THEORIES OF COLOUR-VISION. 1121 



addition to the three kinds of visual substance with their seat in the periphery, 

 there is a central black-white excitation which is the basis of the intrinsic light, 

 and is capable of being set in action by change in the chromatic substances in 

 the periphery as well as by change in the peripheral black-white substance. 



Ebbinghaus 1 attempted to bring the theory into relation with retinal 

 structures. He supposed that visual purple was Hering's blue-yellow substance, 

 that it existed alone in the rods, and together with some other substance 

 which neutralised its colour in the cones. These views were supported by 

 the correspondence of the absorption curves of visual purple and visual yellow 

 with the elementary curves for yellow and blue in the red-green blind. 



Other theories of colour- vision. — The theory of Donders 2 occupies 

 a position intermediate between those of Helmholtz and Hering. 

 Donders recognised the claims of red, yellow, green, and blue to 

 be regarded as primary, or, as he called them, simple colours. On 

 the other hand, he held the facts of colour mixture to show a 

 threefold physiological process. The colours corresponding to these 

 three physiological energies he called fundamental colours, and he 

 adopted as such red, green, and violet. He supposed that these energies 

 had their seat in the periphery, while the processes in the visual centres 

 had a fourfold nature, corresponding to the four simple colours. He 

 further supposed that a single peripheral process might affect more than 

 one central process, so that a fundamental colour, as violet, might corre- 

 spond to two simple colours, as red and blue. Tor his central processes 

 Donders adopts a hypothesis in terms of molecules. The sensation white 

 corresponds to complete, colours to partial, dissociation of the molecules ; 

 this partial dissociation gives rise to molecules whose secondary dissocia- 

 tion is the basis of the colours complementary to those arising through 

 the primary dissociation. Black corresponds to a condition where dis- 

 sociation and repair are equal, but both are reduced to a minimum. 

 Donders did not attempt to apply his theory to the whole range of colour 

 phenomena, but used it chiefly in reference to colour-blindness. A some- 

 what similar molecular hypothesis has been advanced by Ladd Franklin. 3 



In 1881, Preyer 4 advanced a theory which, in many respects, 

 resembled that of Hering. In this theory analogies were drawn with 

 the temperature sense of the skin, and assumptions were made as to 

 the seat of the different kinds of sensation, the rods being regarded as 

 photogenous, and the cones as chromatogenous elements. 



The theory of Wundt 5 has been already mentioned. He assumes 

 the existence of separate substances for light and colour sensations, but 

 supposes that the latter depend on different kinds of change in one com- 

 plex substance. In order to support his theory, attempts have been made 

 to show that colour-blindness may occur in a far greater variety of forms 

 than is commonly supposed (Kirschmann). 6 Contrast is explained psycho- 

 logically, while after-images are referred to fatigue assisted by contrast. 



A theory of a different nature has been advanced by Charpentier. 7 

 He distinguishes between two kinds of retinal elements, — photo-resthetic 

 elements of a rudimentary nature, which have to do with sensations 



1 Zlschr.f. Psychol, u. Physiol, d. Sinnesorg., Hamburg u. Leipzig, 1893, Bd. v. S. 145. 



2 Arch.f. Ophth., 1881, Bd. xxvii. Abth. 1, S. 55 ; and 1884, Bd. xxx. Abth. 1, S. 15. 

 i 3 Mind, London and Edinburgh, 1893>vol. ii. p. 473. 



4 Arch.f. d. yes. Physiol, Bonn, 1881, Bd. xxv. S. 31. 



5 "Grundztige d. phys. Psych.," 1893, Bd. i. S. 535. 



6 Phil. Stud., Leipzig, 1893, Bd. viii. S. 173 and 407. 



7 " La lumiere et les couleurs," Paris, 1888, p. 265. 



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