438 Negative After-images and Successive Contrast. 



the perception of colour be continually receiving impulses which, affecting 

 it and the visual centre, cause the sensation of light (Eigenlicht) which 

 is seen in the absence of all light stimulation, and the whole retino-cerebral 

 apparatus be fatigued by light of a certain wave-length, a negative after- 

 image will appear through simultaneous contrast. If one portion of the 

 visual area be less sensitive for impulses caused by light of a certain wave- 

 length (for instance, red), and the adjoining areas be stimulated by impulses 

 corresponding to light of all wave-lengths, the image corresponding to the 

 fatigued area will be relatively blue-green to the images corresponding to 

 surrounding areas. This explanation on the Edridge-Green theory of 

 colour-vision is in accordance with the other facts of simultaneous contrast.* 

 It would appear as if an inhibitory process took place which prevented light, 

 unless it reached a certain intensity, from being perceived. In the present 

 state of our knowledge it is impossible to say at what point in the nervous 

 track the inhibitory process takes place, whether there be obstruction to the 

 impulse at any point of the nervous track, or whether the effect be chiefly 

 central. It is to be noted that when an effect is produced there is very little 

 change in most cases, except when the intensity of the reacting light is the 

 same or less than that of the fatiguing light. 



The negative after-image is much darker, more difficult to produce, and 

 more evanescent in the absence of all external light as when black velvet and 

 the hands are placed over the eyes. It is obvious, therefore, that external 

 light has an influence on negative after-images. It is impossible to explain 

 these facts on the Hering and Young-Helmholtz theories of colour-vision : — 



I. Hering. — The complementary to the exciting light is never strengthened 

 in the spectrum on the screen by the after-image, and the after-image is not 

 surrounded by the primary colour as it should be according to this theory. 



II. Young-Helmholtz — 



1. The effect of fatiguing the eye with a monochromatic region produces a 

 uniform grey band across this region. On the Young-Helmholtz theory this 

 should vary in colour and luminosity across its breadth. 



2. There is no alteration of the after-image by observation of the second 

 spectrum. 



3. On the Young-Helmholtz theory the after-image should change colour on 

 fading, because of the varying amount of fatigue of the hypothetical colour 

 sensations. This is not the case. 



4. Eegions like violet, after fatigue to red, should be very little affected, 

 but they are the most affected. 



* 'Roy. Soe. Proc.,' 1912, B. vol. 84, p. 546. 



