Reighard. — Senses and Learning in Fishes 161 



is fed ; if he goes to the red plate he is not fed. The fish, 

 after one trial, is returned to the smaller compartment 

 and again the door is opened and he makes a choice of 

 the blue or red plate and is rewarded by food if he se- 

 lects the blue plate. After many trials the fish learns to 

 go at once to the blue plate to be fed. He rarely makes 

 a mistake and there can be no doubt that he has learned 

 to distinguish the blue plate from the red. If the blue 

 plate were always in the same position, always at the 

 left of the red, the fish might learn to tell them apart by 

 position. He might learn no more than to turn to the 

 left in order to get food. It is therefore necessary to 

 shift the plates in an irregular way as the experiments 

 progress, so that the blue is sometimes on the fish's right 

 and sometimes on his left. The plates are of the same 

 form and size and, with shifting, are not to be told apart 

 by their relative positions. It would seem natural to 

 conclude that the fish had therefore distinguished them 

 by their colors. At first sight the evidence seems suffi- 

 cient to show that fish can discriminate the color blue 

 from the color red. But this is an error and one into 

 which earlier investigators were sometimes led. 



Let us suppose now that the fish is actually color blind. 

 In that case both the blue and the red plates appear to 

 him to be gray. But one of these (to him) , gray plates 

 may be lighter than the other. If we should photograph 

 the red and blue plates they would both appear grey in 

 the printed picture, but one might be darker than the 

 other and it would be very easy to distinguish them in 

 the photograph. So the fish may discriminate between 

 the colored plates because they appear to him to differ in 

 brightness although to his eye they may not differ in 

 color. If now we make the red plate of the same bright- 

 ness as the blue and the fish can still tell them apart, we 

 may be sure that he does this by the color alone, for the 

 plates will then be identical in size, shape and brightness 

 and will differ in color only. It is not easy for us to 

 match two colors by the eye or by any of our instruments 

 so that they will be of equal brightness for the fish, but 



