LIGHT OF DIFFERENT WAVE-LENGTHS BY FISH 93 



and by shifting the filters a red or a 

 blue light could be secured on either 

 plate. 



8. By use of slits the intensities could be 

 varied. The aquarium was illuminated 

 by a ceiling light of constant intensity, 

 and other light was excluded from it 

 by a light-tight enclosure. 



9. A curtain was hung about the aquarium 

 within the light-tight enclosure in such 

 a way as to conceal the experimenter 

 from the fish. 



10. Food was released before one of the 

 stimulus plates (blue or white) by 

 means of a device operated by an elec- 

 tro-magnet controlled by a switch, 

 which could be manipulated from out- 

 side the curtain. The sliding door by 

 means of which the stimulus plates 

 were exposed to the fish could also be 

 controlled from outside the curtain. 



B. Method of experimentation 



11. Training methods were used in which 

 a food association was established by 

 feeding the fish before a stimulus- 

 patch of constant intensity in contrast 

 to one of variable intensity. By shift- 

 ing filters and adjusting slits the posi- 

 tions of constant and variable stimuli 

 were irregularly interchanged. In ex- 

 periments with light of restricted wave- 

 length, blue was used as the constant 

 and was the positive stimulus. 



12. The unlearned differential responses of 

 the fish toward lights of different wave- 

 lengths and different intensities were 

 studied by the use of the same appa- 

 ratus. 



