To begin at the beginning, the photoplay consists of a series of flat pictures in contrast to the plastic objects of the real world which surrounds us.  But we may stop at once:  what does it mean to say that the surroundings appear to the mind plastic and the moving pictures flat?  The psychology of this difference is easily misunderstood.  Of course, when we are sitting in the picture palace we know that we see a flat screen and that the object which we see has only two dimensions, right-left, and up-down, but not the third dimension of depth, of distance toward us or away from us.  It is flat like a picture and never plastic like a work of sculpture or architecture or like a stage.  Yet this is knowledge and not immediate impression.  We have no right whatever to say that the scenes which we see on the screen appear to us as flat pictures.

We may become more strongly conscious of this difference between an object of our knowledge and an object of our impression, if we remember a well-known instrument, the stereoscope.  The stereoscope, which was quite familiar to the parlor of a former generation, consists of two prisms through which the two eyes look toward two photographic views of a landscape.  But the two photographic views are not identical.  The landscape is taken from two different points of view, once from the right and once from the left.  As soon as these two views are put into the stereoscope the right eye sees through the prism only the view from the right, the left eye only the view from the left.  We know very well that only two flat pictures are before us; yet we cannot help seeing the landscape in strongly plastic forms.  The two different views are combined in one presentation of the landscape in which the distant objects appear much further away from us than the foreground.  We feel immediately the depth of things.  It is as if we were looking at a small plastic model of the landscape and in spite of our objective knowledge cannot recognize the flat pictures in the solid forms which we perceive.  It cannot be otherwise, because whenever in practical life we see an object, a vase on our table, as a solid body, we get the impression of its plastic character first of all by seeing it with our two eyes from two different points of view.  The perspective in which our right eye sees the things on our table is different from the perspective for the left eye.  Our plastic seeing therefore depends upon this combination of two different perspective views, and whenever we offer to the two eyes two such one-sided views, they must be combined into the impression of the substantial thing.  The stereoscope thus illustrates clearly that the knowledge of the flat character of pictures by no means excludes the actual perception of depth, and the question arises whether the moving pictures of the photoplay, in spite of our knowledge concerning the flatness of the screen, do not give us after all the impression of actual depth.