We are too readily inclined to imagine that our eye can directly grasp the different distances in our surroundings.  Yet we need only imagine that a large glass plate is put in the place of the curtain covering the whole stage.  Now we see the stage through the glass; and if we look at it with one eye only it is evident that every single spot on the stage must throw its light to our eye by light rays which cross the glass plate at a particular point.  For our seeing it would make no difference whether the stage is actually behind that glass plate or whether all the light rays which pass through the plate come from the plate itself.  If those rays with all their different shades of light and dark started from the surface of the glass plate, the effect on the one eye would necessarily be the same as if they originated at different distances behind the glass.  This is exactly the case of the screen.  If the pictures are well taken and the projection is sharp and we sit at the right distance from the picture, we must have the same impression as if we looked through a glass plate into a real space.

The photoplay is therefore poorly characterized if the flatness of the pictorial view is presented as an essential feature.  That flatness is an objective part of the technical physical arrangements, but not a feature of that which we really see in the performance of the photoplay.  We are there in the midst of a three-dimensional world, and the movements of the persons or of the animals or even of the lifeless things, like the streaming of the water in the brook or the movements of the leaves in the wind, strongly maintain our immediate impression of depth.  Many secondary features characteristic of the motion picture may help.  For instance, by a well-known optical illusion the feeling of depth is strengthened if the foreground is at rest and the background moving.  Thus the ship passing in front of the motionless background of the harbor by no means suggests depth to the same degree as the picture taken on the gliding ship itself so that the ship appears to be at rest and the harbor itself passing by.

The depth effect is so undeniable that some minds are struck by it as the chief power in the impressions from the screen.  Vachel Lindsay, the poet, feels the plastic character of the persons in the foreground so fully that he interprets those plays with much individual action as a kind of sculpture in motion.  He says:  “The little far off people on the oldfashioned speaking stage do not appeal to the plastic sense in this way.  They are by comparison mere bits of pasteboard with sweet voices, while on the other hand the photoplay foreground is full of dumb giants.  The bodies of these giants are in high sculptural relief.”  Others have emphasized that this strong feeling of depth touches them most when persons in the foreground stand with a far distant landscape as background—­much more than when they are seen in a room.  Psychologically