The parapet, and the hidden carriers

Between the fire and the prisoners runs a low wall "like the screen which marionette players have before them," and along it people carry figures of men and animals in wood and stone. The carriers stay hidden. Only the objects rise above the wall.

The most-skipped detail, and the whole trick. The prisoners are not looking at nature's shadows — they are looking at the shadows of artefacts, carried deliberately by other people, who are out of sight. The image is not about perception in general. It is about a curated show with a staff.