An illusion is often practised at fancy fairs and bazaars, when a
spectator, looking into what he supposes to be an ordinary
looking-glass, sees his companion instead of himself. Of course the
exhibitor endeavours to show the illusion to two persons at once; and if
they are strangers to each other, and of opposite sex, a great deal of
fun is made out of the trick. Showmen at the fairs have made immense
harvests by showing two such mirrors, one to all the young girls who
wished to see their future husbands, and the other to all the young men
who wished to see their future wives.
_Explanation._—Make a cube-box, fifteen inches, say, each way, and stand
it on a pedestal to bring it to the height of the eye. In each side of
this box let there be an opening of an oval form, ten inches high and
seven wide. In this box place two mirrors with their backs against each
other. Let them cross the box in a diagonal line, and in a vertical
position. Decorate the openings in the side of this box with four oval
frames and transparent glasses, and cover each with a curtain so
contrived that all draw up together.
Place four persons (in the case first mentioned two are confederates) in
front of the four sides, and at equal distances from the box, and then
draw them up that they may see themselves in the mirrors, when each of
them, instead of his own figure, will see that of the person next to
him, but who will appear to him to be placed on the opposite side. Their
confusion will be the greater, as it will be very difficult, if not
impossible, for them to discover the mirrors concealed in the box. The
reason is that though the rays of light may be turned aside by a mirror,
yet they always _appear_ to proceed in right lines.
[Illustration: Fig. 123.]