en
thus mounted, the telescope may be driven by clockwork, or by hand with
the aid of a screw geared to a handle carrying a universal joint.

And now for testing the telescope. It has already been remarked that the
excellence of a telescope depends upon the perfection of the image
formed at the focus of the objective. In what follows I have only a
refractor in mind, although the same principles would apply to a
reflector. With a little practice anybody who has a correct eye can form
a fair judgment of the excellence of a telescopic image. Suppose we have
our telescope steadily mounted out of doors (if you value your peace of
mind you will not try to use a telescope pointed out of a window,
especially in winter), and suppose we begin our observations with the
pole star, employing a magnifying power of sixty or seventy to the inch.
Our first object is to see if the optician has given us a good glass. If
the air is not reasonably steady we had better postpone our experiment
to another night, because we shall find that the star as seen in the
telescope flickers and "boils," and behaves in so extraordinary a
fashion that there is no more definition in the image than there is
steadiness in a bluebottle buzzing on a window pane. But if the night is
a fine one the star image will be quiescent, and then we may note the
following particulars: The real image is a minute bright disk, about one
second of arc in diameter if we are using a four-and-a-half or five-inch
telescope, and surrounded by one very thin ring of light, and the
fragments, so to speak, of one or possibly two similar rings a little
farther from the disk, and visible, perhaps, only by glimpses. These
"diffraction rings" arise from the undulatory nature of light, and their
distance apart as well as the diameter of the central disk depend upon
the length of the waves of light. If the telescope is a really good one,
and both object glass and eyepiece are properly adjusted, the disk will
be perfectly round, slightly softer at the edge, but otherwise equally
bright throughout; and the ring or rings surrounding it will be exactly
concentric, and not brighter on one side than on another. Even if our
telescope were only two inches or two inches and a half in aperture we
should at once notice a little bluish star, the mere ghost of a star in
a small telescope, hovering near the polar star. It is the celebrated
"companion," but we shall see it again when we have more time to s