124 FRAGMENTS OF SCIENCE 



was slowly turned round its axis, the eye of tlie 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 re- 

 fraction the tangent of the polarizing angle, the reflection 

 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 ae'rial molecular reflection.* I have, 

 however, operated upon substances of widely different re- 



' **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, how- 

 ever 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 there- 

 about, 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 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 airl" (Sir John Herschel, * 'Meteorology," par. 233.) 



Any particles, if small enough, will produce both the color and the polar- 

 iBation 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 vapor of the higher atmosphere 

 being therefore in practical contact with the cold of space. 



