people move is not the distance of our real space, such as the theater
shows, and the persons themselves are not flesh and blood. It is a
unique inner experience, which is characteristic of the perception of
the photoplays. _We have reality with all its true dimensions; and yet
it keeps the fleeting, passing surface suggestion without true depth and
fullness, as different from a mere picture as from a mere stage
performance._ It brings our mind into a peculiar complex state; and we
shall see that this plays a not unimportant part in the mental make-up
of the whole photoplay.

While the problem of depth in the film picture is easily ignored, the
problem of movement forces itself on every spectator. It seems as if
here the really essential trait of the film performance is to be found,
and that the explanation of the motion in the pictures is the chief task
which the psychologist must meet. We know that any single picture which
the film of the photographer has fixed is immovable. We know,
furthermore, that we do not see the passing by of the long strip of
film. We know that it is rolled from one roll and rolled up on another,
but that this movement from picture to picture is not visible. It goes
on while the field is darkened. What objectively reaches our eye is one
motionless picture after another, but the replacing of one by another
through a forward movement of the film cannot reach our eye at all. Why
do we, nevertheless, see a continuous movement? The problem did not
arise with the kinetoscope only but had interested the preceding
generations who amused themselves with the phenakistoscope and the
stroboscopic disks or the magic cylinder of the zooetrope and bioscope.
The child who made his zooetrope revolve and looked through the slits of
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 zooetrope
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,