But illusions suggest themselves.  There is no end to the effects which can be had from this simple apparatus, and if the operators are sufficiently well drilled the result is truly remarkable to the uninitiated.  The illusion, as presented by Hermann, was identical with this, only he, of course, had a big stage, and people clothed in black to creep about and do his bidding, while here the power behind the throne is but a black-veiled hand and arm.  It can be made even more complicated by having two assistants, one on each side of the box, and this is the reason why it was advised that two holes be cut.  This enables an absolutely instantaneous change as one uncovers the object at the moment the second assistant covers and removes the other.

[Illustration:  The Magic Cave]

It is important that the assistants remain invisible throughout, and if portieres are impossible, a screen must be used.  But any boy ingenious enough to follow these simple instructions will not need to be told that the whole success of the exhibition depends upon the absolute failure of the audience to understand that there is more than one concerned in bringing about the curious effects which are seen.  The exhibitor should be a boy who can talk; a good “patter”—­as the magicians call it—­is often of more value than a whole host of mechanical effects and helpers.  It is essential that the exhibitor and his confederate be well drilled, so that the latter can produce the proper effects at the proper cue from the former.  Finally, never give an exhibition with the “cave” until you have watched the illusions from the front yourself; so that you can determine whether everything connected with the draping is right, or whether some stray bit of light reveals what you wish to conceal.

** Reversing-Switch for Electrical Experiments [92]

A homemade reversing-switch, suitable for use by students of electrical and engineering courses in performing experiments, is shown in the diagram.

[Illustration:  Fig.2 Suitable for Students’ Use]

Referring to Fig. 1, A represents a pine board 4 in. by 4 in. and a is a circular piece of wood about 1/4 in. square, with three brass strips, b1, b2, b3, held down on it by two terminals, or binding posts, c1, c2, and a common screw, d.  Post c1 is connected to d by means of an insulated wire, making them carry the same kind of current (+ in the sketch).

About the center piece H moves a disk, held down by another disk F (Fig. 2), which is fastened through the center piece to the wooden base, A, by means of two wood screws.  On the disk G are two brass strips, e1 and e2, so arranged that, when handle K is turned to one side, their one end just slips under the strips b1, b2, or b2, b3, respectively, making contact with them, as shown in Fig. 2, at L, while their other ends slide in two half-circular brass plates f1, f2, held down on disk F by two other terminals, c3, c4, making contact with them as shown at y, Fig. 2.