282 COLOUR VISION 



following facts : (1) the tendency for well-defined and homogeneous 

 parts of a complex image, whether a direct image or an after-image, 

 to undergo " complete fading " and to be revived in consciousness as 

 independent wholes ; (2) the fact that when two images are fixated 

 successively and so as to fall partially upon the same area of one 

 retina each forms an after-image distinct and separate from that 

 of the other, so that the two after-images may appear simultaneously 

 or alternately in consciousness ; (3) when an after-image is projected 

 upon a not too bright surface it is not always modified at once by 

 the effects of the light from this surface, but may maintain itself in 

 consciousness unaltered for a certain length of time. Fixation of a 

 patch of light therefore throws the cortical areas affected into a state 

 of preparedness to function again or to continue to function in the same 

 way as during the action of the light. This tendency may override and 

 obscure changes initiated in the retina. 



Hence, it is not proper to say that an after-image has its seat either 

 in the retina or in the brain ; both retain an impress from the original 

 stimulation, and though the persistence of the exciting substances in 

 the retina is perhaps the more important element, yet the cortical 

 impress plays a large part in determining the exact form in which the 

 after-image shall appear. 



McDougall's views on the evolution of the colour-sense are shown 

 in the following extract 1 . 



' It will be generally admitted that if we try to form a conception 

 of the course of development of the colour-processes we must begin by 

 assuming the vision of the lower animals, in which vision is but little 

 developed, to be monochromatic, i.e., we must assume the visual 

 sensations to be of one kind only, varying only in intensity or bright- 

 ness ; and we must suppose one kind of light sensation to be similar 

 to our white or grey sensation or at least to stand to it in the 

 relation of a direct ancestor. If we then seek the probable first step 

 in the development of the colour-processes from this stage of simple 

 grey- vision, we must, I think, assume that it consisted in a differentia- 

 tion of the effects of the light of the warm and the cold ends of the 

 spectrum ; the rays of the cold end would begin to set free, in addition 

 to a white-exciting substance, a substance that by the excitement of a 

 concurrently differentiated retino-cerebral apparatus would add the 

 sensation of blue to that of white ; and in just the same way the rays 

 of the warm end would begin to set free an additional substance that 



1 Mind, N.S. x. pp. 212-214, 1901. 



