VISION 389 



feeling that a colour is pure or mixed is not to be trusted. It 

 may be based upon the chromatic aberration of the eye, or it 

 may be reminiscent of the paint-box. We know that we cannot 

 make yellow by mixing red and green pigments, hence we feel 

 that it is pure. Of green we are not by any means sure ; 

 gamboge and Prussian blue come into our minds.) Except 

 when the light which falls upon the retina is giving rise to one of 

 the four pure colour-sensations, all three substances are simul- 

 taneously affected, although one may be undergoing katabolism 

 while the other two are being built up, or vice versa. Hering 

 accounts for simultaneous contrast by assuming that the 

 activity of any one part of the retina induces an opposite kind 

 of change in the remainder, and especially in the vicinity of 

 the primarily active part. When a certain patch is developing 

 a sensation of red, the rest of the retina develops a sensation 

 of green. 



The great merit of the theory is, however, to be found in its 

 offering an explanation of complementary after-images. The 

 green patch seen with closed eyes after one has stared at a 

 red object is due to the rebound of metabolism. In returning 

 to a condition of chemical equilibrium the retinal substance 

 acts as a stimulant which evokes the antagonistic colour. But 

 it is a theory which makes very large assumptions. It assumes, 

 for example, the possibility of the existence of a substance 

 which is built up by light from one end of the spectrum, and 

 decomposed by light from its centre. Not that Hering regards 

 the existence of three retinal substances as essential to his 

 theory. He is prepared to transfer to the brain the seat of the 

 substances, or the substance, which, by their, or its, anabolism 

 and katabolism, produces antagonistic colour-perceptions ; but 

 in this he is abandoning physiology for metaphysics. We have 

 no warrant for imagining that there exists in the brain any 

 substance which, by undergoing physical changes of various 

 kinds, produces various psychical effects. The problem to be 

 solved is physiological. Rays of light of different wave- 

 lengths excite the retina to discharge impulses which are 

 variously distributed in the brain. The effects which they 

 produce in consciousness depend upon their distribution. The 

 impulses to which the longest rays give rise evoke sensa- 

 tions of red, those due to the shortest, sensations of violet. 



