128 AFTER-IMAGES. [BOOK in. 



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 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 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 produces 

 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 complementary 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. 



Though these negative images only becomes striking after a 

 prolonged or intense excitation of the retina, such as rarely occurs 

 in ordinary vision, still the effect must intervene, even if to a 

 slight extent only, in our daily sight, and proportionately con- 

 tribute to the discrepancy between the perception and the object. 



783. The phenomena of ' simultaneous ' and ' successive 

 contrast ' are further of interest in relation to the theory of colour 

 vision ; and we may venture for a little while to consider them in 

 this connection. The mere occurrence of the negative images can 

 be explained as a result of exhaustion on either hypothesis of 



