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all; consequently, although the fish at O may see the upper part of the man situated at MP, he will do so very indistinctly, and in a new position, because the pencil NEOFM will be very much refracted; he will not see the part, N L, of the man at all, because the pencil, N EL, does not enter the water at all; and he will see probably his legs, LP (in the clear water), because there is neither refraction nor obstruction to prevent him. So that the figure MP will, in the eye of the fish, be cut into two portions, separated from each other by a long unsubstantial 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 distinctness by his game : thus the shelving gravel beds which reach far into the Dove, and other Trout streams, are so many most advantageous positions for the angler. (Pl. I. K.)
* This diagram is constructed on two well-known optical laws, viz. first, the sine a. b. of the angle of incidence, A E f, of a ray 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 Ef (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.