This terminates our experiments on optical illusions and you will now enter upon another field of knowledge altogether.
The Insensible Coin.
Cut a piece of cardboard about six inches long, and by sticking the extremities together with a pin, or with gum, form a circle or ring. Balance it carefully on the neck of a wine bottle or decanter, and on the top of the ring place a dime, exactly over the neck of the bottle. Now the trick to be performed is to take off the ring so that, without touching it, the coin falls into the bottle. On the inner side of the ring give a sharp knock with the finger, or, better still, with the thumb and forefinger, as in shooting a marble, as shown in figure. The ring will come off, and the coin which on account of its inertia, does not participate in the movement, will infallibly fall into the bottle. It is absolutely necessary to strike the interior of the circle, because in striking it from the outside one would not get any result at all, on account of the elasticity of the cardboard.
The Asses’ Bridge.
Every schoolboy knows which is the famous geometrical theorem, commonly called the Asses’ Bridge, and which is propounded as follows: