THE COMPARATIVE PSYCHOLOGY OF COLOUR VLSION 135 



salivary reflex eventually followed the red stimulus only. It was found, 

 however, that the reflex in dogs was conditioned by the change in 

 intensity of the light, not by variations in the wave-length. The experi- 

 ments are not of a nature to demonstrate colour-blindness in the dogs. 



Kalischer^ has published interesting experiments on dogs. His 

 animals were placed in a dark chamber which could be illuminated by 

 means of light from a lamp, transmitted through coloured glass. On 

 the appearance of red light the dog was given food, which was, however, 

 withheld during the period when light was excluded. When a perfect 

 reaction habit had been formed, the experiment was varied sometimes 

 by transmitting the light through blue, instead of through red glass, 

 though the subject was still only fed when a red light was exposed. 

 In order to render the comparison of brightness between the different 

 lights more difficult, their successive exposure was always followed 

 by a dark interval. At first the subjects reacted in a similar 

 manner to either coloured light, but after some further training the 

 difference was duly learnt, and eventually discrimination became so 

 perfect that no matter how the brightness of the red or the blue light 

 might be changed the reaction was almost invariably correct. 



Observations were also made with green, yellow and red-violet lights, 

 but discrimination was found to be the most definite when red and blue 

 were used ; red and yellow proving very difficult to learn, and red with 

 red-violet almost impossible. Colvin and Burford likewise found that 

 when their subjects were required to discriminate between red and violet, 

 only a relatively low percentage of right choices was obtained. 



In later experiments Kalischer left his subjects untested for two 

 or three days, and then presented a blue or a green light previous 

 to the exposure of the food-colour, red. Though these experiments 

 were repeated many times and under varied conditions, the animals 

 never failed to react correctly to the food-colour when it appeared. 



It is of importance to note that not only did the animals learn to 

 discriminate, during the course of the experiments, between different 

 colours irrespective of the degree of their luminosity, but that in addition 

 the behaviour shown on the appearance of each colour was characteristic, 

 e.g., on the exposure of a blue light the subject would appear frightened 

 and quickly withdraw his head from the food ; whereas when a green 

 or a yellow light was exposed great hesitation was shown, the head was 

 allowed to fall gradually nearer and nearer towards the meat as though 

 to seize it, and was then suddenly jerked away. 



1 Arch.f. Anat. p. 316, 1909. 



