old it nearly in front of the mirror between the
latter and the window. When you have adjusted the distance to the focal
length of the mirror, you will see an image of the window projected upon
the paper, and by varying the distance, as before, you will be able to
produce, at will, pictures of nearer or more remote objects. It is in
this way that images are formed at the focus of the mirror of a
reflecting telescope.

Now, you will have observed that the chief apparent difference between
these two methods of forming an image of distant objects is that in the
first case the rays of light, passing through the transparent lens, are
brought to a focus on the side opposite to that where the real object
is, while in the second case the rays, being reflected from the
brilliant surface of the opaque mirror, come to a focus on the same side
as that on which the object itself is. From this follows the most
striking difference in the method of using refracting and reflecting
telescopes. In the refractor the observer looks toward the object; in
the reflector he looks away from it. Sir William Herschel made his great
discoveries with his back to the sky. He used reflecting telescopes.
This principle, again, can be readily illustrated by means of our simple
experiment with a reading glass and a magnifying mirror. Hold the
reading glass between the eye and a distant object with one hand, and
with the other hand place a smaller lens such as a pocket magnifier,
near the eye, and in line with the reading glass. Move the two carefully
until they are at a distance apart equal to the sum of the focal lengths
of the lenses, and you will see a magnified image of the distant object.
In other words, you have constructed a simple refracting telescope. Then
take the magnifying mirror, and, turning your back to the object to be
looked at, use the small lens as before--that is to say, hold it between
your eye and the mirror, so that its distance from the latter is equal
to the sum of the focal lengths of the mirror and the lens, and you will
see again a magnified image of the distant object. This time it is a
reflecting telescope that you hold in your hands.

The magnification of the image reminds us of the second purpose which is
subserved by a telescope. A telescope, whether refracting or reflecting,
consists of two essential parts, the first being a lens, or a mirror, to
form an image, and the second a microscope, called an eyepiece, to
magnify