the black cover in the drum saw through every slit the drawing of a dog
in one particular position. Yet as the twenty-four slits passed the eye,
the twenty-four different positions blended into one continuous jumping
movement of the poodle.

But this so-called stroboscopic phenomenon, however interesting it was,
seemed to offer hardly any difficulty. The friends of the zoötrope
surely knew another little plaything, the thaumatrope. Dr. Paris had
invented it in 1827. It shows two pictures, one on the front, one on the
rear side of a card. As soon as the card is quickly revolved about a
central axis, the two pictures fuse into one. If a horse is on one side
and a rider on the other, 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.