134 COLOUR VISION 



mouse. The method consisted in teaching the animals to associate one 

 of two differently illuminated compartments with a disagreeable electric 

 shock. Light blue and orange, green and red, violet and red, were 

 distinguished even when their luminosities were considerably varied. 

 The possibility that these discriminations were made from brightness 

 rather than colour was not whollv eliminated. Yerkes concluded, how- 



i> 



ever, that the mice have a certain degree of ability to distinguish red, 

 green, and violet as colours. No ability to discriminate green and blue 

 was shown unless there was a great difference in brightness. There 

 is evidence that the red end of the spectrum is much darker to the 

 mouse's than to the human eye. Waugh 1 on the other hand found that 

 his mice could only distinguish red from white and light greys with great 

 difficulty. 



Davis and Cole 2 experimented on racoons by the feeding method 

 and found that while discrimination of black from white, yellow, red, 

 blue or green appeared easy, that of blue from yellow and red from green 

 was difficult. Davis thought it probable that the animals were colour- 

 blind, but Hess does not agree with this conclusion. 



Washburn and Abbott 3 made attempts to discover the brightness 

 value of red for the light-adapted eye of the rabbit. They arrived at 

 the conclusion that this animal can discriminate between a saturated red 

 and a grey paper, and that the discrimination is based upon luminosity 

 rather than colour. Red has a low stimulus value for the rabbit. The 

 experiments furnish no evidence that it sees red as a colour, but do not 

 prove that it does not do so. 



An elaborate series of researches has been carried out in Pawlow's 

 laboratory 4 with a view to establishing " conditional reflexes." The 

 salivary secretion was used as the indicator. An indifferent stimulus, 

 e.g., a strong electrical stimulus to the skin, is at first applied simul- 

 taneously with the placing of food or acid in the mouth. After a short 

 time the salivary secretion is called forth by the application of the 

 indifferent stimulus alone. Orbeli 5 studied the effects of coloured 

 optical stimuli. After exposure of a red square on a white screen 

 reflex salivary secretion was obtained. The same recurred when other 

 colours were exhibited. If red and grey squares were exhibited suc- 

 cessively and the dog fed only after exhibition of the red square the 



1 J. of Comp. Neur. and Psycho! . xv. 549, 1905. 



2 Cole, loc. cit. xvn. 211, 1907. 3 J. of Animal Behavior, 11. 145. 



4 Pawlow, Brit. Med. JL n. 473, 1913. 



5 Comptes rend dc la Sue. we'd, russe, St Petersburg. 1907 ; Arch, de.s Sc. biol. xiv. 

 1908. 



