It may be said offhand that even the complete appearance of depth such as the stereoscope offers would be in no way contradictory to the idea of moving pictures.  Then the photoplay would give the same plastic impression which the real stage offers.  All that would be needed is this.  When the actors play the scenes, not a single but a double camera would have to take the pictures.  Such a double camera focuses the scene from two different points of view, corresponding to the position of the two eyes.  Both films are then to be projected on the screen at the same time by a double projection apparatus which secures complete correspondence of the two pictures so that in every instance the left and the right view are overlapping on the screen.  This would give, of course, a chaotic, blurring image.  But if the apparatus which projects the left side view has a green glass in front of the lens and the one which projects the right side view a red glass, and every person in the audience has a pair of spectacles with the left glass green and the right glass red—­a cardboard lorgnette with red and green gelatine paper would do the same service and costs only a few cents—­the left eye would see only the left view, the right eye only the right view.  We could not see the red lines through the green glass nor the green lines through the red glass.  In the moment the left eye gets the left side view only and the right eye the right side view, the whole chaos of lines on the screen is organized and we see the pictured room on the screen with the same depth as if it were really a solid room set on the stage and as if the rear wall in the room were actually ten or twenty feet behind the furniture in the front.  The effect is so striking that no one can overcome the feeling of depth under these conditions.

But while the regular motion pictures certainly do not offer us this complete plastic impression, it would simply be the usual confusion between knowledge about the picture and its real appearance if we were to deny that we get a certain impression of depth.  If several persons move in a room, we gain distinctly the feeling that one moves behind another in the film picture.  They move toward us and from us just as much as they move to the right and left.  We actually perceive the chairs or the rear wall of the room as further away from us than the persons in the foreground.  This is not surprising if we stop to think how we perceive the depth, for instance, of a real stage.  Let us fancy that we sit in the orchestra of a real theater and see before us the stage set as a room with furniture and persons in it.  We now see the different objects on the stage at different distances, some near, some far.  One of the causes was just mentioned.  We see everything with our right or our left eye from different points of view.  But if now we close one eye and look at the stage with the right eye only, the plastic effect does not disappear.  The psychological causes for this perception of depth with one eye are essentially the differences of apparent size, the perspective relations, the shadows, and the actions performed in the space.  Now all these factors which help us to grasp the furniture on the stage as solid and substantial play their role no less in the room which is projected on the screen.