When, with head fixed, we allow the eyes to wander over a fixed scene before us, we find that the eyes do not move continuously, but by jerks, so that what we see is really a series of separate pictures, each of which is at rest, the jerk from one position to the next being so rapid that we practically see nothing during the change of picture. That this is so can be shown if we have a bright light—say, an incandescent electric lamp—in the field of vision. After allowing the eyes to wander over the scene before us, we find, on closing the eyes, that we see a series of separate sharp secondary images of the bright light. Even when we make an effort to move the eyes continuously we find, by means of the secondary images, that we have not succeeded in doing so.