MOST of us have observed that, if we look for a moment at a light as it is being turned out, we see an image of it a while afterward. That image is an afterimage, or after-sensation. Such images manifest themselves according to traceable laws and play an important, though seldom recognized, rôle in our experience. We shall now make a study of some of their characteristic modes of behavior.
Success in this experiment depends mainly upon concentration of attention and economy in the use of the eyes. The eyes should not be allowed to wander aimlessly over the figures, and it is well to close them for rest as often as possible. Clear images can be obtained only after steady fixation. It is easier to get the after-image with one eye alone, but in the long run it is more restful to use the two eyes together.
1. Existence.a. Brightness.--Place the black square at the center of the upper half of a white sheet of paper; fixate the center of it (i.e., look intently and____________________