scope to adjust itself to out-of-door temperature,
except in the summer time, and it is better to allow an hour or two for
such adjustment in cold weather. Any irregularity in the shape of the
rings which persists after the lenses have been accurately adjusted and
the telescope has properly cooled may be ascribed to imperfections, such
as veins or spots of unequal density in the glass forming the objective.

[Illustration: THE OUT-OF-FOCUS RINGS.

1, Correct figure; 2 and 3, spherical aberration.]

The spherical aberration of an object glass may be undercorrected or
overcorrected. In the former case the central rings inside the focus
will appear faint and the outer ones unduly strong, while outside the
focus the central rings will be too bright and the outer ones too
feeble. But if the aberration is overcorrected the central rings will be
overbright inside the focus and abnormally faint outside the focus.

[Illustration: TWO VIEWS OF MARS IN 1892.

The smaller with a three-and-three-eighths-inch telescope; the larger
with a nine-inch.]

Assuming that we have a telescope in which no obvious fault is
discernible, the next thing is to test its powers in actual work. In
what is to follow I shall endeavor to describe some of the principal
objects in the heavens from which the amateur observer may expect to
derive pleasure and instruction, and which may at the same time serve
as tests of the excellence of his telescope. No one should be deterred
or discouraged in the study of celestial objects by the apparent
insignificance of his means of observation. The accompanying pictures of
the planet Mars may serve as an indication of the fact that a small
telescope is frequently capable of doing work that appears by no means
contemptible when placed side by side with that of the greater
instruments of the observatories.

CHAPTER II

IN THE STARRY HEAVENS

"Now constellations, Muse, and signs rehearse;
In order let them sparkle in thy verse."--MANILIUS.

Let us imagine ourselves the happy possessors of three properly mounted
telescopes of five, four, and three inches aperture, respectively. A
fine midwinter evening has come along, the air is clear, cool, and
steady, and the heavens, of that almost invisible violet which is
reserved for the lovers of celestial scenery, are spangled with stars
that hardly twinkle. We need not disturb our minds about a few thin
clouds here and there floating lazily in the high air; th