LIGHT OF DIFFERENT WAVE-LENGTHS BY FISH 91 



ferential responses toward patches of light of different wave- 

 length but of presumably matched brightness. 



It is a priori not impossible that the responses of the 

 fish to the stimulus plates are determined by the total 

 energy reaching the fish from them, as measured by physi- 

 cal instruments, rather than by the stimulus value of the 

 light rays alone. Tests were therefore made to determine 

 this point but no relation could be detected between total 

 energies and the responses of the fish. (p. 53.) 



Throughout the work the attempt has been made to 

 escape the sources of error enumerated in the last section. 

 The fish have been kept in normal physiological state and 

 it is believed that environmental factors likely to modify 

 normal response or to inhibit it have been detected and 

 eliminated. 



The conclusion is reached that the fish used are capable 

 of responding differentially to light of longer and shorter 

 wave-lengths. This capacity is made evident by the use 

 of a food association. In the absence of modifying or 

 inhibiting environmental factors differential response may 

 be apparent in the unlearned responses of the fish of some 

 species. The two sources of evidence thus supplement 

 each other. Evidence is collated to show that the response 

 to light of longer and shorter wave-lengths is mediated 

 through the eye. We may therefore speak of color vision 

 in fish, but attention is called to the differences found by 

 others between the retinal structures and visual purple of 

 fish and higher vertebrates, differences which indicate that 

 the stimulus value of the longer and shorter wave-lengths 

 of light may not be the same in the two cases. 



The evidence presented in this paper that the longer 

 wave-lengths of light have a physiological or psycholog- 

 ical effect upon some fish different from that produced by 

 the shorter rays is, I think, sufficient, even in the sense 

 demanded by Watson (1914, p. vS5-i) when he writes, 



" Ordinarily we mean when we say that an animal is sensitive to difference in 

 wav^e-length? that such stimuli play a role in the adjustment of the animal to food, 

 sexual objects, shelter, escape from enemies, etc., t. e. that such stimuli initiate 

 activity in arcs which end in striped muscle." 



I prefer to designate what I have found as difference 

 in stimulating value of the longer and shorter wave-lengths 



