OTHER THEORIES 279 



colour sensations. These facts are strikingly in favour of the three- 

 components theory. 



Perhaps the greatest service which McDougall has done to the theory 

 is to show that it can be made to give a reasonable explanation of the 

 facts of induction. His theory of contrast was adumbrated by Rollet 

 in 18671. 



The darkening contrast effect exerted by a white area on adjoining 

 grey areas may be attributed, as shown above, to the inhibitory effect 

 of the more vigorous cortical processes excited by the white area on the 

 feebler cortical processes excited by the grey areas. If the explanation 

 be true, then by the theory it must also hold for colour-contrast. For 

 (photopic) white being the resultant of simultaneous activity of the red, 

 green, and blue colour-systems, inhibition of white by white must be 

 expected to involve inhibition of red by red, green by green, and blue 

 by blue. In the cortical level of each colour system the activity of any 

 one part tends to inhibit the activity of all other parts, and when any 

 one part is more intensely excited than the rest its activity does partially 

 or completely inhibit that of all other parts of the cortical area of the 

 same colour system (see Fig. 75). Suppose then that all parts of all 

 three colour-systems are equally excited to a moderate degree, except 

 that one small part of one system, e.g., the red, is more highly excited. 

 Then the activity of this more highly excited part of the red cortical 

 area depresses that of the rest of the red cortical area, more especially 

 that of the immediately surrounding parts, with the result that in the 

 parts of the field surrounding the red patch the activity of the blue and 

 green systems predominates over that of the red, and the grey ground 

 appears blue-green. When the balance of the activities of the three 

 cortical levels has once been turned in this way in favour of the blue and 

 green systems, the predominance of the blue and green systems must 

 be still further increased by the antagonism between the corresponding 

 parts of the cortical areas of the three colour-systems, i.e., the activity 

 of the red cortical area must be still further depressed or inhibited by 

 that of the blue and green areas. In favour of these views are the facts 

 that coloured light on one retinal area can be inhibited by white light 

 on another area and one of the constituents of the sensation of white 

 can be inhibited by a patch of bright coloured light. 



Simultaneous induction, in Hering's sense of the term {v. p. 125), 

 is the appearance of the same colour around a patch of any colour 

 when it has been steadily fixated for some time, generally 10 sees, or 

 1 Berichit d. Wiener Akad. lv. 344, 424, 741, 18G7. 



