ARTIFICIAL SKY. 125 



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 sud- 

 denly 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 maxi- 

 mum 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 snowstorm, 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 Alpen- 

 glow 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 red- 

 dened by the light of sunset. The shoulder of the 

 Alphubel was similarly coloured, while the great mass 



