erhaps, be commanded at
will, yet, they are turned out with increasing frequency, and the best
artists are generally able, at all times, to approximate so closely to
perfection that any shortcoming may be disregarded.

In what is said above I refer, of course, to the refracting telescope,
which is the form of instrument that I should recommend to all amateurs
in preference to the reflector. But, before proceeding further, it may
be well to recall briefly the principal points of difference between
these two kinds of telescopes. The purpose of a telescope of either
description is, first, to form an image of the object looked at by
concentrating at a focus the rays of light proceeding from that object.
The refractor achieves this by means of a carefully shaped lens, called
the object glass, or objective. The reflector, on the other hand, forms
the image at the focus of a concave mirror.

[Illustration: IMAGE AT THE FOCUS OF A LENS.]

A very pretty little experiment, which illustrates these two methods of
forming an optical image, and, by way of corollary, exemplifies the
essential difference between refracting and reflecting telescopes, may
be performed by any one who possesses a reading glass and a magnifying
hand mirror. In a room that is not too brightly illuminated pin a sheet
of white paper on the wall opposite to a window that, by preference,
should face the north, or away from the position of the sun. Taking
first the reading glass, hold it between the window and the wall
parallel to the sheet of paper, and a foot or more distant from the
latter. By moving it to and fro a little you will be able to find a
distance, corresponding to the focal length of the lens, at which a
picture of the window is formed on the paper. This picture, or image,
will be upside down, because the rays of light cross at the focus. By
moving the glass a little closer to the wall you will cause the picture
of the window to become indistinct, while a beautiful image of the
houses, trees, or other objects of the outdoor world beyond, will be
formed upon the paper. We thus learn that the distance of the image from
the lens varies with the distance of the object whose image is formed.
In precisely a similar manner an image is formed at the focus of the
object glass of a refracting telescope.

[Illustration: IMAGE AT THE FOCUS OF A CONCAVE MIRROR.]

Take next your magnifying or concave mirror, and detaching the sheet of
paper from the wall, h