content of consciousness must be added to such a series of visual
impressions. The mere idea of succeeding phases of movement is not at
all the original movement idea. This is suggested first by the various
illusions of movement. We may believe that we perceive a movement where
no actual changes of visual impressions occur. This, to be sure, may
result from a mere misinterpretation of the impression: for instance
when in the railway train at the station we look out of the window and
believe suddenly that our train is moving, while in reality the train on
the neighboring track has started. It is the same when we see the moon
floating quickly through the motionless clouds. We are inclined to
consider as being at rest that which we fixate and to interpret the
relative changes in the field of vision as movements of those parts
which we do not fixate.

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.