THE SKY OF THE ALPS 133 



the foregoing pages knows, De in great part quenched by 

 a Nicol's prism, while the light of a common cloud, being 

 unpolarized, cannot be thus extinguished. Hence the pos- 

 sibility 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 

 indistinguishable from the residual dull tint which out- 

 lives 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 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 southward. When the sky was quenched behind 

 them these floating masses seemed like dull embers sud- 

 denly 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 re- 

 ferred to may be here studied under the best possible 

 conditions. On August 23, 1869, the evening Alpen-glow 

 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 



