STATEMENT OF THE THEORY 219 



components we cannot explain on the theory the increase in saturation 

 which follows previous activity in the blue components. Hess refuses 

 to admit that the facts can be brought into line with the theory. We 

 must therefore accept the theory as explaining satisfactorily either the 

 phenomena of after-images or those of dichromatic vision, but not both. 

 The large mass of accurate evidence which has now accumulated, chiefly 

 from the observations of Abney and v. Kries and their fellow-workers, 

 showing that all spectral lights do not act upon all the components 

 makes it impossible to accept the explanation as it stands for successive 

 induction. Moreover, successive induction is a complex condition allied 

 to simultaneous induction, and it must be admitted that the facts of 

 simultaneous contrast cannot be explained directly by the trichromatic 

 theory alone. 



With regard to spatial induction v. Helmholtz made no serious 

 attempt to correlate the facts with the theory. In his opinion the 

 phenomena of simultaneous contrast are purely psychical and are 

 explained as "illusions of judgment." The arguments which can be 

 advanced in favour of this view do not concern the three-components 

 theory, and it is only by modifications of the theory, which will be dealt 

 with at a later stage, that any more purely physiological explanation, on 

 the three-components' basis, of spatial induction can be advanced. 



Finally, it may well be asked whether there is any direct positive 

 evidence of the existence of independent visual components answering 

 to the theoretical components. It is in this respect that the electrical 

 researches of Gotch, Himstedt and Nagel, and others, and the fatigue 

 experiments of Burch are so valuable. They offer no crucial proof, 

 but they are suggestively concordant with the view that such com- 

 ponents exist. It has already been mentioned that Burch is inclined 

 to predicate four. Theoretically this is of no importance, though it 

 complicates matters and is unnecessary for the explanation of the facts 

 of colour-mixtures, etc. It does not in any way detract from the 

 support which Burch's experiments afford of an additive theory de- 

 pendent upon processes occurring in certain components. 



As is usually the case, the three-components' theory becomes less 

 plausible the more concrete the form it takes. We have seen that 

 Young suggested three different types of nerve-fibre, others have 

 suggested three substances which undergo chemical change, others 

 again have suggested electrical changes. Each such hypothetical 

 fibre or substance, etc., leads inevitably to further hypothetical con- 

 ceptions of the details of the processes. These become the subject 



