The dioptrical paradox consists of a hard-wood base, A, B, C, D, about
eight inches square, with a groove in which slides coloured prints or
drawings. Connected with the base are a pillar (E), a horizontal bar
(F), with a tube (G) directly over the centre of the base, and
containing a peculiarly shaped glass.
[Illustration: Fig. 122.]
_Performance._—An ace of diamonds, when placed on the base, will be
actually shown as an ace of clubs to anybody looking down through the
glass, or one animal is seen as another; in fact, any shape is presented
as something different.
_Explanation._—The glass (G) is like the common multiplying glass,
except that its sides are flat and diverge from its hexagonal base
upwards to a point in the axis of the glass, like a pyramid, each facet
being an isosceles triangle.
Regulate its distance from the eye so that each side will refract the
various parts of the drawing on the border so as to form one figure, and
the centre object be entirely unreflected.
The ace of clubs is, therefore, drawn mechanically on the circle of
refraction at six different parts of the border, and blended in with it.
So with the other drawings.