OUTLINES OF ICHTHYOLOGY. 155 



different positions in different species, — in some, 

 such as the Sl<ate, being placed high up, near the 

 top of the head, and in others, as the Pike, lower 

 down on the side of the head (the more frequent 

 situation), their position, however, being always 

 adjusted to suit the habits of the particular fish. 

 The external surface of the eye itself is nearly Hat, 

 but the lens is spherical — a structure which, in a 

 dense medium, affords the greatest power of vision 

 at short or moderate distances, rather than a very 

 long sight. When water is clear and undisturbed, 

 however, the sight of fishes is very acute ; and for 

 this reason it is that a ruffled water is always pre- 

 ferable to a smooth one for purposes of fishing. I 

 believe that the sight of fish in clear water is 

 longer than is generally supposed. Standing by 

 the side of a Scotch loch in bright calm weather I 

 have occasionally noticed Pike basking at some 

 not inconsiderable distance from the shore, and 

 could plainly perceive that the observation was 

 mutual. The fish, after apparently regarding me for 

 a few moments, has generally backed slowly away 

 into the deep water, disappearing so motionlessly 

 — if I may use the expression — that the eye was 

 hardly aware of his retreat until becoming conscious 

 that he had vanished. Moreover it is well known 

 that a basking Chub will sink like a stone upon 

 the flitting of a swallow across the river ; that in 

 ponds where Carp are habitually fed by visitors, 

 the former will follow the latter about for the ex- 



