THE COMPARATIVE PSYCHOLOGY OF COLOUR VISION 139 



physiological effects produced by the different colours even when of 

 approximately the same brightness (greater affective value of red 

 compared with blue, etc.) ; (3) the evidence afforded by the " approxi- 

 mate brightness value series " that on the whole no grey was found that 

 was indistinguishable from the standard colours employed, training 

 being still imperfect ; (4) the difficulty and confusion of the ''' grey 

 difference threshold series " as compared with the parallel " approximate 

 brightness value series " ; (5) the results obtained from the ' hue 

 discrimination series " ; (6) the difference in the approximate thresholds 

 of the four colours and grey as determined by the " colour threshold 



series.'' 



Whereas the facts recorded in the first group of items of evidence 

 are not incompatible with the assumption that dogs possess some sort 

 of colour vision, those in the second group are quite irreconcilable with 

 the view that the subjects are limited to brightness vision. 



Miss Smith draws the following conclusions : (1) That while evidence 

 has been obtained to show that some dogs possess a rudimentary power 

 of colour discrimination, such discrimination is highly unstable and 

 cannot be supposed to play any part in the animal's normal existence. 



(2) That the colour sense is, as shown by the high colour threshold, 

 very weak. 



(3) That colour discrimination, even where clearly established, may 

 be readily inhibited by differences of luminosity or position. 



Piper 1 investigated the electrical reactions of dogs, cats and rabbits. 

 In dogs and cats he found the reactions in both light- and dark-adapted 

 eyes to correspond with the achromatic scotopic values for man. In 

 rabbits the strongest photopic value was 570 /u,/z, scotopic 540 //,//,. 



Hess tested a monkey in the following mannner. Grain was scattered 

 over a black surface, upon part of which a spectrum was thrown. The 

 monkey gathered all the grains from the extreme red to the extreme 

 violet, leaving those unilluminated. When the animal was dark-adapted 

 and the intensity of the light diminished until the grains were visible 

 to the dark-adapted experimenter only in the yellow and green, only 

 these grains were gathered. Hess concluded that the extent of the 

 visible spectrum is the same for the monkey as for man, and that it is 

 brightest for the dark-adapted monkey in the region where it is also 

 brightest for the dark-adapted man. 



Hess also studied the effects of different spectral lights and the lights 

 transmitted by coloured glasses on the pupil reactions of cats and rabbits. 



1 Arch. f. Aiiat., Suppl. 1905. 



