ARTIFICIAL SKY 125 



fractive indices, and therefore of very different polarizing 

 angles as ordinarily defined, but the polarization of the 

 beam, by the incipient cloud, has thus far proved itself to 

 be absolutely independent of the polarizing angle. The 

 law of Brewster does not apply to matter in this condi- 

 tion, and it rests with the undulatory theory to explain 

 why. Whenever the precipitated particles are sufficiently 

 fine, no matter what the substance forming the particles 

 may be, the direction of maximum polarization is at right 

 angles to the illuminating beam, the polarizing angle for 

 matter in this condition being invariably 45. 



Suppose our atmosphere surrounded by an envelope 

 impervious to light, but with an aperture on the sunward- 

 side through which a parallel beam of solar light could 

 enter and traverse the atmosphere. Surrounded by air 

 not directly illuminated, the track of such a beam would 

 resemble that of the parallel beam of the electric lamp 

 through an incipient cloud. The sunbeam would be blue, 

 and it would discharge laterally light in precisely the same 

 condition as that discharged by the incipient cloud. In 

 fact, the azure revealed by such a beam would be to all 

 intents and purposes that which I have called a "blue 

 cloud." Conversely our "blue cloud" is, to all intents 

 and purposes, an artificial sky. 1 



1 The opinion of Sir John Herschel, connecting the polarization and the 

 blue color of the sky, is verified by the foregoing results. "The more the sub- 

 ject [the polarization of skylight] is considered," writes this eminent philoso- 

 pher, "the more it will be found beset with difficulties, and its explanation 

 when arrived at will probably be found to carry with it that of the blue color 

 of the sky itself, and of the great quantity of light it actually does send down 

 to us." "We may observe, too," he adds, "that it is only where the purity of 

 the sky is most absolute that the polarization is developed in its highest degree, 

 and that where there is the slightest perceptible tendency to cirrus it is mate- 

 rially impaired." This applies word for word to our "incipient clouds." 



