the
foregoing pages knows, be in great part quenched by a Nicol's prism,
while the light of a common cloud, being unpolarised, cannot be thus
extinguished. Hence the possibility of very remarkable variations,
not only in the aspect of the firmament, which is really changed, but
also in the aspect of the clouds, which have that firmament as a
background. It is possible, for example, to choose clouds of such a
depth of shade that when the Nicol quenches the light behind them,
they shall vanish, being undistinguishable from the residual dull tint
which outlives the extinction of the brilliancy of the sky. A cloud
less deeply shaded, but still deep enough, when viewed with the naked
eye, to appear dark on a bright ground, is suddenly changed to a white
cloud on a dark ground by the quenching of the light behind it. When
a reddish cloud at sunset chances to float in the region of maximum
polarisation, the quenching of the surrounding light causes it to
flash with a brighter crimson. Last Easter eve the Dartmoor sky, which
had just been cleansed by a snow-storm, wore a very wild appearance.
Round the horizon it was of steely brilliancy, while reddish cumuli
and cirri floated southwards. When the sky was quenched behind them
these floating masses seemed like dull embers suddenly blown upon;
they brightened like a fire.

In the Alps we have the most magnificent examples of crimson clouds
and snows, so that the effects just referred to may be here studied
under the best possible conditions. On August 23, 1869, the evening
Alpenglow was very fine, though it did not reach its maximum depth and
splendour. The side of the Weisshorn seen from the Bel Alp, being
turned from the sun, was tinted mauve; but I wished to observe one of
the rose-coloured buttresses of the mountain. Such a one was visible
from a point a few hundred feet above the hotel. The Matterhorn also,
though for the most part in shade, had a crimson projection, while a
deep ruddy red lingered along its western shoulder. Four distinct
peaks and buttresses of the Dom, in addition to its dominant head--all
covered with pure snow--were reddened by the light of sunset. The
shoulder of the Alphubel was similarly coloured, while the great mass
of the Fletschorn was all a-glow, and so was the snowy spine of the
Monte Leone.

Looking at the Weisshorn through the Nicol, the glow of its
protuberance was strong or weak according to the position of the
prism. The summ