260 COLOUR VISION 



Further, whilst red-green and yellow-blue show a null-point or 

 absence of sensation, black-white shows no such null-point. Moreover 

 the autonomous and allonomous equilibrium of the black- white are very 

 different greys. In the light of the duplicity theory Hering's mean 

 grey must be referred to a different mechanisn from the chromatic 

 mechanism. Whilst long fixation of a toned light certainly tends to 

 produce a tone-less sensation, the behaviour of black and white is quite 

 different. Yet on no theory can the chromatic mechanism be deprived 

 of its capacity to produce a tone-less sensation. 



We have already stated that a given colour can be defined by its 

 hue, its luminosity and its degree of saturation. Hering hypothecates 

 three ratios : white to black, red to green, and yellow to blue. The 

 question arises how the hue, the luminosity and the saturation are 

 determined by these three ratios. Hering's early hypothesis that the 

 luminosity was determined by the white to black ratio proved untenable 

 and gave place to the specific brightness of colours. Similarly, the hue 

 cannot be determined simply by the two chromatic ratios, since we have 

 seen that the hue changes for most colours on admixture with white. 

 These are serious difficulties attending acceptance of the theory, but it 

 must be admitted that the relationship of luminosity to hue and saturation 

 is not satisfactorily explained either by the three components or any 

 other theory. 



Hering's theory starts from the choice of psychologically pure colour 

 sensations. If, however, spectral red and green are chosen and mixed 

 they prove not to be complementary colours^. The colour adopted 

 by Hering and his followers as fundamental red is a distinctly carmine 

 or bluish-red {v. p. 258). Psychological analysis, however, does not 

 afford a very secure foundation for the determination of the purity or 

 simplicity of a colour sensation 2. Most people would say that green 

 is not a simple sensation, doubtless owing to experience derived from 

 the mixture of pigments. On the other hand, most people would agree 

 that yellow is a simple sensation, and this assumption is a necessary part 

 of Hering's theory. Yet there are many arguments, apart from those 

 derived from colour mixtures and fatigue already discussed, against 

 such a view. McDougalP brings forward the following. (1) If patches 

 of red and green light be thrown upon one area of one retina, they may 

 under suitable conditions show the phenomenon of struggle as well as 

 of fusion, i.e., at one moment they may appear as yellow, at another 



^ Ladd- Franklin, Psychol. Rev, vtn. ^ See Section I. Chap. ni. 



3 Mind, X. N.S. 380. 1901. 



