162 Shifting and Swinging
which an error of refraction is produced. Hence, when the eye shifts normally no error of refraction is mani fest. The more rapid the unconscious shifting of the eye, the better the vision; but if one tries to be con scious of a too rapid shift, a strain will be produced.
Perfect sight is impossible without continual shifting, and such shifting is a striking illustration of the mental control necessary for normal vision. It requires perfect mental control to think of thousands of things in a frac tion of a second; and each point of fixation has to be thought of separately, because it is impossible to think of two things, or of two parts of one thing, perfectly at the same time. The eye with imperfect sight tries to accomplish the impossible by looking fixedly at one point for an appreciable length of time; that is, by staring. When it looks at a strange letter and does not see it, it keeps on looking at it in an effort to see it better. Such efforts always fail, and are an important factor in the production of imperfect sight.
One of the best methods of improving the sight, there fore, is to imitate consciously the unconscious shifting of normal vision and to realize the apparent motion pro duced by such shifting. Whether one has imperfect or normal sight, conscious shifting and swinging are a great help and advantage to the eye; for not only may imper fect sight be improved in this way, but normal sight may be improved also. When the sight is imperfect, shifting, if done properly, rests the eye as much as palm ing, and always lessens or corrects the error of refraction.
The eye with normal sight never attempts to hold a point more than a fraction of a second, and when it shifts, as explained in the chapter on "Central Fixation," it always sees the previous point of fixation worse. When it ceases to shift rapidly and to see the point