552 



Dr. F. W. Edridge-Green. 



[Dec. 4 



is difficult, to recognise as coloured without special comparison with a known 

 white surface, as, for instance, if a small piece of grey paper be placed upon 

 a large piece of very pale green paper. If the paper be regarded as white, 

 the light reflected from the grey paper must be regarded as rose, for the 

 subtraction of the small quantity of the green light from the light of the 

 pale green paper in order to make this white is sufficient, when subtracted 

 from the white light reflected from the grey paper, to make this appear rose. 

 When it is recognised that the green paper is not white the contrast colour 

 disappears, and the grey paper is seen as grey. The contrast colour is most 

 •developed on grey paper, and not nearly so well, if at all, on white or black 

 paper. It is therefore produced in exactly those conditions in which the 

 subtraction of a small quantity of green light will be most effective in 

 altering the appearance of the colour. It is well known that in most contrast 

 ■experiments, if a direct comparison be made with a known white surface, the 

 contrast colour disappears. There is no reason why this should occur if the 

 contrast colour were an actually induced colour. 



My conclusion, therefore, is that the contrast colour developed in 

 simultaneous contrast is due to the perception of an actual objective relative 

 difference — in fact, the greatest difference which is perceptible in the circum- 

 stances, white being not a fixed objective quality, but a sensation produced 

 by admixture of light of certain wave-lengths. If the sensation of one 

 colour induce that of the complementary in the adjacent portion of the 

 retina, there are many circumstances in which the colour ought to be visible 

 and yet is not found. I have never yet, for instance (excluding negative 

 after-images), seen the faintest trace of green on the dark surfaces in a 

 photographic dark room illuminated by a pure red light. Neither have I 

 come across any other person who has seen green in these circumstances. 

 On coming out of the dark room white objects only appear slightly tinged 

 with green, if any change be noticed at all. Not the faintest trace of green 

 is to be seen round red lights at night, and I find the greatest difficulty in 

 obtaining an after-image even by staring fixedly at the red light, if this be 

 not of considerable intensity. 



The subject of induction of colour by simultaneous contrast can be 

 investigated in another way, that is by entirely eliminating red or any other 

 spectral colour and then studying the effects of simultaneous contrast. This 

 may be accomplished by viewing objects through coloured glasses which 

 are opaque to light of certain wave-lengths. In the case of red light we 

 can use blue-green glasses, which are impermeable to the red rays. I have 

 therefore had a pair of spectacles glazed with blue-green glass. This blue- 

 green glass is absolutely opaque to the red rays from the termination of the 



