ON THE TROUT. 11 



all ; consequently, altliongli the fisli at may see 

 the upper part of the man situated at MP, he 

 will do so very indistinctly, and in a new posi- 

 tion, because the pencil N E F M will be very 

 much refracted ; he will not see the part, N L, 

 of the man at all, because the pencil, N E L, 

 does not enter the water at all ; and he will see 

 probably his legs, L P (in the clear water), be- 

 cause there is neither refraction nor obstruction 

 to prevent him. So that the figure M P will, 

 in the eye of the fish, be cut into two portions, 

 separated from each other by a long unsub- 

 stantial interval.^ 



The application of those two little theorems 

 to the use of the fisherman is very obvious. 



In the first place, a low bank, almost on a 

 level with the water, is a great advantage to 

 the fisher, who is there seen with less distinct- 

 ness by his game : thus the shelving gravel 

 beds which reach far into the Dove, and other 

 Trout steams, are so many most advantageous 

 positions for the angler. (PI. I. K.) 



^ This diagi'am is constructed on two well-known optical 

 laws, viz. first, the sine a. b. of the angle of incidence, A E f , of 

 a I'ay of light passing out of air into water, is always to the sine, 

 c d, of the angle of refraction, C E e, as about four to three ; and 

 secondly, light will not pass out of air into water, if the angle of 

 incidence, N E f (fig. 2) exceeds about 88 degrees, but will be 

 reflected. 



The old experiment of the shilling and the basin of water 

 affords an easy practical demonstration of the first theorem in 

 the text. 



