servation regions near the limb at one
time, and at another bringing into view others beyond the limits of the
maps, which represent the moon in the mean state of libration. The area,
in fact, thus brought into view, or taken out of it, is between 1/12th
and 1/13th of the entire area of the moon, or about the 1/6th part of the
hemisphere turned away from the earth. It is convenient to bear in mind
that we see an object under nearly the same conditions every 59 d. 1 h.
28 m., or still more accurately, after the lapse of fifteen lunations, or
442 d. 23 h. Many observers avoid the observation of objects under a high
light. This, however, should never be neglected when practicable, though
in some cases it is not easy to carry out, owing to the difficulty in
tracing details under these circumstances.

Although to observe successfully the minuter features, such as the rills
and the smaller craterlets, requires instruments of large aperture
located in favourable situations, yet work of permanent value may be
accomplished with comparatively humble telescopic means. A 4 inch
achromatic, or a silver-on-glass reflector of 6 or 6 1/2 inches aperture,
will reveal on a good night many details which have not yet been
recorded, and the possessor of instruments of this size will not be long
in discovering that the moon, despite of what is often said, has not been
so exhaustively surveyed that nothing remains for him to do.

Only experience and actual trial will teach the observer to choose the
particular eyepiece suitable for a given night or a given object. It will
be found that it is only on very rare occasions that he can accomplish
much with powers which, perhaps only on two or three nights in a year in
this climate, tell to great advantage; though it sometimes happens that
the employment of an eyepiece, otherwise unsuitable for the night, will,
during a short spell of good definition, afford a fleeting glimpse of
some difficult feature, and thus solve a doubtful point. It has often
been said that the efficiency of a telescope depends to a great extent on
"the man at the eye end." This is as true in the case of the moon as it
is in other branches of observational astronomy.

Observers, especially beginners, frequently fall into great error in
failing to appreciate the true character of what they see. In this way a
shallow surface depression, possibly only a few feet below the general
level of the neighbouring country, is often desc