STATEMENT OF THE THEORY 261 



the red or the green may alone affect consciousness, thus proving that 

 the actions of the red and green rays on the retina are not mentally 

 antagonistic or destructive, but that they proceed side by side in the one 

 area of the retina. (2) On fixation of a bright yellow light it becomes 

 after a few seconds a bright pure red or a bright pure green or shows 

 struggle of red and green. (3) In the after-image of very bright light, 

 whether white, or yellow, or of other colour, yellow never appears 

 except as an incident in the struggle between red and green, just as purple 

 occurs as an incident in the struggle of red and blue : it never appears 

 as do red, green, and blue, forming one of the phases of constant colour 

 of the recurring cycle, red, green, blue, red, green, blue (v. p. 111). 

 (4) On diminishing the illumination of a patch of yellow after fixating 

 it for 30 sees, or more, it usually becomes red unless the period of fixation 

 has been considerably prolonged, when it usually becomes blue. (5) The 

 after-image of bright yellow light may be yellow in the first phase, but 

 this yellow always resolves itself into green struggling upon red as in 

 the case of an initial yellow phase following bright white light. In the 

 after-image of less bright yellow light blue usually appears, but red and 

 green usually predominate. In fact "it is impossible to determine 

 from the character of a sensation that the physiological process under- 

 lying it is simple and that the sensation is not the result of psychical 

 fusion of the effects on consciousness of two or more separate physio- 

 logical processes." 



Although the theory of opponent colours offers an easily com- 

 prehensible explanation of the most prominent facts of induction, 

 both successive and simultaneous, it is incapable of accounting for 

 many other facts of the same nature. McDougall 1 brings forward the 

 following formidable list. 



(1) The occurrence of the sense of absolute darkness or blackness 

 in the absence of any stimulation of white light which could produce 

 it by simultaneous or successive contrast. If a dull or moderately 

 bright patch of white or coloured light on a dark ground is fixated very 

 steadily it will suddenly disappear. If the patch be upon a background 

 not quite dark, but feebly illuminated, it happens not infrequently 

 that the whole field of vision disappears, leaving a sense of complete 

 and extreme darkness which is more complete than is experienced on 

 excluding all light from the eyes. This " complete fading " occurs more 

 readily if accommodation is relaxed. 



1 Loc. cit. 



