264 COLOUR VISION 



continuation for a certain period of the anabolic or katabolic process 

 after the stimulus has been withdrawn. 



v. Kries, however, pointed out that the theory fails to explain 

 certain details adequately. Thus, since stimulation with colourless 

 light does not act upon the chromatic substances it was to be expected 

 that after previous stimulation with white light the same quantities of 

 coloured light would be required on both the stimulated and resting 

 areas to produce a colour-match. He found that it was not the case. 

 Hering 1 made many objections to v. Kries's experiments and explained 

 away his results (v. p. 108). He appears to hold that whilst previous 

 stimulation with white does not affect colour valency, chromatic stimula- 

 tion markedly affects white valency. As Greenwood points out 2 , Bering's 

 hypothesis becomes unsatisfactory by multiplying its detailed sub- 

 hypotheses. 



On simultaneous contrast Hering himself has done much work and 

 has been the means of initiating more. The researches of Hess and 

 Pretori have already been discussed. Those of Pretori and Sachs 3 on 

 colour contrast are scarcely comprehensible except in terms of the 

 theory and may now be mentioned. These authors endeavoured to 

 discover the quantitative relations underlying colour contrast. Accord- 

 ing to Hering toned lights possess both colour and white valencies. 

 In addition to these exogenous valencies there is an endogenous black 

 valency dependent upon autonomous processes and manifesting itself 

 in the intrinsic grey of the resting eye. There exists therefore between 

 a coloured " contrast- exciting " (kontrasterregend) and a colourless 

 " contrast-responding " (kontrasterleidend) field besides the colour con- 

 trast also a colourless brightness contrast so long as the white valency 

 of the contrast-responding field is not exactly equal to the black induction 

 due to the white valency of the contrast-exciting field. The saturation 

 of the contrast colour depends therefore upon the relationship of the 

 colour contrast to the simultaneous colourless contrast. Hering 4 has 

 demonstrated the disturbing influence of simultaneous brightness con- 

 trast on colour contrast by special experiments. 



Pretori and Sachs used a rotating disc with variable sectors. Three 

 sets of circular papers were mounted concentrically on the disc. The 

 diameters of the three sets were : upper 8 cm., middle 11-4 cm., lower 

 19-6 cm. ; so that the middle set formed a ring 17 mm. in breadth and 



1 Arch.f. d. ges. Physiol. xciv. 533, 1903. 



2 Physiology of the Special Senses, p. 200. 



3 Arch. f. d. ges. Physiol. LX. 71, 1895. 4 Loc. cit. XLI. 27, 1887. 



