vii EETINAL EXCITATION 3"79 



as equal as possible. It would consequently bo better to take 

 dark blue instead of violet for the third colour, as did A. Fick and 

 Konig. With red, green, and blue we can obtain all the inter- 

 mediate colours of the spectrum, which is not possible with red, 

 green, and violet. 



It was long supposed that Young's three-colour theory was 

 supported by facts observed in partial colour-blindness (dyschroma- 

 toptia). Helmholtz assumed three different kinds of dyschroma- 

 topsia red-blindness (anerythropsia),greQn-\ lind ness (achloropsia), 

 and violet-blindness (acyanopsia) due to deficiency of one or other 

 of the three receptor substances. But more accurate and com- 

 prehensive study of these cases has shown the impossibility of 

 explaining them on this theory. As Wundt pointed out, it is 

 inadequate to explain cases of total colour-blindness (achroma- 

 topsia} in which the solar spectrum appears colourless, brightest 

 in the middle (in the yellow-green), and less bright at the ends, 

 while paintings appear as photographs. Again the fact that in 

 red- and green-blindness white light appears white and not 

 coloured, as it should according to the Young-Helmholtz theory, 

 is irreconcilable with it. The colour-blind, moreover, declare that 

 they see yellow and blue, while on this theory they should see 

 green and violet (anerythropsia) or red and violet (achloropsia). 



To obviate this and other difficulties inseparable from the 

 thre'e-colour hypothesis, Wundt, as early as the first edition of 

 his Psychological Physiology (1874), brought forward another theory 

 based on the assumption that two different stimulation processes, 

 chromatic and achromatic, are set in action by every retinal 

 irritation. The chromatic excitation is a function of the wave- 

 length of the vibrations ; the achromatic is a function of the 



O 



amplitude of the vibrations. 



Chromatic stimulation, according to Wundt, is a polyform 

 photochemical process which changes with the wave-length ; 

 achromatic stimulation, on the contrary, is a uniform photo- 

 chemical process, which changes only in intensity and not in 

 quality with alteration of the wave-length. 



Achromatic excitation can be aroused by the weakest and the 

 strongest stimuli; chromatic excitation can only be evoked by 

 stimuli of moderate intensity. Wundt assumed that achromatic 

 sensations preceded chromatic sensations in the phylogenetic and 

 ontogenetic development of vision, and were not therefore the 

 product of the latter. 



Wundt's hypothesis is obviously too vague and indefinite, and 

 scarcely deserve* the name of a theory. Have the achromatic 

 and chromatic excitatory processes any distinct physiological 

 substrate in the retina ? This could be answered by means of the 

 " duplicity theory " which Schultze, Pariuaud, and v. Kries put 

 forward in regard to the functions of the rods and cones. Is it, 



