Problems of peripheral vision.

THE FIRST points to be considered in a study of peripheral vision are whether light can reach the periphery at all; and, if so, whether it suffers any loss of intensity as compared with the light reaching the fovea, and also whether the dioptric system is capable of producing a good image with the light which reaches the periphery. The reduction in intensity due to light-absorption in the media, which may be suffered by a ray of light striking the anterior pole of the lens, is roughly independent of the direction of passage when the pupil is not dilated. But the effective area of the pupil, acting as a stop in peripheral, as in foveal, vision, depends on obliquity. The first loss of intensity, however, occurs at the cornea. Does it vary appreciably with the perimetric angle? When a ray of light I strikes the cornea, the finite size of even a 1700 dilated pupil limits the visually effective angle of incidence at the cornea to 700 (Fig. la). Larger angles can, of course, be formed by the ray and the visual axis; but, in order to be visually effective, the ray will have to enter at a point other FIG. 1 (a).-Section through anterior part of human eyeball, than the corneal pole. showing that the maximum angle of incidence of a Hence, it generally visually effective ray of light on the cornea is about 70 f from the perpendicular. forms an angle of less