Now a certain amount of eye-movement does not hurt the muscles; it is, on the contrary, rather pleasant, because their business is to attend to those maters. But the eye will become fatigued by a great amount of movement, especially when it is forced upon us at unexpected moments, just as any other part of the body will become fatigued when it is forced to perform a great number of sudden, unexpected tasks.
A simple experiment will illustrate this further. Suppose that we are sitting in our door-yard, gazing across a valley at a group of trees a mile or so away. It is more restful to look at those distant trees than at a single tree only fifty feet away; and the reason is simple. When we look at any object our eyes have a tendency to follow its outline. Now, of course, it requires more rolling of the eyes to follow the outline of a tree near by than one in the distance. This rolling movement involves muscular work. And, if we look first at the near, large object and then shift to the distant, small ones, we immediately experience the restfulness of reduced work. There are other reasons why distant objects are restful to the eyes, but they do not concern us here.
Have you ever noticed the pleasing effect in the motion pictures when the thing of interest, say, a train or a band of horsemen disappearing in the distance, narrows itself down to a small space? All images on the screen are, of course, equally distant from the spectator; yet there is a sense of restfulness, as we have just explained, because the rolling of the eyes decreases with the diminishing of the image and its area of movement on the screen.
But suddenly there comes a close-up of a face
