NEW CHEMICAL REACTIONS. 95 



of its protuberance was strong or weak according to the 

 position of the prism. The summit also underwent 

 striking changes. In one position of the prism it exhibited 

 a pale white against a dark background; in the rectangular 

 position it was a dark mauve against a light background. 

 The red of the Matterhorn changed in a similar manner; 

 but the whole mountain also passed through wonderful 

 changes of definition. The air at the time was filled with 

 a silvery haze, in which the Matterhorn almost disappeared. 

 This could be wholly quenched by the Nicol, and then the 

 mountain sprang forth with astonishing solidity and 

 detachment from the surrounding air. The changes of 

 the Dom were still more wonderful. A vast amount of 

 light could be removed from the sky behind it, for it 

 occupied the position of maximum polarization. By a 

 little practice with the Nicol it was easy to render the ex- 

 tinction of the light, or its restoration,, almost instan- 

 taneous. When the sky was quenched, the four minor 

 peaks and buttresses, and the summit of the Dom, to- 

 gether with the shoulder of the Alphubel, glowed as if set 

 suddenly on fire. This was immediately dimmed by turn- 

 ing the Nicol through an angle of 90 degrees. It was not 

 the stoppage of the light of the sky behind the mountains 

 alone which produced this startling effect; the air between 

 them and me was highly opalescent, and the quenching of 

 this intermediate glare augmented remarkably the dis- 

 tinctness of the mountains. 



On the morning of August 24 similar effects were finely 

 shown. At 10 A.M. all three mountains, the Dom, the 

 Matterhorn, and the Weisshorn,were powerfully affected 

 by the Nicol. But in this instance also, the line drawn to 

 the Dom being very nearly perpendicular to the solar beams, 

 the effects on this mountain were most striking. The 

 gray summit of the Matterhorn, at the same time, could 

 scarcely be distinguished from the opalescent haze around 

 it; but when the Nicol quenched the haze, the summit 

 became instantly isolated, and stood out in bold definition. 

 It is to be remembered that in the production of these 

 effects the only things changed are the sky behind, and the 

 luminous haze in front of the mountains; that these are 

 changed because the light emitted from the sky and from 

 the haze is plane polarized light, and that the light from 

 the snows and from the mountains, being sensibly unpolar- 



