NATURAL HISTORY. 



" The sounds and seas, with all their finny drove, 

 Now to the moon in wavering' morrice move." MILTON. 



te removed backwards, and resembles the optician's double convex lens ; but in the 

 fLh it is a sphere, and being brought in contact with the transparent cornea, it 

 not only has the power to concentrate the rays of light coming through the water, 

 but by its altered position it increases greatly the sphere of vision (fig. 2). It 

 may be added that it is not exactly the cornea that is deficient in the fish, but tha 

 aqueous humour behind it. An aqueous fluid being thus both behind and before 

 the cornea, and that membrane being in a very slight degree thicker in the 

 centre than in the margin, this part of the organ which is so efficient in the atmo- 

 sphere is rendered useless in water. A man diving, for example, sees imperfectly, 

 somewhat in the condition of an aged person who requires spectacles. 



1139. Why does a, fish lie with his head against the stream? 



Because when a fish is situated with Ins head down the 

 stream, he is compelled to travol more rapidly than the waters, 

 or the latter will find its way into the gills, and, by becoming 

 stationary, suffocate him. 



1140. A trout may be seen lying for hours stationary, while the stream is running 

 past him ; and it sometimes appears to remain so for whole days and nights. In 

 salmon-fishing the fly is played upon the broken water in the midst of a torrent, 

 and there the fish shows himself, rising from a part of the river where men could 

 not preserve their footing, though assisted by poles, or locking their arms 

 together. 



1141. Why do the jack and stickleback keep up a continual 

 motion of the fins nearest their gills ? 



Because they frequent still shallows, and require the water to be 

 perpetually brought to their gills. In this case, the water does not 

 come of itself, and, therefore, the fish moves his pectoral fins con- 

 tinually to create a perpetual change in the water, propelling 

 that which has already passed through the gills, bringing fresh in 

 its place, and thus keeping up a constant current. 



1142. Neither to the jack nor the stickleback does the motion appear to cause 

 any exertion ; it seems natural to them, and a distinct function apart from the 

 motion of the fins for swimming purposes. It is, in fact, somewhat analogous to 

 the perpetual motion of the heart, lungs, and internal viscera in the human 

 body. 



