274 COLOUR VISION 



V. Kries^ raises the fundamental objection that the complete decom- 

 position of the differentiated molecule gives rise to the same sensation 

 as the undifferentiated. This objection may be surmounted by suppos- 

 ing that the differentiated molecule still retains a core or substratum of 

 undifferentiated atoms, which are in a more stable condition than the 

 rest of the differentiated molecule. 



Whilst the explanation of after-images may pass muster, that of 

 simultaneous contrast is unsatisfactory. We know of no " retinal 

 circulation " Avhich is so rapid as to account for the phenomena. It 

 would have to be almost instantaneous and it is impossible on physical 

 grounds to conceive of a mechanism so potent as to transfer molecules 

 so instantaneously through a finite distance. The reciprocal action of 

 retinal areas must be explained on some more plausible grounds. 



The suggestion that both the scotopic and photopic luminosities 

 are to be attributed to the rods and their contained grey substance is 

 attractive, but the fact that the luminosity curve of the totally colour- 

 blind remains the same as the normal achromatic scotopic curve even 

 when the intensities of the incident light are raised far above the 

 scotopic level is against the conjecture. 



III. McDougall's Theory 



McDougall^ strongly supports the three-components theory of colour 

 vision. He prefers to term it Young's theory rather than the Young- 

 Helmholtz theory, because he considers that v. Helmholtz did it scant 

 service by his far-fetched psychological explanations of the facts of 

 induction. 



McDougall's views of colour vision are part of a general theory of 

 psycho-physical processes^. He defines a psychological process as that 

 part of the total process of physiological excitation within the nervous 

 system which stands in a relation of immediate interaction with psychical 

 process or consciousness. He thus rejects the hypothesis of psycho- 

 physical parallelism held by Fechner, G. E. Miiller and others. He 

 adduces strong evidence derived from anatomy, physiology and psycho- 

 logy in favour of the view that the seat of the psycho-physical process 

 is in the synapses or arborisations at the sites of contact of nerve cells 

 or neurones. Many of his arguments on this part of his theory receive 



^ In Nagel's Hayidb. d. Physiol, d. Menschen, p. 277. 



2 Mind, X. N.S. 52, 210, 347. 1901. 



3 Brain, xxiv. 577, 1901 ; xx\^. 153, 1903. 



