84 THE SCIENCE OF DRY FLY FISHING. 



Although a man's size is relatively reduced the more nearly he 

 approaches the water level, still, under ordinary circumstances, 

 the man is clearly visible on the trout's horizon. If, then, 

 this appearance is accompanied by an invariable agitation of 

 the surface, or the violent appearance of lines or flies, over a 

 trout's head, even the most unsophisticated trout will quickly 

 learn to associate these two phenomena, and be increasingly 

 ready to take fright when a man is seen. Hence the necessity 

 for caution, delicacy and finesse on the part of the fisherman 

 not only on his own account, but that of his brother 

 anglers. 



All other downward rays coming to the eye of the trout, 

 save those which enter the arc subtended by the cone of 

 the trout's vertical vision, are external to the cone, 

 and are the reflections from the under surface of the water 

 of sub-aqueous objects, the water acting as a huge 

 mirror of all bodies below its surface. (See E d D, E d D, 

 Diagram g.) 



It may be taken for granted that, under ordinary circum- 

 stances, when a fisherman can see the trout the trout can 

 see him, with the exception of three factors, however, which 

 must always be considered. 



i The amount of light falling on either. 



2. The glint or glare in the eyes of one or the other. 



3. The background of each. 



i. The first may be considered as sometimes favouring 

 the fish and sometimes the angler. 



