280 COLOUR VISION 



more. It may appear on a surrounding grey ground in the absence of 

 or as a sudden reversal of the contrast- colour. It is most readily 

 induced on an intensely black background, a fact which militates 

 strongly against Hering's explanation of it {v. p. 255). The longer 

 fixation is continued the brighter the induced light becomes and the 

 further it spreads away from the inducing patch of light over the ground. 

 McDougall attributes simultaneous induction to retinal changes. The 

 light rays decompose certain retinal mother substances. Thus red 

 light sets free more of the red substance than of the blue and green 

 substances. The substances diffuse into adjoining areas of the retina, 

 and as the red substance is in excess its excitatory effect upon the nerve- 

 endings of the red system manifests itself beyond that of the green and 

 blue substances upon their respective systems. It is to be noted that 

 in the phenomenon of simultaneous induction (in Hering's meaning of 

 the term) there is time for such diffusion to occur^. The effects of these 

 peripheral stimuli, which are regarded as the basis of simultaneous 

 induction, may of course be abolished or altered by inhibitory effects in 

 accordance with the principles suggested above as the basis of contrast. 



The explanation of after-images, founded on the same principles, 

 is as might be expected much more complicated. All after-images are 

 primarily due to the persistence in the retina of substances set free in 

 it by the action of the light rays on stored-up mother substances. 

 These specific substances continue to act upon the retinal nerve-endings 

 and thereby to be used up gradually. In the dark the relative intensity 

 of the action of any one of the different specific substances is chiefly 

 a function of the quantity of that substance present in unit area of the 

 retina. The frequently recurring changes in the brightness and colour 

 effects of after-images are, with the exception of the gradual diminution 

 of intensity due to the using up of the substances, all determined by 

 changes in the cortex and not by changes in the retina. 



There are two stages in the chemical changes, (a) the setting free of 

 the specific substances (red, green, blue, and (scotopic) white substances), 

 (b) the excitation of the nerve-endings by the substances. Red light, 

 for example, may be conceived to cause the activity of the red system 

 to predominate in three ways : (1) by setting free the red substance in 

 larger quantity than the blue and green substances while reinforcing 

 the exciting action of all three equally ; (2) by setting free the three 

 substances in equal amounts while reinforcing the action of the red more 

 than that of the blue and green substances ; (3) by exerting a more 

 1 Contrast Ladd-Franklin, p. 274 ; Edridge-Green, p. 297. 



