Stabilized images on the retina.

I n normal vision the eye is constantly in motion. Small involuntary move­ ments persist even when the eye .is "fixed" on a stationary object. As a result the image of the object on the retina of the eye is kept in constant motion. One movement of the eyeball makes the im­ age drift slowly away from the center of the fovea, the region of maximum visual acuity in which the cone receptor cells are most densely concentrated. The drifting motion terminates in a flick that brings the image back toward the cen­ ter of the fovea. Superimposed on the drift motion is a tremor with frequencies up to 150 cycles per second and an am­ plitude of about half the diameter of a single cone receptor. These three involuntary movements of the eyebalJ, alJ much smaller than the voluntary movements involved in look­ ing at the visual world or in reading, have been known to physiologists for many years. During the past decade Lorrin A. Riggs of Brown University and R. W. Ditchburn of the University of Reading in England succeeded in measuring them with great accuracy. Though the movements cannot be stopped without incapacitating the sub­ ject or endangering the eye, Ditchburn and Riggs found ways to circumvent them and so make an image stand still on the retina. They were thereby able to show that the motion of the image plays a significant role in the sensory function of the eye. When an image is stabilized on the retina by one means or another, it soon fades and disappears. Just how this happens is not yet completely under­ stood. It was also observed, however, that the stabilized image regenerates after a time and again becomes visible to the subject in whole or in part. The image­ or fragments of it-alternately fades and