254 COLOUR VISION 



an ascending change, that corresponding to the white a descending 

 change, and the condition of uniform grey of the whole surface corre- 

 sponds to equihbrium, 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 sub- 

 stances. 



" 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 

 contribute 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 Kght. 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 comple- 

 mentary 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 



