THEORIES OF COLOUR- VISION. 1115 



changing very slowly, and in any definite interval of time as corresponding 

 to the mean illumination during that period. The white-black substance 

 is then in a condition of allonomous equilibrium at a low potential. 

 The brighter the illumination to which the eye is adapted, the lower is 

 the potential at which equilibrium occurs. During the daytime the 

 white-black substance is always to be regarded as in a condition of relat- 

 ively low potential. The condition of allonomous equilibrium at a high 

 potential is exceptional in the case of the black-white substance, owing 

 to the fact that there is no external stimulus to anabolism. Black 

 proper, according to Hering, only occurs under the influence of simul- 

 taneous or successive contrast ; and the best example of allonomous 

 equilibrium at relatively high potential, in the case of the black- white 

 substance, occurs in Hering's "simultaneous induction." If a black 

 square on a white ground is fixed, the whole surface after a time is seen 

 as a uniform grey. The black- white substance corresponding to the black 

 square will have undergone an ascending change, that corresponding to 

 the white a descending change, and the condition of uniform grey of the 

 whole surface corresponds to equilibrium, which in the former case is at 

 a high potential, and in the latter at a low potential. The potential of 

 the former will only, however, be higher than that of the eye completely 

 adapted to the dark, if the experiment is carried out with the eye adapted 

 to the dark. In the case of the white-black substance, it would seem as if 

 allonomous equilibrium might in ordinary life take place at very different 

 levels of potential, less than that of the autonomous condition ; and that 

 allonomous equilibrium, at a potential higher than that of the dark-adapted 

 eye, only occurs under very exceptional conditions. This might be urged 

 as an objection to the terminology adopted by Hering. It is due, however, 

 to the absence of proper external stimuli to anabolism, and this difficulty 

 does not occur in Hering's treatment of the chromatic substances. 



When the red -green and yellow-blue substances undergo the descend- 

 ing change, the corresponding sensations are red and yellow respectively. 

 When they undergo the ascending change, they are green and blue 

 respectively. The rapidity of the change (or the predominance of one 

 process over the other) partly determines the " weight " of the particular 

 element in question in the sensational complex, or, in other words, the 

 purity or saturation of the colour; the other factor determining the purity 

 being the degree of simultaneous stimulation of the other substances. 



The conditions of adaptation to coloured light may be readily 

 referred to different conditions of allonomous equilibrium. When the 

 chromatic substances are in a condition of equilibrium, they do not con- 

 tribute to the quality of the sensational complex. They are equally in a 

 condition of autonomous equilibrium, whether the eye has been wholly 

 unstimulated or whether the stimulation has been exclusively by mixed 

 colourless light. When the eye, after exposure to red light, no longer 

 sees objects red, the red-green substance has become adapted to the 

 light, and is in a condition of allonomous equilibrium, but at a low 

 potential ; on the other hand, in adaptation to green light, this substance 

 is in allonomous equilibrium at high potential. With removal of the light 

 to which the eye has become adapted, the complementary after-image 

 colour is due to the autonomous change back to a condition of mean 

 potential. After-images occupying limited portions of the visual field are 

 due to local adaptation, and are explained on the same lines as general 

 adaptation. The after-images seen with complete exclusion of light from 



