LIGHT OF DIFFERENT WAVE-LENGTHS BY FISH 3 



determine to which of the two variables involved, intensity 

 or wave-length, the response was to be attributed. In 

 order so to control the experiment that response could be 

 made to but one of these variables the brightness of the 

 negative plate was varied until it was equal to that of the 

 positive for the human eye. Continued response to the 

 positive plate might then be due to the quality of the 

 stimulus light. But since it could not be assumed, without 

 proof, that the relative brightness values of the spectrum 

 were similar for fish and man the response might be due 

 to a brightness difference between the two plates. To 

 follow the usual practice of equating the brightness of the 

 plates by the human eye only could not lead to dependable 

 results, and so a means was sought by which it might be 

 possible to determine by the behavior of the fish when the 

 two plates were of equal brightness for them. It is believed 

 that such means has been found. 



As an aid in interpreting the responses obtained when 

 two stimuli of different wave-lengths were used, an effort 

 was made to determine the minimum intensity differences 

 between two white light stimuli capable of producing 

 differential response. For this purpose an attempt was 

 made to form a food association with the duller one of 

 two white plates, which was kept at constant intensity 

 while its companion plate was varied in intensity. If 

 there should occur any difference in the relative ease of 

 discrimination of the stimulus plates when the quality 

 difference was absent then the value of difference in wave- 

 lengths for discrimination would be shown. The minimum 

 intensity difference necessary for the discrimination of 

 two lights should be the same for white and for colored 

 lights if wave-length differences are not able to differen- 

 tially stimulate fish. 



' For normal activity it is essential that the fish be kept 

 in a normal environment; otherwise response is often in- 

 hibited by unnatural conditions or by manipulations that 

 induce fright. The water should be of constant tempera- 

 ture from one container to another, shifting of containers 

 from place to place, variations in intensity of the general 

 illumination, and manipulations likely to frighten the 



