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.