94 TONE OF INVISIBILITY 



it may be visible, while any object situated in the 

 remaining 60 degrees of that circle must be invisible 

 as a general rule. Mr. Sheringham reminded me that 

 trout can apparently see at times directly behind 

 themselves. I admit that trout do become aware of 

 a man's presence at times, when in the supposed 

 zone of invisibility, but not in the plane in which 

 the trout is lying. It will generally be found that 

 an object thus seen by the trout will be elevated 

 at some height above the surface, and the bulge of its 

 shoulders would not intervene between the eye and 

 the object, as it would if the object and the trout's 

 eye were in one plane. 



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, its eyes are naturally diverted up- 

 stream, and it can see when in this position any object 

 in the unshaded portion A B C E D, in its own plane, 

 and cannot, certainly without moving its position, 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 generally 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 

 considered, and an entirely new factor presents itself 

 this is the refractive influence of the water on all rays 

 entering it. 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 



