One of the numerous optical illusions which have from time to time been
evolved by scientific minds, is that of making an image or picture
appear in the air. This is produced by means of a mirror, and an object
in relief, upon which a strong light is thrown--the mirror being set at
such an angle as to throw up the reflection of the image to a certain
point in the view of the spectator. This illusion is produced as
follows: Let a screen be constructed in which is an arched aperture,
the center of which may be five feet from the floor; behind the screen
is placed a large mirror of an elliptical form. An object is now placed
behind the screen, upon which the light of a strong lamp is thrown from
a point above the mirror, and is received by the mirror and reflected
to the center of the arched cavity in the screen, where it will appear
to the spectator. Care should be taken to place the image in an
inverted position, and the light, which must be very powerful, should
be so placed that none of it may reach the opening.