THEORIES OF COLOUR VISION 587 



in finding an explanation of these cases which show an inability to respond to a part 

 of the spectrum. Although there does not seem to be any special difficulty in so 

 modifying this hypothesis that it fits in with all the varieties of colour blindness, yet 

 it would seem that this would cause it only to be more and more like the theory of Young, 

 with this important difference, that according to Young's theory the three substances 

 by which light is selectively absorbed, according as its wavelength corresponds to the 

 red, green or blue part of the spectrum, is in the retina, whereas the three mechanisms 

 required by Edridge Green are in the brain. Changing their situation would not appear 

 to have added to our knowledge of them, but would on the other hand appear to add 

 greatly to our difficulties, for it is impossible to understand how impulses of the enor- 

 mous frequency of light could be transmitted intact up the optic nerves, as Edridge 

 Green requires. 



McDOUGALL'S HYPOTHESIS is not antagonistic to Young's theory, as the two 

 previous views have been, but adds valuable suggestions as to the causation of contrast 

 and after image phenomena, points to which the original theory gave little or no atten- 

 tion. McDougall also accepts the duplicity theory. He commences by stating that 

 there are four centres for the two eyes, namely red, green, blue and white (the mechanism 

 of which is the rods), and that these centres tire distinct and have no anatomical iden- 

 tity. Between these centres there is antagonism, the red centre of one eye against the 

 green and blue centres of the other and also to a less extent against those of itself. 

 In this way one can explain not only binocular, but also monocular rivalry. Contrast 

 is explained in a somewhat similar manner ; thus if the object looked at consists of a 

 red area on a grey field, the red stimulus inhibits the appreciation of red in the surround- 

 ing field, and therefore causes it to have a blue-green colour, a deduction which is 

 confirmed by experiment. After images are dealt with in a somewhat similar manner. 



With the evidence that has accumulated up to the present before us, there appears 

 to be more in favour of Young's hypothesis than is to be found for its rivals. Further 

 than that, it is not at present advisable to go. 



