COLOUR PERCEPTION IN FISH 



15 



which exist in the natural fly, the colour 

 of the imitation is quite immaterial. 



The facts upon which he based this 

 theory were (1) that during the May-fly 

 season he used several artificial May-flies, 

 some of which were coloured scarlet, some 

 bright blue, and some coloured to imitate 

 the natural fly, all of them being similarly 

 graduated with regard to the shade of 

 their various component parts ; (2) that 

 he caught trout with all these flies, no 

 particular one of them being decidedly 

 more successful than the others. 



This experience of his no doubt would 

 at first strike one as being very strongly 

 in favour of his theory ; but on going 

 deeply into the matter, its bearing on the 

 fish's powers of vision is not so gr-eat as 

 it appears. 



To begin with, we must consider 

 whether, judging from experience in the 

 past, trout have been known to rise at 

 things on the water which were not only 

 unlike in colour to any flies on the water, 

 but also unlike them in shape and grada- 

 tions of shade. This we know they will 

 sometimes do. I have on several occasions 



