closed. 



Eyes of Fishes. In the Fishes, the cornea is nearly 

 flat, 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 refrac- 

 tive power in this part, were its convexity ever so great. 

 The refraction 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 designed 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. 



This structure is shown by Fig. 110, which repre- 

 sents the eye of the Perch ; c, being the flat cornea ; Z, 

 Fi 109 ^ e s P^ er ical lens ; v, the vitreous hu- 

 mor ; 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 Fishes being continually 

 washed by the element in which they 

 live, require no glands to secrete a fluid 

 for moistening them ; or any eyelids to prevent the 

 dust from flying into them. 



Remarkable structure of the Lens of the Cod-fish. 

 Sir David Brewster has recently made an analysis of 

 the structure of the crystalline lens of the Cod-fish, to 

 which he was led by noticing some remarkable optical 

 appearances presented by thin layers of this substance. 



