i88 HERMANN VON HELMHOLTZ 



a colour once established, even when the conditions which 

 produced it are removed; in homogeneous red illumination 

 the parts that are poorly lighted take on the complementary 

 green in consequence of retinal fatigue. Helmholtz conjec- 

 tured that the appearance of true contrast phenomena depends 

 upon an error of judgement : we can compare correctly when 

 the points to be compared are adjacent in the field of vision ; 

 spatial separation and succession in time on the contrary 

 weaken the positiveness of the impression. He opposes this 

 view to the earlier explanations which assumed actual alteration 

 of the nervous excitation. 



Starting from Fechner's Theory of After-images, which 

 only fails to give a positive explanation of phenomena in 

 cases where the circumstances are very complicated, Helmholtz 

 attempts to give a theoretical exposition of the temporal 

 sequence of visual impressions. Since Fechner gives two 

 grounds of explanation, to which he refers the complexity 

 of the phenomena relating to this subject, i. e. survival of 

 excitation, and fatigue of the nervous mechanism of the 

 eye owing to previous excitation, it is obvious that in the 

 colour phenomena of after-images, each of these processes 

 must come into play for each of the three kinds of nerve-fibres 

 assumed by Young's Colour Theory. Accordingly there must 

 be six quantities of alterable magnitude, on which depend the 

 brilliancy and colour of the after-image observed under certain 

 given external conditions of illumination. As an after-image 

 is positive when the after-excitation more than counteracts the 

 fatigue, negative in the opposite case, an explanation of the 

 complex processes with several colours is only possible under 

 definite quantitative assumptions as to the time phenomena 

 of excitation and fatigue in the nerve apparatus. As at the 

 time of his investigation there were very few real quantitative 

 determinations, Helmholtz confined himself to finding the mathe- 

 matical functions, the variation of which with time corresponds, 

 at least in direction, with the course of the phenomena, even 

 if no exact correspondence of actually measured magnitudes 

 be demonstrable. 



'We find/ he says in a note on these observations, 'that 

 two kinds of alterations are brought about in the living eye 

 by light, apart from any distinctions of colours, i. e. excitation 



