176 EXPERIMENTAL PHYSIOLOGY. 



emphasized the fact that this is not the case and so far it has not been possible 

 to cover adequately the conditions of the defect with this, or indeed with any 

 other, theory. 



HERING'S THEORY OF COLOUR VISION. The only other theory which has 

 rivalled that of Young as a starting point for experimental work is the one brought 

 forward by Hering. This is also based on the laws of colour mixing which we have 

 already outlined, but it affords as well some physiological explanation for the 

 important fact that in our sensations light may not only be divided into different 

 colours, but these again appear to fall into two great classes, the 'warm' or 

 'bright', cheerful colours and the 'cold' or 'dull' ones. To the 'warm' class 

 belong reds, oranges, and yellows; they are bright of their own nature as it were, 

 quite apart from the brilliancy with which they are lighted. The dull ones are 

 green, blue and violet. There are many examples which show that we recognize 

 this inherent brightness or dullness of colours in everyday life. We "see things 

 through rosy spectacles" or we "feel blue". Decorators use yellow in north 

 rooms with no sunlight, but blue or green are said to be too 'cold'; children are 

 dressed in red in winter because it is "such a warm colour". The suggestion 

 which Hering made was this: there are three substances in the retina each of 

 which may be changed by light stimuli in two ways, being either built up or 

 broken down. One of them is the white-black substance; light waves of more 

 than a certain intensity break this down, no matter what their length, and give 

 as a result the sensation of white light. Darkness causes it to be built up and a 

 sensation of blackness arises from this. Of the other two components, one is 

 acted on only by red light, which breaks it down or katabolises it, or by green, the 

 complementary to red, which builds it up. Katabolism of the third substance 

 is caused by yellow light, anabolism by blue, its complementary. All other colour 

 sensations are mixtures of the sensations arising from the anabolism or katabolism 

 of the three substances. All colours which break down the components are 

 similar to white, which also does this; that is why they are specifically bright. 

 Colours which build up are like darkness and hence in themselves are dull. Com- 

 plementary colours give white light because, since they act equally on a single 

 substance, the katabolic effect of one cancels the anabolic effect of the other and 

 the only reaction left over is the katabolism which they both bring about in the 

 white-black substance. 



According to this theory the reason why the after-image of, for instance, a 

 green object is coloured red is that the action of the green light having built up a 

 great deal of the red-green substance, the red waves contained in the white light 

 from the bright field which follows have more substance to act on than the other 

 waves. The after-image of a yellow object is blue because the yellow waves have 

 broken down the yellow-blue substance to a great extent and the anabolic reaction 

 to the blue waves in the succeeding light is more intense, tending to restore the 

 equilibrium, than it would be if the equilibrium had been undisturbed. 



This theory gains some support from other observations on vision. Blackness 

 seems to us to be, not a negative, but a positive thing. In the ordinary resting 

 state of the retina we do not see blackness; our visual field is covered with vague 



