944 A MANUAL OF PHYSIOLOGY 



image of the object is formed to be more or less fatigued. And this 

 fatigue will extend to all three kinds of fibres ; so that white light 

 of a given intensity will now cause less excitation in this part than 

 in the rest of the retina. It is easy to understand that the negative 

 after-image of a coloured object will be seen, upon a white ground, 

 in the 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 meta- 

 bolic change is set up in it which is of the opposite kind to that 

 caused by the original excitation (see p. 945). 



The phenomena of negative after-images are often included 

 together as examples of successive contrast, the name implying 

 mutual influence 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, uhen set in 

 rotation, ought, by 7'albot's law, to show sharply marked and uni- 

 form 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 pheno- 

 mena 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 of the grey paper ; the 

 white light coming from the latter, therefore, affects mainly the 

 component connected with the sensation of the complementary 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 illu- 

 mination 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. 



