116 FKAGMENTS OF SCIENCE. 



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 JSTicol 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 down- 

 wards or upwards. 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 polarisation of sky light, 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 re- 

 quire 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 



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

 the sun's light upon something. The question is on what I 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 unevapo- 

 rated molecules (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 polarised by a 

 single reflection, the polarising angle must be 45, and the index 

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



