RESEARCHES BASED UPON THE THEORY 265 



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^. 

 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. 



