"Things You Wouldn't Know If We Didn't Blog Intermittently."
Quite an interesting comment on politics and politicians I think...
It is dependent on both the curvature of the glass and the placement of the camera and paper. The reversal happens because the lens is set up to be a relay imagining system. The relative position of the two determines the magnification which needs to be 1 to 1 for this effect to work.
In layman's language, "it only works at one distance."