Lunar Attraction.
Jacques Ozanam, the famous French mathematician, invented this startling illusion, which I will describe for the benefit of the Round Table.
Make a box three feet square, or of any convenient size, and place a board of the same dimensions in the bottom, slightly inclined, with a serpentine groove in it, so that a ball of lead can roll in it freely. Extend a plain mirror from the elevated end of the board to the opposite upper corner, with the reflecting side down. Cut a small hole in the end of the box facing the mirror, and in such a position that the grooved board itself cannot be seen. If a ball of lead rolls along the groove, it will appear to ascend.
Vincent V. M. Beede.