276 COLOUR VISION 



field is presented to one eye and a blue field to the corresponding area 

 of the other eye, one eye be closed or covered for a brief period one 

 second will suffice the colour presented to that eye always predominates 

 over and inhibits the colour presented to the other eye as soon as the 

 eye is uncovered, i.e., the rested tract predominates over the relatively 

 fatigued tract, even if the period of rest be not more than one second ; 

 (3) if by any one of several devices, as, for example, slight movements 

 of objects in the field (the drawing of a hair across the field a little before 

 the eye will suffice), the predominance of one field, say the red field, 

 be prolonged, the blue field of the other eye tends more and more 

 strongly to assert itself until, no matter how vigorous the movements 

 in the red field, the blue predominates and the red field disappears from 

 consciousness ; all such devices merely direct attention to the one field, 

 i.e., they cause the excitation of the one cortical conduction-path to be 

 reinforced by the activity of higher levels, and we see that as the 

 excitation continues to pass through this path the resistance of the path 

 increases until, in spite of such reinforcement, it yields to the inhibitory 

 influence of the other rested tract ; (4) if, accommodation being relaxed, 

 a white field be presented to the right eye and a much less bright white 

 or grey field to the corresponding area of the left eye, and if the left eye 

 be covered over for some ten to fifteen seconds, while the right eye 

 continues to fixate the brighter field, then on uncovering the left eye 

 the bright image of the right eye yields to the much less bright image of 

 the left eye, and disappears from consciousness for some seconds. 

 McDougall gives many examples of uniocular struggle 1 . 



So far as colour vision is concerned McDougall supplements the 

 original Young theory by the addition of an independent white mechan- 

 ism, the end organ of which is the rods. In other words he adopts the 

 duplicity theory. It is a difficulty, even of these combined theories, 

 that there is no place in them for a special black-exciting process. 

 McDougall boldly accepts the view that such a process is unnecessary 

 to explain the facts, and adduces cogent arguments in its favour 2 . 

 The sensation of black, according to him, is experienced when " com- 

 plete fading " occurs. The visual cortex is then at complete rest, as 

 opposed to the condition of normal tone which manifests itself as the 

 intrinsic light (Eigenlicht). 



This combined theory therefore assumes a separate retino-cerebral 

 apparatus or system for each of the primary or simple colour-affections, 

 red, green, blue, and (scotopic) white. The next point of importance 

 Mind, x. N.S. Sec. V. 2 Cf. also Ward, Brit. Jl. of Psychol. i. 407, 1905. 



