Page:Popular Science Monthly Volume 14.djvu/23

Rh 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 illuminated with pure white light. Instead, however, of appearing to the eye white, the shadow 2 will seem to be colored decidedly 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 regulating 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 following 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



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 white, 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 vanishes, 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