846 THE EYE AND VISION [CH. LVIlL 



complementary colour. So, too, if a point midway between Figs. I. 

 and II. is fixated, and the plate held at a sufficient distance for 

 both figures to be simultaneously visible, the after-image of the grey 

 strip of II. will appear darker than that of I. 



Seeing that simultaneous contrast persists in after-images, and 

 seeing how generally recognised are its effects (for instance, by the 

 painter, who depicts in Hue the shadows cast by an object on the yellow 

 sand), it seems far more probable that the part played by the higher 

 mental processes consists, not, as Helmholtz supposed, in causing the 

 illusion, but in reducing or overcoming it. According to this view, 

 experience educates us in seeing objects in what we know to be 

 their real colour, instead of in the colour which would result from the 

 operation of simultaneous contrast. Some support is lent to this 

 view by the fact that contrast is much enhanced when all irregu- 

 larities are, as far as possible, eliminated from the surface of the 

 object (here, the grey oblong) in which the contrast colour is 

 induced, or when that object is made to appear, e.g. by covering the 

 whole with tissue paper to combine with the object (the coloured 

 square) which induces the contrast colour, so as to form an apparently 

 single object. On the other hand, colour contrast is very markedly 

 reduced, if the grey object is outlined in pencil on the tissue paper 

 through which it is viewed. Thus, whatever tends to the apparent 

 independence of the object in which the contrasting colour is induced 

 tends to the reduction of the contrast effect. 



Insisting on the sensory nature of simultaneous contrast, Hering 

 explained it in the following way. He supposed that excitation 

 of an area of the retina by a stimulus of given colour or brightness 

 simultaneously induces an opposite metabolic process in the same 

 colour apparatus in neighbouring areas of the retina. When, for 

 example, a part of the retina is being stimulated by blue, the 

 anabolic change thus evoked in the yellow-blue apparatus simultane- 

 ously is supposed to induce a katabolic change in the same apparatus 

 in the neighbouring retinal area which is being excited by a grey 

 stimulus. Consequently, the grey acquires a yellowish tinge. 



Binocular colour-mixture. By means of the stereoscope, binocular 

 combinations of colour can be obtained. Thus, if one eye is exposed 

 to a red disc, and the corresponding portion of the other eye to a 

 yellow one, the mind usually perceives one disc of an orange tint ; 

 but frequently, especially if there be differences of brightness or of 

 form in the two objects, we notice that "rivalry of the fields of 

 vision " occurs, first one then the other disc rising into consciousness. 

 A stereoscopic combination of black and white produces the appear- 

 ance of metallic lustre ; this is very beautifully shown with figures of 

 crystals, one black on a white ground, the other white on a black 

 ground. Probably the combination of black and white is interpreted 



