﻿of the Sky. and on the Polarization of Light. 389 



That water-particles, if they could be obtained in this exceedingly 

 fine state of division, would produce the same effects, does not admit 

 of reasonable doubt. And that they must exist in this condition in 

 the higher regions of the atmosphere is, I think, certain. At all 

 events, no other assumption than this is necessary to completely ac- 

 count for the firmamental blue and the polarization of the sky *. 



Suppose our atmosphere surrounded by an envelope impervious to 

 light, but with an aperture on the sunward side through which a pa- 

 rallel beam of solar light could enter and traverse the atmosphere. 

 Surrounded on all sides by air not directly illuminated, the track of 

 such a beam through the air 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 pur- 

 poses that which I have called a " blue cloud" t. 



But, as regards the polarization of the sky, we know that not only 

 is the direction of maximum polarization at right angles to the track 

 of the solar beams, but that at certain angular distances, probably 

 variable ones, from the sun " neutral points " (or points of no polari- 

 zation) exist, on both sides of which the planes of atmospheric polari- 

 zation are at right angles to each other. 



I have made various observations upon this subject which I reserve 

 for the present ; but, pending the more complete examination of the 

 question, the following facts and observations bearing upon it are 

 submitted to the Royal Society. 



The parallel beam employed in these experiments marked its way 

 through the laboratory-air exactly as sun-beams are seen to do in 

 the dusty air of London. I have reason to believe that a great por- 



76°, we should look to water or ice as the reflecting body, however inconceivable 

 the existence in a cloudless atmosphere on a hot summer's day of unevaporated 

 molecules (particles?) of water. But though we were once of this opinion, careful 

 observation has satisfied us that 90°, or thereabouts, is a correct angle, and that 

 therefore, whatever be the body on which the light has been reflected, if polarized 

 by a single reflection, the polarizing angle must be 45°, and the index of refraction, 

 which is the tangent of that angle, unity ; in other words, the reflection would 

 require to be made in air upon air !" (' Meteorology,' par. 233). 



* Any particles, if small enough, will produce both the colour and the polari- 

 zation of the sky. But is the existence of small water-particles on a hot summer's 

 day in the higher regions of our atmosphere inconceivable? It is to be remem- 

 bered that the oxygen and nitrogen of the air behave as a vacuum to radiant 

 heat, the exceedingly attenuated vapour of the higher atmosphere being therefore 

 in practical contact with the cold of space. 



•f The opinion of Sir John Herschel, connecting the polarization and the blue 

 colour of the sky, is verified by the foregoing results. " The more the subject [the 

 polarization of skylight] is considered," writes this eminent philosopher, " 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 colour 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 abso- 

 lute that the polarization is developed in its highest degree, and that where there 

 is the slightest perceptible tendency to cirrus it is materially impaired." T!as 

 applies, word for word, to the "incipient clouds." 



