e when one-quarter _d_ away from it. B already magnifies the
image twice; the eye-piece again magnifies it four times; so that the
total magnification is 2 x 4 = 8 times. This result is arrived at
quickly by dividing the focus of B (which corresponds to the
object-glass of a telescope) by the focus of the eye-piece, thus:--

20
____ = 8
2-1/2

The ordinary astronomical telescope has a very long focus object-glass
at one end of the tube, and a very short focus eye-piece at the other.
To see an object clearly one merely has to push in or pull out the
eye-piece until its focus exactly corresponds with that of the
object-glass.

THE TERRESTRIAL TELESCOPE.

An astronomical telescope inverts images. This inversion is inconvenient
for other purposes. So the terrestrial telescope (such as is commonly
used by sailors) has an eye-piece compounded of four convex lenses which
erect as well as magnify the image. Fig. 125 shows the simplest form of
compound erecting eye-piece.

[Illustration: FIG. 125.]

THE GALILEAN TELESCOPE.

[Illustration: FIG. 126.]

A third form of telescope is that invented by the great Italian
astronomer, Galileo,[24] in 1609. Its principle is shown in Fig. 126.
The rays transmitted by the object-glass are caught, _before_ coming to
a focus, on a concave lens which separates them so that they appear to
meet in the paths of convergence denoted by the dotted lines. The image
is erect. Opera-glasses are constructed on the Galilean principle.

THE PRISMATIC TELESCOPE.

In order to be able to use a long-focus object-glass without a long
focussing-tube, a system of glass reflecting prisms is sometimes
employed, as in Fig. 127. A ray passing through the object-glass is
reflected from one posterior surface of prism A on to the other
posterior surface, and by it out through the front on to a second prism
arranged at right angles to it, which passes the ray on to the compound
eye-piece. The distance between object-glass and eye-piece is thus
practically trebled. The best-known prismatic telescopes are the Zeiss
field-glasses.

[Illustration: FIG. 127.]

THE REFLECTING TELESCOPE.

We must not omit reference to the _reflecting_ telescope, so largely
used by astronomers. The front end of the telescope is open, there being
no object-glass. Rays from the object fall on a parabolic mirror
situated in the rear end of the tube. This reflects them forwards to a
focus. In the Newtonian reflec