66 COLOUR VISION 



It has been found in experiments on the colour threshold that if 

 the colour is just extinguished very slight illumination of the retina 

 with white light will cause it to reappear without making any alteration 

 in the intensity of the coloured light. Boswell 1 , working under Nagel's 

 direction, showed that the same phenomenon occurred on the addition 

 of any light of considerable scotopic value, such as green. Since the 

 eye is dark adapted the summation of the colourless scotopic values of 

 the coloured lights throws the value of the stronger above the chromatic 

 threshold. It is not a question of contrast, as is shown by the action 

 of white light, nor of complementary colours. The same fact is borne 

 out by Abney's experiments, where the extinction intensity for retinal 

 illumination of 0'2 foot-candle is 2 '6 times that of illumination of 

 U'00078 foot-candle. 



1 Ztsck. f Sinnesphysiol XLI 364, 1907. 



