COLOUR VISION AND COLOUR-VISION THEORIES 4 79 



relation to this false estimation. That is to say, the missing 

 (or added) colour is deducted from (or added to) both. 



8. A complementary contrast colour does not appear in the 

 absence of objective light of that colour. 



9. The negative after-images of contrasted colours are com- 

 plementary to the colours seen. 



VII. Colour Adaptation (25) 



Colour adaptation is the term applied to the changes that 

 take place when the eye is subjected to light in which certain 

 wave-lengths predominate. Colour adaptation is the means by 

 which colours appear to remain the same when the physical 

 conditions are quite different. Daylight differs chiefly from the 

 Osram electric light in that it contains many more blue rays and 

 less red. A piece of bright blue paper appears a darker blue by 

 an Osram light than by daylight. The fact that it appears blue at 

 all is due to colour adaptation, for if we place the blue paper in 

 a photometer and illuminate it by Osram light it will be matched 

 exactly by a piece of chocolate-brown paper illuminated by day- 

 light. It will be noticed that the theory of perception of 

 relative difference accounts for all the facts. The following are 

 the facts of colour adaptation : 



1. In colour adaptation, the retino-cerebral apparatus appears 

 to become less and less sensitive to the colour corresponding to 

 the dominant wave-length, and to set up a new system of 

 differentiation. 



2. When light of a composition differing from that of day- 

 light is employed to illuminate objects, an immediate and 

 unconscious estimation of the colours of these objects is made 

 in relation to this light, the light employed being considered as 

 white light. 



3. No colour is seen of which the physical basis is not present 

 in the light employed. 



4. When spectral regions are examined with a colour-adapted 

 eye, that of the dominant wave-length appears colourless, whilst 

 those immediately on either side of it appear to be shifted higher 

 and lower in the scale respectively. 



5. There is immediate colour adaptation, as well as colour 

 adaptation after a longer stimulation with the adapting light. 



6. Colours which correspond to the dominant wave-length of 



