if a cage is on one and a bird on the other,
we see the rider on the horse and the bird in the cage. It cannot be
otherwise. It is simply the result of the positive afterimages. If at
dark we twirl a glowing joss stick in a circle, we do not see one point
moving from place to place, but we see a continuous circular line. It is
nowhere broken because, if the movement is quick, the positive
afterimage of the light in its first position is still effective in our
eye when the glowing point has passed through the whole circle and has
reached the first position again.

We speak of this effect as a positive afterimage, because it is a real
continuation of the first impression and stands in contrast to the
so-called negative afterimage in which the aftereffect is opposite to
the original stimulus. In the case of a negative afterimage the light
impression leaves a dark spot, the dark impression gives a light
afterimage. Black becomes white and white becomes black; in the world of
colors red leaves a green and green a red afterimage, yellow a blue and
blue a yellow afterimage. If we look at the crimson sinking sun and then
at a white wall, we do not see red light spots but green dark spots.
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 zooetrope, 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 zooetrope 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 s