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.