RESEARCHES BASED UPON THE THEORY 2Gr, 



constituted the contrast-responding field. The inner circle (upper set) 

 and outer ring (lower set) always had the same sectors and together 

 constituted the contrast-exciting field. If the exciting field was red, 

 for example, and the responding ring black, the black appeared reddish 1 . 

 If a white sector was introduced into the responding field the red dis- 

 appeared, and if the white sector was increased in size the grey ring 

 became tinged with the contrast colour, green. By further adding a 

 suitable red sector to the responding field the contrast green could be 

 counteracted. The size of the red sector afforded a measure of the 

 contrast effect. If the red sector in the responding field was increased 

 by 10 each time from zero to 120, the white and black sectors being 

 altered suitably to eliminate the contrast colour, it was found that with 

 a constant coloured exciting field the grey responding field remained grey 

 when its red and white valencies increased proportionately. 



The contrast-exciting field was then varied in three ways : (1) the 

 colour valency was changed while the white valency was kept constant ; 

 (2) the white valency was changed while the colour valency was kept 

 constant ; (3) both colour and white valencies were changed, but the 

 ratio between them was kept constant. 



(1) In the contrast-exciting field there were a coloured sector and 

 sectors of white and black such that the grey produced by rotation looked 

 exactly like the colour when seen by achromatic scotopic vision. In 

 the contrast-responding field a definite grey made up of black and white 

 sectors was used and a coloured sector was introduced until the initial 

 contrast colour was eliminated. In the experiments the coloured sector 

 in the exciting field was gradually increased ; that in the responding field 

 was kept constant ; the contrast colour was eliminated by gradually 

 increasing the white sector from zero. It was found that with constant 

 white valency of the contrast-exciting field (and therefore constant 

 black induction due to it), combined with increase of colour valency, 

 the same amount of colour contrast is caused by a simply proportional 

 diminution of white valency in the contrast-responding field. If there- 

 fore the white valency of the responding field is kept constant the con- 

 trast effect increases proportionally to the increase in colour valency. 

 If the exciting light is kept constant and the white valency of the respond- 

 ing field increased from zero the saturation of the contrast colour in- 

 creases up to a certain optimum value. 



(2) In the contrast- exciting field the coloured sector was kept 

 constant and the black- white sectors varied. In the contrast-responding 



1 Cf. v. Helmholtz, 1st ed. p. 400. 



