§ 10. The Negative AfterImage, etc.—"If, after looking 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 the image of a white patch. These afterimages, which follow the removal of the primary stimulation, are called negative images. "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 indigoblue, negative image, and so on."* The conditions for the production of the negative image are the more favourable, the more intense and persistent is the primary stimulation. When the primary stimulation is very transient, it may give rise in the first instance to a positive image, as we shall see later. Negative images arise also when the eye is simply closed after the primary stimulation as well as when it is turned to a different background.

It is not absolutely necessary for the occurrence of negative images that the primary stimulus should be removed. The same result may be brought about by diminishing its intensity. If we steadfastly gaze at a red spot on a yellow ground, and then diminish the intensity of the illumination by turning down the light or otherwise, a green spot upon a blue ground will appear instead of the red spot on a yellow ground.

*Foster, TextBook of Physiology, part iv., book iii., chap, iii., p. 1266.

The same process is manifested in a different way while the eye is actually subject to the primary stimulation in undiminished intensity. If we gaze long and steadfastly at any colour, it gradually becomes less saturated; the effect of steadfastly gazing at yellow is the same as that produced by gradually mingling the yellow light with more and more of its complementary blue. It becomes paler. We may gather these facts under one formula. The continuance of the same mode of stimulation tends to produce a contrast effect, not only on adjoining portions of the retina, but also on that portion which the stimulus directly excites. This contrast effect takes the form of a negative image when the primary stimulation is withdrawn or sufficiently weakened. When the stimulus is continued so as to maintain its positive effect, the contrast effect mingles with this, so as to produce loss of saturation. In this way, the yellow illumination of a gaslight or candle practically becomes equivalent to white light when it is long continued. It is noteworthy that negative images modify each other's colourtone by contrast, and this even in cases in which it is difficult to obtain a contrast effect under ordinary conditions. The negative image of a red patch on a white ground is bluegreen; the negative image of the white ground which surrounds it is reddened by contrast. This is important, because it shows that contrast phenomena are not due to errors of judgment, as has been maintained by Helmholtz.