spot upon
the moon are to be observed from hour to hour (or from day to day,
according to the size of the spot) as the sun's light gradually sweeps
across it, until the spot is fully lighted; then as the moon wanes and the
sun's light gradually passes from the spot, the series of observations is
to be renewed. A comparison of them is likely--especially if the observer
is a good artist and has executed several faithful delineations of the
region under observation, to throw much light upon the real contour of the
moon's surface at this point.

In the two lunar views in Plate 7 some of the peculiarities I have
described are illustrated. But the patient observer will easily be able
to construct for himself a set of interesting views of different
regions.

It may be noticed that for observation of the waning moon there is no
occasion to wait for those hours in which only the waning moon is
visible _during the night_. Of course for the observation of a
particular region under a particular illumination, the observer has no
choice as to hour. But for generally interesting observations of the
waning moon he can wait till morning and observe by daylight. The moon
is, of course, very easily found by the unaided eye (in the day time)
when not very near to the sun; and the methods described in Chapter V.
will enable the observer to find the moon when she is so near to the sun
as to present the narrowest possible sickle of light.

One of the most interesting features of the moon, when she is observed
with a good telescope, is the variety of colour presented by different
parts of her surface. We see regions of the purest white--regions which
one would be apt to speak of as _snow-covered_, if one could conceive
the possibility that snow should have fallen where (now, at least) there
is neither air nor water. Then there are the so-called seas, large grey
or neutral-tinted regions, differing from the former not merely in
colour and in tone, but in the photographic quality of the light they
reflect towards the earth. Some of the seas exhibit a greenish tint, as
the Sea of Serenity and the Sea of Humours. Where there is a central
mountain within a circular depression, the surrounding plain is
generally of a bluish steel-grey colour. There is a region called the
Marsh of Sleep, which exhibits a pale red tint, a colour seen also near
the Hyrcinian mountains, within a circumvallation called Lichtenburg.
The brightest portion of the whole