1869.] 



and on the Polarization of Light, 



227 



Now, as regards the polarization of skylight, the greatest stumblingblock 

 has hitherto been that, in accordance with the law of Brewster, which 

 makes the index of refraction the tangent of the polarizing angle, the re- 

 flection which produces perfect polarization would require to be made in 

 air upon air ; and indeed this led many of our most eminent men, 

 Brewster himself among the number, to entertain the idea of molecular 

 reflection. I have, however, operated upon substances of widely different 

 refractive indices, and therefore of very different polarizing angles as ordi- 

 narily 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 condition, and 

 it rests with the undulatory theory to explain why. Whenever the preci- 

 pitated particles are sufficiently fine, no matter what the substance form- 

 ing 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°. This I consider to be a point of capital 

 importance with reference to the present question*. 



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

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

 able 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 account for the firmamental 

 blue and the polarization of the sky t. 



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 on all 

 rides by air not directly illuminated, the track of such a beam through the 

 dr would resemble that of the parallel beam of the electric lamp through 

 in incipient cloud. The sunbeam would be blue, and it would discharge 

 .aterally light in precisely the same condition as that discharged by the in- 



* The difficulty referred to above is thus expressed by Sir John Herschel : — " The 

 cause of the polarization is evidently a reflection of the sun's light upon some- 

 thing. The question is on what ? Were the angle of maximum polarization 76°, we 

 should look to water or ice as the reflecting body, however inconceivable the existence 

 in a cloudless atmosphere, and 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). ' 



f Any particles, if small enough, will produce both the colour and the polarization 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 remembered 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. 



