94 FRAGMENTS OF SCIENCE. 
pages knows, be in great part quenched by a Nicol's prism, 
while the light of a common cloud, being unpolarized, 
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 undistingtiishable 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 polarization, the quench- 
ing 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 south- 
ward. 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 
splendor. 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-colored buttresses of the 
mountain. Such a one was visible from a point a few 
hundred feet above the hotel. The MaUerhorn also, 
though for the most part in shade, had a crimson projec- 
tion, while a deep ruddy red lingered along its western 
shoulder. Four distant 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 colored, while the great mass 
of the Fletschorn was all aglow, and so was the snowy 
spine of the Monte Leone. 
Looking at the Weisshorn through the Nicol, the glow 
