tor a plane mirror or prism is situated in
the axis of the tube, at the focus, to reflect the rays through an
eye-piece projecting through the side of the tube. Herschel's form of
reflector has the mirror set at an angle to the axis, so that the rays
are reflected direct into an eye-piece pointing through the side of the
tube towards the mirror.

THE PARABOLIC MIRROR.

This mirror (Fig. 128) is of such a shape that all rays parallel to the
axis are reflected to a common point. In the marine searchlight a
powerful arc lamp is arranged with the arc at the focus of a parabolic
reflector, which sends all reflected light forward in a pencil of
parallel rays. The most powerful searchlight in existence gives a light
equal to that of 350 million candles.

[Illustration: FIG. 128.--A parabolic reflector.]

THE COMPOUND MICROSCOPE.

We have already observed (Fig. 110) that the nearer an object
approaches a lens the further off behind it is the real image formed,
until the object has reached the focal distance, when no image at all is
cast, as it is an infinite distance behind the lens. We will assume that
a certain lens has a focus of six inches. We place a lighted candle four
feet in front of it, and find that a _sharp_ diminished image is cast on
a ground-glass screen held seven inches behind it. If we now exchange
the positions of the candle and the screen, we shall get an enlarged
image of the candle. This is a simple demonstration of the law of
_conjugate foci_--namely, that the distance between the lens and an
object on one side and that between the lens and the corresponding image
on the other bear a definite relation to each other; and an object
placed at either focus will cast an image at the other. Whether the
image is larger or smaller than the object depends on which focus it
occupies. In the case of the object-glass of a telescope the image was
at what we may call the _short_ focus.

[Illustration: FIG. 129.--Diagram to explain the compound microscope.]

Now, a compound microscope is practically a telescope with the object at
the _long_ focus, very close to a short-focus lens. A greatly enlarged
image is thrown (see Fig. 129) at the conjugate focus, and this is
caught and still further magnified by the eye-piece. We may add that the
object-glass, or _objective_, of a microscope is usually compounded of
several lenses, as is also the eye-piece.

THE MAGIC-LANTERN.

The most essential features of a mag