STUDIES IN SPECIAL SKNSE PHYSIOLOGY 419 



stances " have just as much and just as little real existence as the 

 three components of our other theory. It is idle to say that the 

 postulated anabolic and katabolic processes are essentially unlike 

 any chemical mechanisms with which we are acquainted. It is 

 equally vain to object that stimulation processes in animal nature 

 are, to all appearance, bound up with katabolic changes ; this 

 would be a valid objection if we tried to identify the hypothetical 

 substances with any known retino-cerebral constituent. No such 

 identification is attempted ; the suggestion that Hering and his 

 school identify the black-white substance with visual purple is 

 altogether false. The fact is that this theory can only be judged 

 on the grounds of scientific expediency, and in no other way. Does 

 this method of presenting the facts give us a clearer insight into 

 the phenomena of vision than the method based upon stimulus 

 relations ? This is the only question which requires to be 

 answered. 



We notice at once that the hypothesis, in the form in which 

 it has just been epitomised, offers two considerable advantages. 

 Firstly, it deals with the immediate data of vision, the visual 

 sensations of colour, directly, and not in terms of stimulation 

 magnitudes. In this sense it might be called a primary hypo- 

 thesis. In the second place it is essentially simple and easy of 

 comprehension, which is a very strong point in its favour. 



But if we find, on testing -this simple hypothesis with such 

 facts of experiment as have previously been detailed, that it 

 rapidly ceases to be simple ; if we find that its apparently direct 

 grip upon the objects of investigation is perceptibly weakened ; in 

 one word, if it ceases to be definite, then its characteristic advan- 

 tages will have disappeared. 



Let us first of all attempt to express the facts of successive 

 contrast, after-images in terms of the hypothesis. After resting 

 the eye on a white object, we should expect, under certain con- 

 ditions, a positive, under others a negative after-effect. The 

 experimental results agree perfectly with the theory. In the case 

 of colour stimuli, we can understand why complementary reacting 

 lights gain in saturation. For example, if the eye be stimulated 

 with green, anabolism will occur in the red-green substance, an 

 anabolism which will lead to the formation of a large quantity of 

 " material." If now red light stimulates the retina, it produces 

 not only katabolism of the normal quantity of substance, but the 



