The screen must, of course, hide all this from the company, who will see nothing but the light on the wall.
To write your message is now a very simple matter. Dip a coarse brush into some lamp-black water color, and, writing backwards, inscribe what you wish upon the face of the mirror. The message will then appear legibly upon the wall, seemingly written by a mysterious hand.
By dipping the brush into clean water and washing out what you have written upon the glass, the message on the wall will disappear as inexplicably as it appeared in the first place.
Fig. 2.—Thread passed round prongs of the bone.
Fig. 3.—Match through twisted thread.
A peculiar optical illusion is accomplished as follows. The wishbone of a fowl or duck should be thoroughly cleaned, and a thread passed several times around the prongs of the fork, as shown in [Fig. 2]. Having secured the thread tightly, pass a strong wooden match between the strands, twisting it several times until the prongs of the bone have been drawn closer together ([Fig. 3]).
Now, pulling out the match sufficiently to allow of one end catching against the fork, hold the bone firmly. Releasing the match it immediately describes a circle, striking against the under part of the fork, but so rapidly has it completed this revolution that the eye has been quite unable to follow it. This causes an illusion that induces all who witness the experiment to imagine that the match passes through the fork of the bone at A.
But if the eye was too slow in the last experiment, it is so officious in what is next to be described that it sees something which really does not take place.