174 EXPERIMENTAL PHYSIOLOGY. 



as well as responding to violet, is sensitive to blue, but little to the other wave 

 lengths (Fig. 48). If any one of these components is stimulated much more 

 than the others the result is a sensation of the primary colour corresponding to it. 

 Sensations of colours which lie intermediate to the primary colours in the spectrum 

 are simply the result of less unequal stimulation of two of the primary substances; 

 that explains why you can get such a sensation either by using a wave length (such 

 as blue) which affects two components (violet and green), one more than the 

 other, or by stimulating the retina with waves of two other colours (violet and 

 green) but unequal intensities, one of which stimulates mainly one, and one 

 mainly the other, of the same components. If any two components are stimulated 

 equally a sensation of white light results; complementary colours are therefore 

 merely those which have an equal effect on two of the components. Whether 

 or not a colour is more or less saturated depends on whether the components other 

 than the one mainly involved are responding much to the light or not. 



Experiment 70. After Images. Look fixedly for a few seconds 

 at a bright object (the filament of an electric light will do) and 

 then transfer your gaze to a black background. The bright 

 image does not disappear at once but only fades gradually 

 away. This is a POSITIVE -AFTER-IMAGE. Repeat, after the 

 image has quite faded, but this time move your eyes to a brilli- 

 antly lighted white background. There is a persistent image 

 in this case also, of the same shape as the object, but whereas 

 the object was a bright one the image looks dark. This is a 



NEGATIVE AFTER-IMAGE. 



Look steadily at a tiny spot in the centre of a square of 

 bright red paper, well lighted. After a few minutes shift the gaze 

 to a white background. You see the square still, but tinged 

 with greenish-blue, the colour complementary to red. Repeat, 

 after waiting until the after-image has faded, with a yellow 

 square, then try with a green one. The negative after-images 

 are coloured with the shades complementary to those of the 

 objects. 



The explanation of the positive after-image of a white object v 

 probably lies in the fact, which we have already noticed in another 

 connection, that the reaction of the retina does not cease imme- 

 diately the stimulus is removed. The positive image is merely the 

 after-discharge of the light-sensitive cells in the retina. The pheno- 

 menon of the negative after-image is comparable to the "rebound" 

 which occurs after a reflex, as Sherrington showed. When a reflex 

 arc has been for some time under the influence of one reflex, its 



