he base to a point in the axis of the glass
like a cone; it has six sides, and each side, from its angular
position to the eye, has the property of refracting from the border of
the print F, such a portion of it (designedly placed there) as will
make a part in the composition of the figure to be represented; for
the hexagonal and conical figure of this glass prevents any part of
the ace of diamonds being seen; consequently the ace of clubs being
previously and mechanically drawn in the circle of refraction in six
different parts of the border, at 1, 2, 3, 4, 5, 6, and artfully
disguised in the ornamental border, by blending them with it, the
glass in the tube at E will change the appearance of the ace of
diamonds, F, into the ace of clubs, G. In the same manner many other
prints undergo similar changes, according to the will of an ingenious
draughtsman who may design them. The figure of the glass is shown at
H.

[Illustration: Fig. 25.]

_To show the Spots in the Sun's Disk by its Image in the Camera
Obscura._

Put the object-glass of a ten or twelve feet telescope into the
scioptric ball, and turn it about till it be directly opposite the
sun. Then place the pasteboard mentioned in page 16, in the focus of
the lens, and you will see a clear bright image of the sun, about an
inch diameter, in which the spots on the sun's surface will be exactly
described.

As this image is too bright to be seen with pleasure by the naked eye,
you may view it through a lens whose focus is at six or eight inches
distance, which, while it prevents the light from being offensive,
will, by magnifying both the image and the spot, make them appear to
greater advantage.

_The Diagonal Opera Glass._

By the diagonal position of a plane mirror, a curious opera-glass is
constructed, by which any person may be viewed in a theatre or public
company without knowing it. It consists only in placing a concave
glass near the plane mirror, in the end of a short round tube, and a
convex glass in a hole in the side of the tube, then holding the end
of the tube with the glass to the eye, all objects next to the hole in
the side will be reflected so as to appear in a direct line forward,
or in a position at right angles to the person's situation who is
looked at. Plane glasses, instead of a convex and concave, may be
used; in this case the size of the object will not be increased, but
it will appear brighter.

_To observe an Eclipse of the Su