ARTIFICIAL SKY. 127 



the line drawn to the Dom being very nearly perpen- 

 dicular to the solar beams, the effects on this mountain 

 were most striking. The grey summit of the Matter- 

 horn, 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 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 rela- 

 tions of the mountains as to distance unmistakable. 

 When this haze is regarded through the Nicol perpen- 

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

 quenched, because the light which it emits in this direc- 

 tion 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. 



