160 Shifting and Swinging
strain and the vision is lowered. This can readily be demonstrated by trying to hold one part of a letter for an appreciable length of time. No matter how good the sight, it will begin to blur, or even disappear, very quickly, and sometimes the effort to hold it will produce pain. In the case of a few exceptional people a point may appear to be held for a considerable length of time; the subjects themselves may think that they are holding it; but this is only because the eye shifts uncon sciously, the movements being so rapid that objects seem to be seen all alike simultaneously.
The shifting of the eye with normal vision is usually not conspicuous, but by direct examination with the oph thalmoscope it can always be demonstrated. If one eye is examined with this instrument while the other is re garding a small area straight ahead, the eye being ex amined, which follows the movements of the other, is seen to move in various directions, from side to side, up and down in an orbit which is usually variable. If the vision is normal these movements are extremely rapid and unaccompanied by any appearance of effort. The shifting of the eye with imperfect sight, on the contrary, is slower, its excursions are wider, and the movements are jerky and made with apparent effort.
It can also be demonstrated that the eye is capable of shifting with a rapidity which the ophthalmoscope can not measure. The normal eye can read fourteen letters on the bottom line of a Snellen test card, at a distance of ten or fifteen feet, in a dim light, so rapidly that they seem to be seen all at once. Yet it can be demonstrated that in order to recognize the letters under these condi tions it is necessary to make about four shifts to each letter. At the near-point, even though one part of the