104 COLOUR VISION 



described by different people, but all agree that it is not an impenetrable 

 darkness. There are waves or points of light, and the general sensation 

 is one of very dark greyness. So far as negative after-images are 

 concerned this " light chaos " or " light dust " acts as a grey surface 

 on which the black image is projected. Many explanations of this 

 " intrinsic light " have been suggested. That endogenous stimuli are 

 the cause can scarcely be doubted, but whether these are primarily 

 retinal or of central nervous origin remains uncertain 1 , though it is now r 

 generally agreed that they are central. The question is of theoretical 

 importance and will receive further treatment in Part III. 



McDougall 2 found certain features which, he claims, are quite 

 constant in the after-images of coloured lights. They are as follow : 

 (1) The after-images show in nearly every case a play or succession of 

 colours, in which each of the three simple colours, red, green, and blue, 

 makes itself felt in some phase. (2) The brighter the coloured light 

 fixated, the brighter are the colours of the after-image, and the more 

 keen is the antagonism between the three simple colours, so that these 

 colours fuse less than in the after-images of duller colours. When the 

 primary stimuli are very bright the three simple colours tend to appear 

 pure and saturated in turn in a recurring scale, the unchanging phases 

 of pure colour being separated by periods of struggle between the fading 

 and the succeeding colour, just as is the case in the after-image of 

 bright white light. (3) In the case of fixation of one of the three simple 

 colours, red, green, and blue, the lower intensities are followed by after- 

 images in which the other two colours predominate, more or less fused, 

 i.e., the after-image is predominantly complementary ; while when the 

 colour fixated is very bright the first phase of the after-image is usually 

 homochrornatic and of considerable duration. The like holds good for 

 the compound colours, but in their case the phases of the after-image 

 tend to be rather more varied. (4) The order of occurrence and the 

 duration of the different phases vary readily with slight variations in 

 the conditions. (5) With any given light, the vividness and duration 

 of the after-image, observed in the dark, increase with increase of 

 duration of fixation of the light from about 10 sees, to 90 100 sees., 

 but with further increase of the period of fixation, the duration and 

 vividness of the after-image diminish, so that after very prolonged 

 fixation the after-image is either dull or of short duration or is not seen 

 at all. 



As with white light, a sharply bounded patch of coloured light, 



1 See v. Helmholtz, 3rd ed. p. 12 sqq. 2 Mind, x, N. S. 1901. 



