90 FLY FISHING AND SPINNING 



invisible. Mr. Sheringham told me some time ago of a 

 case in which he had proved that trout can apparently see 

 at times directly behind themselves. I admit that trout 

 do become aware of a danger at times, when in the supposed 

 zone of invisibility, but not in the horizontal plane in which 

 the trout is lying. It will generally be found that if an 

 object be thus seen by the trout it will have been because 

 the object has been lifted at some height above the surface, 

 where the bulge of its shoulders would not intervene 

 between its eye and the object, as it would if the object 

 and the trout's eye were in one plane. 



In Diagram 1, 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 directed up-stream, 

 and when in this position it can see any object in its own 

 plane in the unshaded portion D A B C, and cannot 

 directly see, without moving its position, any object in the 

 shaded portion C E D. Hence it is that the fly fisherman, 

 when within this latter zone, can generally approach his 

 fish without being detected. 



VERTICAL SIGHT 



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 is 

 the refractive influence of the water on all rays entering 

 it. I need not here enter into the 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, may be regarded as being 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, meeting 

 the surface of the water at an angle of 42 degrees. So far 

 as the fish is concerned, within this hollow cone which, 



