116 FRAGMENTS OF SCIENCE. 



beam being therefore also vertical. As the light con- 

 tinued 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 posi- 

 tions mentioned, utterly disappeared when looked at 

 vertically downwards, or vertically upwards. Keflec- 

 tion 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 extinc- 

 tion 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 

 downwards or upwards. This truly fine experiment, 

 which I contemplated making on my own account, was 

 first definitely suggested by a remark in a letter ad- 

 dressed to me by Professor Stokes. 



As regards the polarisation 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 polarising angle, the re- 

 flection which produces perfect polarisation 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. 1 I have, however, operated upon 



1 ' The cause of the polarisation is evidently a reflection of the 

 sun's light upon something. The question is on what ? Were the 

 angle of maximum polarisation 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 there- 

 abouts, is the correct angle, and that therefore whatever be the 

 body on which the light has been reflected, if polarised by a single 

 reflection, the polarising angle must be 45, and the index of refrac- 

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



