OTHER OPTICAL ILLUSIONS.
By varying the disposition of mirrors, prisms, lenses,
and light, an infinite number of the most surprising
effects may be shown, with a comparatively small
amount of trouble and expense. We shall, therefore,
devote this chapter to the explanation of a large number
of allusions, which have been devised by Robertson and
other adepts in the art of honest deception.
One of Robertson's most famous delusions was the "Dance of Demons," an effect he discovered quite accidentally. One evening, while experimenting with the phantasmagoria, he suddenly found himself in the dark, when two persons, each bearing a light, crossed the room on the other side of the screen. A little window which happened to be between the lights and the screen, immediately threw its double image on the cloth, and the method of multiplying shadows was discovered.
The figures used in this experiment are cut out of fine cardboard, and may be made a foot high or thereabouts. They are placed on a second screen in front of the principal one, and by multiplying the lights, as shown in fig. 55, you may have as many shadows as you please. The effect is much heightened if the figures are cut out so as to show as lights when thrown on the screen. A little ingenuity shown in the arrangement of the distance and movements of the lights, will produce an
