94 FRA GMENTS OF SCIENCE. 



pages knows, be in great part quenched by a NicoFs 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 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 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 Matterhorn 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 



