ARTIFICIAL SKY. 127 



most striking. The grey summit of the Matterhorn, 

 at the same time, could scarcely be distinguished frcm 

 the opalescent haze around it; but when the Nicol 

 quenched the haze, the summit became instantly iso- 

 lated, 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 lumi- 

 nous haze in front of the mountains; that these are 

 changed because the light emitted from the sky and 

 from the haze is plane polarised light, and that the 

 light from the snows and from the mountains, being 

 sensibly unpolarised, 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 renders 

 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 coloured covered with pines, for example 

 every peak and ridge is separated from the mountains 

 behind it by a thin blue haze which renders the relations 

 of the mountains as to distance unmistakable. When 

 this haze is regarded through the Nicol perpendicular 

 to the sun's rays, it is in many cases wholly quenched, 

 because the light which it emits in this direction is wholly 

 polarised. When this happens, aerial perspective is 

 abolished, and mountains very differently distant appear 

 to rise in the same vertical plane. Close to the Bel 

 Alp, for instance, is the gorge of the Massa, and beyond 

 the gorge is a high ridge darkened by pines. This 

 ridge moy be projected upon the dark slopes at the 

 opposite side of the Rhone valley, and between both 



