SECTION V 



OTHER THEORIES 



I. DoNDERs' Theory 



Donders^ accepted the principle of three " fundamental " colours, 

 derived from Young and v. Helmholtz, and later elaborated by Konig 

 and others. He also accepted four " simple " colour-sensations on 

 psychological grounds, — red, yellow, green and blue. He further 

 accepted the absolute correspondence of psychical and physical elements 

 in Fechner's sense, that they may be compared to the concave and 

 convex sides of the same curve. He was thus led, with Mach and 

 Hering, to postulate four corresponding specific processes, the sensations 

 of white and black excluded. 



Donders placed the seat of these processes in the brain. The 

 peripheral or retinal processes are trichromatic, the stimuli acting upon 

 the three mechanisms of the " fundamental " sensations. Here each 

 process acts upon a single form-element. In the cerebral centres more 

 than one process can act upon the same element. From the combined 

 action of two colours a third, containing none of the characteristics of 

 either, can arise, e.g., yellow from red and green, so that from two pro- 

 cesses a third, sui generis, is evolved. 



The processes may be regarded as chemical dissociations. Complete 

 dissociation of the molecules gives rise to the sensation of white. It 

 must be complete because the sensation continues unchanged and does 

 not dispose to secondary sensations ; this is the case with white only. 

 Restoration and dissociation occur somewhat as described in Hering's 

 theory. 



The sensations of the " simple " colours are attributed to partial 

 dissociation of the same molecules. In opposition to white they call 



1 Arch. f. Ophth. xxvn. 1, 155, 1881. 



