In the case of the picture on the screen this conflict is much stronger. We certainly see the depth, and yet we cannot accept it. There is too much which inhibits belief and interferes with the interpretation of the people and landscape before us as truly plastic.  They are surely not simply pictures.  The persons can move toward us and away from us, and the river flows into a distant valley.  And yet the distance in which the 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, if a cage is on one and a bird on the other, we see the rider on