374 PHYSIOLOGY CHAP. 



saturated and brighter by contrast ; colours which resemble the 

 original or are very near it in tone are dulled and feeble. 



The same effects appear even more plainly when a small 

 square of the inducing colour is placed on a large surface of a 

 colour that will be altered by contrast with the induced colour. 

 After gazing at the square for a few seconds it is removed, and 

 the contrast-colour may then be observed on the square which it 

 occupied. 



Under certain conditions the subjective contrast-colour may 

 be so strong as to predominate over the objective colour upon 

 which the after-image is projected. If, e.g., a small square of 

 dark orange paper is stuck in the middle of a red glass field, and 

 the bright sky looked at through it, the intensity of the induced 

 blue-green will be so pronounced that the orange square appears 

 blue. 



An experiment suggested by Johannes Miiller is instructive 

 owing to its simplicity. If, after fixating a red square on a 

 white ground, the gaze is turned to one of the 

 angles of the square so that its objective image 

 R and the subjective after image V are parti- 

 ally superposed (Fig. 177), it will be seen that 

 the greater part of R remains red, and the 

 greater part of V assumes the green comple- 

 mentary colour, while the part in which the 

 FIG. 177. Joh. Mailer's two images overlap appears to be pink shading 

 X-iniages 011 col ured to grey. This effect is the natural result of 



the fusion of the two complementary colours. 

 All the phenomena of colour -contrast can be repeated by 

 Miiller 's device, using small squares of different colours on a 

 background of the complementary colours or of a colour approxi- 

 mating to that of the square. In the first case the colour of the 

 background is reinforced by contrast; in the second it becomes 

 dim and pale. 



The effects of simultaneous contrast are more difficult to 

 interpret. It is well known to painters, whose colour -sense is 

 specially developed, that two colours in juxtaposition affect each 

 other. A grey figure seems brighter and more luminous on a 

 black than on a white ground ; a coloured figure seems brighter 

 and more saturated when the complementary or contrast-colour 

 predominates round it. 



Under special conditions the phenomena of simultaneous 

 contrast are surprisingly plain and obvious. One of the most 

 interesting experiments is that of the so-called coloured shadows, 

 already known to v. Guericke (1612) and Buffon (1743). When 

 the shadows a, b, produced by two candles A, B, of a vertical rod 

 or pencil c are thrown on to a white screen (Fig. 178), with a 

 red glass in front of A, shadow 5, which is illuminated only by 



