262 COLOUR VISION 



(2) Simultaneously induced light appearing : 



(a) on a ground that has shown no previous contrast effect ; 

 (6) almost or quite as bright as the inducing light ; 

 (c) failing to appear on a ground on which a marked contrast is 

 produced. 



(3) Binocular contrast, whether of white or coloured light. 



(4) Binocular fusion of complementary colours to give a white 

 sensation. 



(3) and (4) are only to be reconciled to the theory of opponent 

 colours on the assumption, in the face of strong evidence to the contrary, 

 that the cerebral areas for the two eyes are identical. 



(5) Certain features of uniocular contrast : 



(a) the total inhibition of a patch of colour by bright white light 

 falling on an area of the retina at some distance from that affected by 

 the coloured light ; 



(&) the inhibition of colour of low saturation by a white ground ; 



(c) the fact that contrast only appears as a modification of an 

 existing sensation. 



(6) Positive after-images in general : 



(a) the phenomena of fading of after-images, especially of white 

 light, with its recurring cycle of phases of pure and highly saturated 

 red, green, and blue (v. p. Ill) ; 



(6) the fact that fixation of white light is invariably followed 

 by an after-image brighter than the ground, if the conditions that lead 

 to the production of a relatively bright halo are avoided ; 



(c) The same- coloured initial phase of the after-image of all very 

 bright coloured lights (v. p. 102) ; 



(d) the frequent appearance of a same-coloured phase in the after- 

 image of less bright coloured lights (v. p. 102). 



(7) The variety of colours that may appear in the after-image 

 of any light save the dullest -that will give any after-image, e.g., pure 

 blue phases in the after-image of red light. 



(8) The array of facts indicating that the yellow sensation is due to 

 psychical fusion of red and green (v. p. 260). 



