hat of fresh-fallen snow being
0.78, and of white paper 0.70.[1048] But the disc of Jupiter is by no
means purely white. The general ground is tinged with ochre; the polar
zones are leaden or fawn coloured; large spaces are at times stained or
suffused with chocolate-browns and rosy hues. It is occasionally seen
ruled from pole to pole with dusky bars, and is never wholly free from
obscure markings. The reflection, then, by it, as a whole, of about 70
per cent. of the rays impinging upon it, might well suggest some
original reinforcement.

Nevertheless, the spectroscope gives little countenance to the
supposition of any considerable permanent light-emission. The spectrum
of Jupiter, as examined by Huggins, 1862-64, and by Vogel, 1871-73,
shows the familiar Fraunhofer rays belonging to reflected sunlight. But
it also shows lines of native absorption. Some of these are identical
with those produced by the action of our own atmosphere, especially one
or more groups due to aqueous vapours; others are of unknown origin;
and it is remarkable that one among the latter--a strong band in the
red--agrees in position with a dark line in the spectra of some ruddy
stars.[1049] There is, besides, a general absorption of blue rays,
intensified--as Le Sueur observed at Melbourne in 1869[1050]--in the
dusky markings, evidently through an increase of depth in the atmospheric
strata traversed by the light proceeding from them.

All these observations, however (setting aside the stellar line as of
doubtful significance), point to a cool planetary atmosphere. One
spectrograph, it is true, taken by Dr. Henry Draper, September 27,
1879,[1051] seemed to attest the action of intrinsic light; but the
peculiarity was referred by Dr. Vogel, with convincing clearness, to a
flaw in the film.[1052] So far, then, native emissions from any part of
Jupiter's diversified surface have not been detected; and, indeed, the
blackness of the shadows cast by his satellites on his disc sufficiently
proves that he sends out virtually none but reflected light.[1053] This
conclusion, however, by no means invalidates that of his high internal
temperature.

The curious phenomena attending Jovian satellite-transits may be
explained, partly as effects of contrast, partly as due to temporary
obscurations of the small discs projected on the large disc of Jupiter.
At their first entry upon its marginal parts, which are several times
less luminous than those near the ce