But it is different when we come, for instance, to those illusions in which movement is forced on our perception by contrast and aftereffect. We look from a bridge into the flowing water and if we turn our eyes toward the land the motionless shore seems to swim in the opposite direction. It is not sufficient in such cases to refer to contrasting eye movements. It can easily be shown by experiments that these movements and counter-movements in the field of vision can proceed in opposite directions at the same time and no eye, of course, is able to move upward and downward, or right and left, in the same moment. A very characteristic experiment can be performed with a black spiral line on a white disk. If we revolve such a disk slowly around its center, the spiral line produces the impression of a continuous enlargement of concentric curves. The lines start at the center and expand until they disappear in the periphery. If we look for a minute or two into this play of the expanding curves and then turn our eyes to the face of a neighbor, we see at once how the features of the face begin to shrink. It looks as if the whole face were elastically drawn toward its center. If we revolve the disk in the opposite direction, the curves seem to move from the edge of the disk toward the center, becoming smaller and smaller, and if then we look toward a face, the person seems to swell up and every point in the face seems to move from the nose toward the chin or forehead or ears. Our eye which watches such an aftereffect cannot really move at the same time from the center of the face toward both ears and the hair and the chin. The impression of movement must therefore have other conditions than the actual performance of the movements, and above all it is clear from such tests that the seeing of the movements is a unique experience which can be entirely independent from the actual seeing of successive positions. The eye itself gets the impression of a face at rest, and yet we see the face in the one case shrinking, in the other case swelling; in the one case every point apparently moving toward the center, in the other case apparently moving away from the center. The experience of movement is here evidently produced by the spectator's mind and not excited from without.