BY WAY OF INTRODUCTION vj 



As a fish swims in the water the sides and 

 the under parts of the fish reflect the general 

 colour and shade of its surroundings, .while the 

 amount of light reflected from the back of the 

 fish is regulated by the relaxation or contraction 

 of the dark pigment cells in the skin of the back. 

 A fish is therefore a swimming mirror, and so 

 long as it swims on a level keel it is inconspicuous 

 wherever it goes. 



If, in turning, the fish goes over ever so 

 slightly on to one side, the opposite side catches 

 the light from above, and there is a momentary 

 flash. 



Next, a fish maintains its balance in the 

 water by a concerted movement of its paired 

 fins. When a fish becomes diseased or weak, 

 this perfect balance is interfered with, and as 

 the fish swims it rolls on to one side, and only 

 rights itself with an effort. These movements 

 result in a protracted flash. The track of this 

 weak fish, as it proceeds on its wobbly course, is 

 marked by a series of intermittent protracted 

 flashes. 



Later I will show how intermittent pro- 

 tracted flashes of light are emitted from the 

 plumage of divers under the water, and are 



