286 COLOUR VISION 



Arguments from Regional Effects. I. For the completely developed 

 colour sense only three visual substances are necessary, the red, the 

 green, and the blue. For the red-green-blindness of the middle zone 

 only two are necessary, the blue and the yellow. For the total colour 

 blindness of the outermost zone only one is necessary, the white. 



II. The three developmental phases of the visual substances 

 increase from without towards the centre. W is maximal at the 

 periphery and diminishes steadily towards the centre ; B and Y are 

 slight at the periphery, maximal in the middle zone and slight at the 

 centre ; R and G are minimal at the periphery and increase to a maximum 

 at the centre. 



III. The validity of colour matches for centre and periphery is 

 associated with a reduction of sensation. Hence visual substances of 

 the same nature are present throughout, though perhaps in smaller 

 number in the periphery, i.e., either (a) one or two of the visual sub- 

 stances are absent, or (6) all the visual substances are present but initiate 

 a smaller number of sensations. 



IV. The validity of luminosity matches for centre and periphery 

 necessitates the presence of all the visual substances associated with a 

 smaller number of sensations. If one visual substance were absent the 

 spectral colour which specially stimulates it would appear darker in 

 peripheral vision, which is not the case. In the middle zone, of the 

 fundamental colour sensations only blue is present ; red and green are 

 replaced by yellow. Hence the red and green substances here give 

 rise to a single sensation, yellow, and must be represented by the yellow 

 substance. In the outer zone all three fundamental sensations are 

 absent, and all three must be represented by the white substance. 

 These deductions are legitimate because each pair of derivative sub- 

 stances together combine the peculiarities of the respective mother 

 substances. 



On this basis the theory may be extended. Each visual substance 

 must be compounded of two functionally distinct parts, one for lumino- 

 sity, the other for hue. The first part, which is first affected by the 

 incident light, may be called the stimulus-receptor {Reizempf anger). 

 The second part, which determines the nature and intensity of the aroused 

 sensation, may be called the sensation-stimulator (Empfindungserreger). 

 The amount of energy set free by the receptor determines the excitation 

 of the stimulator and thus the luminosity of the resultant sensation. 



In the fully developed colour-sense the receptor for long waves in 



