ioi6 THE SENSES 



complementary colour, for the components chiefly excited by the latter 

 will have been least fatigued. The negative after-images seen when 

 the eye, after receiving the positive impression, is turned upon a coloured 

 ground, vary with the colour of the object and ground in a manner which 

 has been explained as due to fatigue of one or other component. It 

 is difficult, however, to reconcile the fatigue hypothesis of the after- 

 image with all the facts. Hering supposes that the retina is not 

 passively fatigued, but that a metabolic change is set up in it which is 

 of the opposite kind to that caused by the original excitation (see 

 p. 1017). 



The phenomena of negative after-images are often included together 

 as examples of successive contrast, the name implying mutual in- 

 fluences of the portions of the retina (or retino-cerebral apparatus) 

 successively stimulated. We have now to consider simultaneous 

 contrast, often spoken of simply as contrast. 



Contrast. A small white disc in a black field appears whiter, and a 

 small black disc in a white field darker, than a large surface of exactly 

 the same objective brightness. A disc with alternate sectors of white 

 and black, so arranged that the proportion of white to black increases 

 in each zone from centre to circumference, when set in rotation, ought, 

 by Talbot's law, to show sharply marked and uniform rings, of which 

 each is brighter than that internal to it. But each zone appears 

 brightest at its inner edge, where it borders on a zone darker than itself, 

 and darkest at its outer edge, where it borders on a brighter zone. A 

 plausible explanation of this is based on the assumption that in the 

 neighbourhood of an excited area of the retina, as well as within the 

 area itself, the excitability is diminished ; and the same explanation has 

 been extended to the contrast phenomena of coloured objects. A small 

 piece of grey paper, e.g., is placed on a green sheet. The grey patch 

 appears in the complementary colour of the ground viz., pink or 

 rose-red (Meyer). The red colour is much stronger if the whole is 

 covered with translucent tracing-paper. Here we may suppose that 

 the fatigue of the substance or component chiefly affected by the ground 

 colour spreads into the portion of the retina occupied by the image oi 

 the grey paper; the white light coming from the latter, therefore, 

 affects mainly the component connected with the sensation of the com- 

 plementary colour. 



The curious phenomenon of coloured shadows is also an illustration 

 of contrast. They may be produced in various ways. For example, 

 when a lamp is lit in a room in the twilight, before it has yet grown 

 too dark, the shadows cast by opaque objects on a white window-blind 

 are coloured blue. The yellow light of the lamp overpowers the feeble 

 daylight which passes through the blind, and the general ground is 

 yellowish; but wherever a shadow is thrown it appears of a bluish tint 

 in contrast to the yellow ground. Here the only illumination the eye 

 receives from the region occupied by the shadow is the feeble daylight. 

 Falling upon an area in which the component chiefly affected by yellow 

 rays is more or less fatigued, it causes a sensation of the complementary 

 colour. As darkness comes on, the shadows become black, for now 

 practically no light at all comes from them. 



Helmholtz looked upon simultaneous contrast as a result of false 

 judgment, and not of a change of excitability in parts of the retina 

 bordering on the actually excited parts. For the sake of perspective, 

 t will be worth while to apply this theory by way of illustrating it, to 

 the explanation of the case of contrast we have just been considering, 

 from the other point of view, in Meyer's experiment. Helmholtz's ex- 

 planation of this experiment is as follows: When a coloured surface is 



