290 COLOUR VISION 



so far as its relative intensity is concerned, is also dependent upon the 

 wave-length, and reaches its maximum in the yellow. Both stimula- 

 tions follow different laws with increasing stimulus intensities. At low 

 intensities the achromatic surpasses the chromatic ; at moderate 

 intensities the chromatic is the relatively stronger ; at the highest 

 intensities the achromatic regains superiority. 



The achromatic stimulation consists in a uniform photochemical 

 process, the intensity of which is partly dependent upon the objective 

 light intensity and partly upon the wave-length, since it reaches its 

 maximum in the yellow and falls off towards both ends of the spectrum. 



The chromatic stimulation consists in a poly form photochemical 

 process, which changes continuously with the wave-length. Thus 

 extreme differences of wave-length produce effects which are of almost 

 the same nature, whilst the effects of certain different intervening 

 wave-lengths are related in such a manner that they completely com- 

 pensate each other like opposed phases of one and the same movement. 



Every photochemical process of stimulation outlasts the excitation 

 for a certain period and exhausts the sensibility of the nerve-substance 

 for the particular form of excitation. Positive and homochromatic 

 after-images are to be explained by the persistence, negative and com- 

 plementary after-images by the exhaustion of the sensibility. The 

 phenomena of contrast are to be explained by the general principle 

 that all impressions of light and colour are experienced in relation to 

 each other, i.e., by the general law of relativity. 



VI. G. E. MULLER'S THEORY 



G. E. Miiller's theory 1 is a modification of Hering's theory in the 

 light of psychophysical principles. It is set forth at great length and 

 would already require further qualification to account for the facts 

 which have been discovered since its promulgation. It is based on the 

 fundamental psychophysical hypothesis, which he wrongly terms an 

 axiom, that " an equality, similarity, or difference in the condition of 

 sensations corresponds to an equality, similarity, or difference in the 

 condition of the psychophysical processes." 



Miiller's essential disagreement with Hering is that the antagonistic 

 action of white and black, red and green, yellow and blue, would be 

 different from what it is if Hering's assumption were true that the 



1 Ztsch. f. Psychol. u. Pliysiol. d. Sin-nc.torg. x. 1 and 321, 1896 ; xiv. 1 and 161, 

 1897 



