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.