532 COLOUR SENSATIONS. [BOOK in. 



than those of green and red. If the stimulus be increased the 

 maximum of violet stimulation will be reached, while the stimula- 

 tion of green will continue to be increased and even that of red to 

 a slight degree. The result will be that the light appears violet 

 mixed with green, that is blue. If the stimulus be still further 

 increased while the green and violet are both excited to the 

 maximum, the red stimulation may be increased until the result is 

 violet, green, and red in the proportions which make white light. 

 And so with light of other colours. 



After-images. We have already seen that in vision the 

 sensation lasts much longer than the stimulus. Under certain 

 circumstances, such as particular conditions of the eye, an intense 

 stimulus, &c., the sensation is so prolonged, that it is spoken of as 

 an after-image. Thus, if the eye be directed to the sun, the image 

 of that body is present for a long while after; and if, on early 

 waking, the eye be directed to the window for an instant and then 

 closed, an image of the window with its bright panes and darker 

 sashes, the various parts being of the same colour as the object, will 

 remain for an appreciable time. These images, which are simply 

 continuations of the sensation, are spoken of as positive after- 

 images. They are best seen after a momentary exposure of the eye 

 to the stimulus. 



When, however, the eye has been for some time subject to a 

 stimulus, the sensation which follows the withdrawal of the sti- 

 mulus is of a different kind ; what is called a negative after-image, 

 or negative image, is produced. If, after looking stedfastly at a 

 white patch on a black ground, the eye be turned to a white 

 ground, a grey patch is seen for some little time. A black patch 

 on a white ground similarly gives rise on a grey ground to a 

 negative image in the form of a white patch. This may be 

 explained as the result of exhaustion. When the white patch 

 has been looked at steadily for some time, that part of the retina 

 on which the image of the patch fell becomes tired; hence the 

 white light, coming from the white ground subsequently looked 

 at, which falls on this part of the retina, does not produce so much 

 sensation as in other parts of the retina; and the image, con- 

 sequently, appears grey. And so in the other instance, the whole 

 of the retina is tired, except at the patch; here the retina is 

 for a while most sensitive, and hence the white negative image. 



When a red patch is looked at, the negative image is a green 

 blue, that is, the colour of the negative image is complementary to 

 that of the object. Thus also orange produces a blue, green a 

 pink, yellow an indigo-blue, negative image ; and so on. This too 

 can be explained as a result of exhaustion on either hypothesis of 

 colour vision. When the coloured patch is looked at, one of the 

 three primary colour sensations is much exhausted, and the other 

 two less so, in varying proportions, according to the exact nature of 



