this is not surprising either.  If the scene were a real room, every detail in it would appear differently to the two eyes.  In the room on the screen both eyes receive the same impression, and the result is that the consciousness of depth is inhibited.  But when a far distant landscape is the only background, the impression from the picture and life is indeed the same.  The trees or mountains which are several hundred feet distant from the eye give to both eyes exactly the same impression, inasmuch as the small difference of position between the two eyeballs has no influence compared with the distance of the objects from our face.  We would see the mountains with both eyes alike in reality, and therefore we feel unhampered in our subjective interpretation of far distant vision when the screen offers exactly the same picture of the mountains to our two eyes.  Hence in such cases we believe that we see the persons really in the foreground and the landscape far away.

Nevertheless we are never deceived; we are fully conscious of the depth, and yet we do not take it for real depth. Too much stands in the way.  Some unfavorable conditions are still deficiencies of the technique; for instance, the camera picture in some respects exaggerates the distances.  If we see through the open door of the rear wall into one or two other rooms, they appear like a distant corridor.  Moreover we have ideal conditions for vision in the right perspective only when we sit in front of the screen at a definite distance.  We ought to sit where we see the objects in the picture at the same angle at which the camera photographed the originals.  If we are too near or too far or too much to one side, we perceive the plastic scene from a viewpoint which would demand an entirely different perspective than that which the camera fixated.  In motionless pictures this is less disturbing; in moving pictures every new movement to or from the background must remind us of the apparent distortion.  Moreover, the size and the frame and the whole setting strongly remind us of the unreality of the perceived space.  But the chief point remains that we see the whole picture with both eyes and not with only one, and that we are constantly reminded of the flatness of the picture because the two eyes receive identical impressions.  And we may add an argument nearly related to it, namely, that the screen as such is an object of our perception and demands an adaptation of the eye and an independent localization.  We are drawn into this conflict of perception even when we look into a mirror.  If we stand three feet from a large mirror on the wall, we see our reflection three feet from our eyes in the plate glass and we see it at the same time six feet from our eye behind the glass.  Both localizations take hold of our mind and produce a peculiar interference.  We all have learned to ignore it, but characteristic illusions remain which indicate the reality of this doubleness.