Flies and HooAs. 93 



to judge, I can detect no difference of structure between 

 the eye of the trout and that of the salmon, which would 

 lead me to infer that there is any material difference in 

 their functions. 



Some time ago I caused a tank to be constructed for 

 the purpose of experimenting on the colors of leaders, the 

 results of which investigation were given in my former 

 volume. The reasons and experiments there given, which 

 led me to devise that apparatus, led me to use it in the 

 present instance, and to have faith in the results so ob- 

 tained. 



Fig. 7. 



In the preceding figure A represents the tank, which 

 was five feet long and fourteen and a half inches deep, 

 measured on the inside. Where the bottom met the 

 ends, apertures were formed three inches high and the 

 width of the tank, which were closed with plate-glass, B. 

 When the tank was in use, the head of the observer and 

 the end of the tank, as well as the glass at the opposite 

 end, were so wrapped in black rubber-cloth as to exclude 

 all light except such as entered through the surface of 

 the water. 



To the end of a salmon-rod tip a piece of copper wire, 



