s of this palace, the
objects there contained, being reflected six times, will seem entirely
to fill up the whole of the building. This illusion will appear very
remarkable, especially if the objects chosen are properly adapted to
the effect which the mirrors are intended to produce.

If you place between two of these mirrors part of a fortification, as
a curtain, and two demi-bastions, you will see an entire citadel with
six bastions; or if you place part of a ball-room, ornamented with
chandeliers and figures, all these objects being here multiplied, will
afford a very pleasing prospect.

_Opaque Bodies seemingly Transparent._

Within the case A B C D, place four mirrors O P Q R, Fig. 22, so
disposed, that they may each make an angle of 45 degrees, that is,
that they may be half-way inclined from the perpendicular, as in the
figure. In each of the two extremities A B, make a circular overture;
in one of which fix the tube G L, in the other the tube M F, and
observe, that in each of these is to be inserted another tube, as H
and I. [_Observe._ These four tubes must terminate in the substance of
the case, and not enter the inside, that they may not hinder the
effect of the mirrors. The four-fold reflection of the rays of light
from the mirrors, darkens in some degree the brightness of the object;
some light is also lost by the magnifying power of the perspective.
If, therefore, instead of the object-glass at G, and concave eye-glass
at F, plain glasses were substituted, the magnifying power of the
perspective will be taken away, and the object appear brighter.]

[Illustration: Fig. 22.]

Furnish the first of these tubes with an object-glass at G, and a
concave eye-glass at F. You are to observe, that in regulating the
focus of these glasses with regard to the length of the tube, you are
to suppose it equal to the line G, or visual pointed ray, which
entering at the aperture G is reflected by the four mirrors, and goes
out at the other aperture F, where the eye-glass is placed. Put any
glass you please into the two ends of the moveable tubes H and L; and
lastly, place the machine on stand E, moveable at the point S, that it
may be elevated or lowered at pleasure.

When the eye is placed at F, and you look through the tube, the rays
of light that proceed from the object T, passing through the glass G,
are successively reflected by the mirrors O P Q and R to the eye at F,
and there point the object T in its proper