____________The Shift That Bests_________163
shifted from worse, the sight ceases to be normal, the swing being either prevented or lengthened, or (occa sionally) reversed. These facts are the keynote of the treatment by shifting.
In order to see the previous point of fixation worse, the eye with imperfect sight has to look farther away from it than does the eye with normal sight. If it shifts only a quarter of an inch, for instance, it may see the previous point of fixation as well as or better than be fore; and instead of being rested by such a shift, its strain will be increased, there will be no swing, and the vision will be lowered. At a couple of inches it may be able to let go of the first point; and if neither point is held more than a fraction of a second, it will be rested by such a shift and the illusion of swinging may be produced. The shorter the shift the greater the benefit; but even a very long shift-as much as three feet or more - is a help to those who cannot accomplish a shorter one. When the patient is capable of a short shift, on the contrary, the long shift lowers the vision. The swing is an evidence that the shifting is being done properly, and when it occurs the vision is always im proved. It is possible to shift without improvement; but it is impossible to produce the illusion of a swing without improvement, and when this can be done with a long shift, the movement can gradually be shortened until the patient can shift from the top to the bottom of the smallest letter, on the Snellen test card or elsewhere, and maintain the swing. Later he may become able to be conscious of the swinging of the letters without con scious shifting.
No matter how imperfect the sight, it is always pos sible to shift and produce a swing, so long as the pre-