THE SKY OF THE ALPS 135 



turning the Nicol through an angle of 90. It was not 

 the stoppage of the light of the sky behind the mountains 

 alone which produced this startling effect; the air be- 

 tween them and me was highly opalescent, and the 

 quenching of this intermediate glare augmented remark- 

 ably the distinctness 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 strik- 

 ing. 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 pro* 

 duction of these effects the only things changed are the 

 sky behind, and the luminous haze in front of the moun- 

 tains; 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 moun- 

 tains, being sensibly unpolarized, is not directly affected 

 by the Nicol. It will also be understood that it is not 

 the interposition of the haze as an opaque body that ren- 

 ders the mountains indistinct, but that it is the light of 

 the haze which dims and bewilders the eye, and thus 

 weakens the definition of objects seen through it. 



These results have a direct bearing upon what artists 

 call "aerial perspective." As we look from the summit 

 of Mont Blanc, or from a lower elevation, at the serried 

 crowd of peaks, especially if the mountains be darkly 



