Compared with these negative pictures, the positive afterimages are
short and they last through any noticeable time only with rather intense
illumination. Yet they are evidently sufficient to bridge the interval
between the two slits in the stroboscopic disk or in the zoötrope, the
interval in which the black paper passes the eye and in which
accordingly no new stimulus reaches the nerves. The routine explanation
of the appearance of movement was accordingly: that every picture of a
particular position left in the eye an afterimage until the next picture
with the slightly changed position of the jumping animal or of the
marching men was in sight, and the afterimage of this again lasted until
the third came. The afterimages were responsible for the fact that no
interruptions were noticeable, while the movement itself resulted simply
from the passing of one position into another. What else is the
perception of movement but the seeing of a long series of different
positions? If instead of looking through the zoötrope we watch a real
trotting horse on a real street, we see its whole body in ever new
progressing positions and its legs in all phases of motion; and this
continuous series is our perception of the movement itself.

This seems very simple. Yet it was slowly discovered that the
explanation is far too simple and that it does not in the least do
justice to the true experiences. With the advance of modern laboratory
psychology the experimental investigations frequently turned to the
analysis of our perception of movement. In the last thirty years many
researches, notably those of Stricker, Exner, Hall, James, Fischer,
Stern, Marbe, Lincke, Wertheimer, and Korte have thrown new light on the
problem by carefully devised experiments. One result of them came
quickly into the foreground of the newer view: the perception of
movement is an independent experience which cannot be reduced to a
simple seeing of a series of different positions. A characteristic