OTHER THEORIES 277 



is the necessity for assuming that the cortical areas of the two eyes are 

 separate, i.e., that each retina contains the peripheral endings of the 

 retino-cerebral nerve-fibres of its own set of four colour systems, and 

 these are distinct from the four systems of the other retina. The 

 view that the central connections of corresponding points of the 

 two retinae are anatomically identical is untenable. It was rejected 

 by v. Helmholtz 1 chiefly from a consideration of the influence of 

 voluntary attention in effecting the predominance of one or other of 

 two struggling fields. As already mentioned McDougall has shown that 

 momentary stimulation of the retina by dim equally diffused light will 

 bring back to consciousness after-images which have completely faded. 

 This must be due to the second stimulus raising the excitability of the 

 visual cortex, so that the feeble impulses coming from the retina are 

 enabled to break down the resistance in these highest levels. Now, 

 if the cortical areas of the two retinae were identical, when the after- 

 image is formed on one retina only, the admission of dim light to either 

 eye should be equally effective in reviving the faded after-image. 

 But it is found that though it has a perceptible effect of this sort in both 

 cases, that effect is very much slighter in degree, and ceases to be effective 

 much more rapidly in the case of the eye in which there is no after-image. 

 The slight effect of stimulating this eye shows that there is some close 

 and sympathetic connection between the cortical areas of the two 

 retinae, although they are not identical. McDougall represents the 

 retino-cerebral systems demanded by the theory diagrammatically in 

 Fig. 75. 



It is to be noted that Hering's theory requires the same explanation 

 of the cortical areas of the two eyes, with the greater complication of 

 three pairs of colour-systems for each eye. If, however, the opponent 

 metabolic processes are peripheral the theory then fails to explain the 

 facts of binocular struggle with red and green lights and so on ; and 

 wherever the processes occur requires much modification to meet the 



case 2 . 



McDougall's experiments on the fading of after-images, already 

 mentioned (v. p. Ill), demonstrate the recurrence of colour-sensations 

 attributable to four primary colour-systems. They further exhibit 

 instances of uniocular struggle between the primary or simple colours. 

 Per contra, experiments devised to exhibit uniocular struggle afford 

 confirmatory examples of the spontaneous development of pure primary 



1 771, 3rd Ed. m. p. 407. 



2 Cf . G. E. Miiller, Ztsch. /. Psychol. u. Physiol. d. Sinnesorg. xiv. 32, 1897 



