NEW CIItiMICAL REACTIONS. 87 



emergent beam being therefore also vertical. As the light 

 continued to act, a superb blue cloud, visible to both my 

 assistant and myself, was slowly formed. But this cloud, 

 so deep and rich when looked at from the positions men- 

 tioned, utterly disappeared when looked at vertically down- 

 ward or vertically upward. Reflection from the cloud 

 was not possible in these- directions. When the large 

 Nicol was slowly turned round its axis, the eye of the 

 observer being on the level of the beam, and the line of 

 vision perpendicular to it, entire extinction of the light 

 emitted horizontally occurred when the longer diagonal of 

 the large Nicol was vertical. But now a vivid blue cloud 

 was seen when looked at downward or upward. This 

 truly fine experiment, which I contemplated making on my 

 own account, was first definitely suggested by a remark in 

 a letter addressed to me by Professor Stokes. 



As regards the polarization of skylight, the greatest 

 stumbling-block has hitherto been, that, in accordance 

 with the law of Brewster, which makes the index of 

 refraction the tangent of the polarizing angle, the reflec- 

 tion 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 aerial molecular reflection.* I 

 have, however, operated upon substances of widely differ- 



* " The cause of the polarization is evidently a reflection of the 

 sun's light upon something. 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 mole- 

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

 careful observation has satisfied us that 90, or thereabouts, is the 

 correct angle, and that therefore whatever be the body on which the 

 light has been reflected, if polarized by a single reflection, the polar- 

 izing angle must be 45, and the index of refraction, which is the 

 tangent of that angle, unity; in other words, the reflection would re- 

 quire to be made in air upon air I" (Sir John Herschel, " Meteorology," 

 par. 233.) 



Any particles, if small enough, 'will produce both the color 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 incon- 

 ceivable? It is to be remembered that the oxygen and nitrogen of 

 the air behave as a vacuum to radiant heat, the exceedingly attenuated 

 vapor of the higher atmosphere being therefore in practical contact 

 with the cold of space. 



