934 AFTER-IMAGES. [BOOK in. 



to a stimulus, the sensation which follows the withdrawal of the 

 stimulus is of a different kind ; the result is what is called a 

 negative after-image, or negative image, to distinguish it from 

 a positive after-image, like the one mentioned above, which is 

 simply a continuation of the sensation primarily excited with all 

 its characters unchanged except that of intensity. If, after look- 

 ing steadfastly 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 

 when the eye is subsequently turned towards 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 has become 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, 

 consequently, appears grey. And so in the other instance ; in 

 this case, 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. In speaking of the retina being tired we are 

 using these words for simplicity's sake. We have no right to 

 suppose that the exhaustion takes place in the retinal structures 

 only ; it may occur in the central cerebral structures during the 

 development of visual impulses into sensations ; indeed the chief 

 part of it is probably of such a cerebral origin. , 



When a red patch is looked at, and the eye subsequently 

 turned to a white or to a grey ground, the negative image is a 

 greenish blue ; that is to say, the colour of the negative image 

 is complementary to that of the object. Thus also orange pro- 

 duces a blue, green a pink, yellow an indigo-blue, negative image, 

 and so on ; the negative image is in each case complementary to 

 the primary one. 



Similarly, when the eye, after looking at a coloured patch, 

 is turned not to a white or grey but to a coloured ground, the 

 colour of the negative image is a mixture of the colour comple- 

 mentary to the primary image with the colour of the ground ; if 

 a yellow ground be chosen after looking at a green object, the 

 negative image will appear as a mixture of red and yellow, a 

 reddish yellow ; and so on. 



Again, when a patch of coloured light is made to travel 

 through the visual field with sufficient rapidity, as when a patch 

 of light or of colour placed near the margin of a rotating disc is 

 looked at, the image of the patch as the disc revolves is followed 

 by a negative image in the shape of a sort of ghost having a 

 colour more or less but not exactly complementary to that of 

 the patch. 



Though these negative images only become striking after a 



