form a part of the same sphere. Though they are more or less of a
greyish-slaty hue--some of them approximating very closely to that of the
pigment known as "Payne's grey"--the tone, of course, depends upon the
angle at which the solar rays impinge on that particular portion of the
surface under observation. Speaking generally, they are, as would follow
from optical considerations, conspicuously darker when viewed near the
terminator, or when the sun is either rising or setting upon them, than
under a more vertical angle of illumination. But even when it is possible
to compare their colour by eye-estimation under similar solar altitudes,
it is found that not only are some of the Maria, as a whole, notably
darker than others, but nearly all of them exhibit _local_ inequalities
of hue, which, under good atmospheric and instrumental conditions, are
especially remarkable. Under such circumstances I have frequently seen
the surface, in many places covered with minute glittering points of
light, shining with a silvery lustre, intermingled with darker spots and
a network of streaks far too delicate and ethereal to represent in a
drawing. In addition to these contrasts and differences in the sombre
tone of these extended plains, many observers have remarked traces of a
yellow or green tint on the surface of some of them. For example, the
Mare Imbrium and the Mare Frigoris appear under certain conditions to be
of a dirty yellow-green hue, the central parts of the Mare Humorum dusky
green, and part of the Mare Serenitatis and the Mare Crisium light green,
while the Palus Somnii has been noted a golden-brown yellow. To these may
be added the district round Taruntius in the Mare Foecunditatis, and
portions of other regions referred to in the catalogue, where I have
remarked a very decided sepia colour under a low sun. It has been
attempted to account for these phenomena by supposing the existence of
some kind of vegetation; but as this involves the presence of an
atmosphere, the idea hardly finds favour at the present time, though
perhaps the possibility of plant growth in the low-lying districts, where