THE VISION OF TROUT n 



a trout with a fly, whether natural or artificial, it may be 

 worth while to spend a little time in an endeavour to see 

 what can be deduced from known facts about the nature 

 and characteristics of the eyesight of the trout. 



The nature and the needs of trout differ greatly from 

 those of man, and it need not therefore surprise us if 

 examination should lead us eventually to the conclusion 

 that his perception by eyesight differs materially from that 

 of man. Indeed, I think it would be remarkable if, living 

 in a different medium that is subject to certain optical laws 

 from which the air is free, and having different needs and 

 modes of being from man, the trout were to see things in 

 all respects as man sees them — even after making all 

 allowance for the correcting and co-ordinating effects of 

 tactile experience. 



To begin with, while man's eyes are placed in front of 

 his head and operate together so that his vision is stereo- 

 scopic, the trout's eyes are on the sides of his head, slanting 

 slightly forward and operating separately, so that it may be 

 inferred that in most cases his vision of an object is mon- 

 ocular. It may be that in the act of taking a fly, whether on 

 the surface or below it, both e^es may be trained forward 

 upon the object, or, alternatively, that one eye only may be 

 on the object and the other attending to business in another 

 direction. 



Then, though the vision of a trout is astonishingly quick, 

 enabling him as it does to pick out and capture minute living 

 objects, often in rapid and turbulent streams, it does not 

 seem to be greatly concerned with a sense of form or detail. 

 Otherwise it is extremely difficult to conceive how he can 

 take a hackled fly, such, for instance, as the Straddlebug May 

 fly, with its long straggling fibres of summer duck, in cir- 

 cumstances which can leave no doubt in any unprejudiced 

 mind that he takes it for May fly or a hatching nymph. In 



