THE CONTRAST OF COLORS. 13 



shadow 2 — that is, the candle-light will illuminate all parts of the screen 

 except those occupied by the shadow 2 ; this portion will be illumi- 

 nated with pure white light. Instead, however, of appearing to the 

 eye white, the shadow 2 will seem to be colored decidedlj^ blue. 



For the production of the most powerful effect, it is desirable that 

 the shadows should have the same depth, which can be effected by regu- 

 lating the size of the aperture admitting daylight. Now, although the 

 shadow cast by the candle is actually pure white, yet, by contrast with 

 the surrounding orange-yellow ground, it is made to appear decidedly 

 blue. So strong is the illusion that, even after the causes which gave 

 rise to it have disappeared, it still persists, as can be shown by the fol- 

 lowing experiment of Helmholtz : 



While the colored shadows are falling on the screen, they are to be 

 viewed through a blackened tube of cardboard held in such a way that 

 the observer has both the shadows in his field of view ; the appearance, 

 then, will be like that represented in Fig. 15. After the blue shadow 



Fig. 15.— Blue and Yellow Shadows viewed thkough a Tube. 



has developed itself in full intensity, the tube is to be moved to the 

 left, so that the blue shadow may fill the whole field. The tube being 

 held steadily in the new position, the shadow will still continue to 

 appear blue instead of Avhite, even though the exciting cause, viz., the 

 orange-yellow candle-light, is no longer acting on the eye. The candle 

 may be blown out, but the surface will still appear blue, as long as the 

 eye is at the tube. On removing the tube, the illusion instantly van- 

 ishes, and it is perceived that the color of the surface is identical with 

 that of the rest of the screen, which is at once recognized as white. In 

 a case like this, the fatigue of the retinal elements can play no part, as 

 the illusion persists for a far longer time than is necessary for their 

 complete rest ; we must hence attribute the result to a deception of the 

 judgment. 



The simple experiments of H, Meyer are less troublesome than 

 those just described, and at the same time highly instructive. A small 

 strip of gray paper is placed on a sheet of green paper, as indicated 



