enter 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.

We may approach the same result also from experiments of very different
kind. If a flash of light at one point is followed by a flash at another
point after a very short time, about a twentieth of a second, the two
lights appear to us simultaneous. The first light is still fully visible
when the second flashes, and it cannot be noticed that the second comes
later than the first. If now in the same short time interval the first
light moves toward the second point, we should expect that we would see
the whole process as a lighted line at rest, inasmuch as the beginning
and the end point appear simultaneous, if the end is reached less than a
twentieth of a second after the starting point. But the experiment shows
the opposite result. Instead of the expected lighted line, we see in
this case an actual movement from one point to the other. Again we must
conclude that the movement is more than the mere seeing of successive
positions, as in this case we see the movement, while the isolated
positions do not appear as successive but as s