We notice the movement more quickly than the whole background in which the movement is executed.

The fluttering of kerchiefs at a far distance or the waving of flags for signaling is characteristic.

All indicate that the movement is to us something different from merely seeing an object first at one and afterward at another place.

We can easily find the analogy in other senses.

If we touch our forehead or the back of our hand with two blunt compass points so that the two points are about a third of an inch distant from each other, we do not discriminate the two points as two, but we perceive the impression as that of one point.