380 PHYSIOLOGY CHAP. 



however, possible to conceive the chromatic excitations of the 

 cones as dependent on a polyform photochemical process, which 

 alters with the wave-length of the vibrations, without assuming 

 the presence of an indefinite number of chemical substances that 

 differ specifically, inasmuch as they are exclusively sensitive to 

 rays of a given wave-length ? 



Yet, notwithstanding its indefiniteness at this crucial point, 

 Wundt's theory must be given the credit of having clearly 

 pointed out that achromatic sensations arise from simple primitive 

 excitatory processes and are entirely independent of the chromatic 

 sensations that are developed later. 



Hering's theory of colour-vision, which, after that of Young- 

 Helmholtz, has been most widely accepted by physiologists, has 

 many points of resemblance with Wundt's theory, particularly 

 in assuming achromatic to be independent of chromatic perception ; 

 but it differs fundamentally in seeking to apply Johannes Miiller's 

 law of the specific energies, by reducing the fundamental qualities 

 of visual sensation to six white, black, red, green, yellow, 

 and blue. 



Hering assumes that there are three different photochemical 

 visual substances in the sensory elements of the retina, which are 

 continually broken down and built up again, like Boll's visual 

 purple. One of these substances is the physiological substrate 

 of achromatic sensations, i.e. black and white ; the other two give 

 rise to chromatic sensations. He replaces the three primary 

 colours of Young-Helmholtz by the four principal colours of the 

 spectrum (already recognised by Aubert, and much earlier by 

 Leonardo da Vinci), viz. red, yellow, green, and blue, which he 

 arranges in two pairs of opponent colours red-green and yellow- 

 blue, to each of which he assigns a specific photochemical visual 

 substance. 



Two antagonistic and simultaneous processes, the one assimi- 

 lative or anabolic, the other dissimilative or katabolic, are 

 constantly taking place in the three kinds of visual substance. 



When dissimilation prevails, sensations of white, red, and 

 yellow (katabolic sensations) are excited; when assimilation 

 predominates, the sensations are black, green, and blue (anabolic 

 sensations) ; when the two opponent processes balance (autonomous 

 equilibrium) there is a sensation of grey (mixture of black and 

 white) or white (mixture of the antagonistic pairs), by which the 

 colours are annulled. On these elaborate studies of vision Hering 

 founded his general [theory of the metabolism of living matter 

 (Vol. I. pp. 42, 86). - 



Hering assumes that the retina contains more of the white- 

 black than of the red-green and yellow-blue substances. Conse- 

 quently the assimilatory and dissimilatory changes are more 

 conspicuous in the first achromatic substance than in the two 



