THE SENSES 827 



a manner which can be readily explained as due to fatigue of one or 

 other fibre group. 



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 successively stimu- 

 lated. 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. The most natural explanation of this is 

 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 holds for the contrast phenomena of coloured objects. 

 A small piece of grey paper, e.g., is placed on a green sheet, and 

 the whole covered with translucent tracing-paper. The grey patch 

 appears in the complementary colour of the ground, viz., rose-red 

 (Meyer). Here we may suppose that the fatigue of the group of 

 fibres chiefly excited 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, excites mainly the fibres which 

 give the sensation of the complementary colour. 



The curious phenomenon of coloured shadows is also an illus- 

 tration 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 fibres chiefly excited by yellow rays are 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, 

 it 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 consider- 

 ing, from the other point of view in Meyer's experiment. Helm- 

 holtz's explanation of this experiment is as follows : When a coloured 

 surface is covered with translucent paper, the latter appears as a 



