THE VISION OF A TROUT. 81 



zone of invisibility, but I doubt if this is due to direct vision, 

 and it would be an interesting point for discussion. 



In Diagram 7, if A B C D represents the horizontal 

 plane in which the trout is lying, E the eye and T the 

 tail of the fish, it can see when in this position any 

 object in the unshaded portion A B C E D, and cannot, with- 

 out moving, see any object in the shaded portion C E D, 

 and hence it is that the dry fly fisherman, when within 

 this latter zone, can approach his fish without being 

 detected. 



In any vertical plane passing through the eye of the 

 trout, however, a different range of sight has to be con- 

 sidered, and an entirely new factor presents itself this 

 factor is the refractive influence of the water on all rays 

 entering it, from objects situated above its level. I need 

 not here enter into the optical laws of refraction, but will 

 ask my readers to accept as a fact that the vertical range 

 of the vision of a trout, as regards all objects external to 

 the water, is confined to the interior of a hollow cone, the 

 apex of which cone is situated at the eye of the trout, and 

 the sides of which rise upward at an angle of 42 degrees 

 to the surface of the water. So far as the fish is con- 

 cerned, within this hollow cone which, therefore, subtends 

 an angle of 96 degrees in every upward direction is con- 

 fined the view of all objects within the 180 degrees 

 vertically above the water. In other words, the trout 

 sees, as it were, all objects above the surface of the water 



D.F, a 



