210 SIR DAVID BREWSTER ON THE POLARISATION OF LIGHT, ETC. 



The most important application of the preceding experiments is to the polari- 

 sation of the atmosphere. Aeago, Babinet, and others, in their theory of the 

 neutral points, and of the partial polarisation of the atmosphere, took no account 

 of the rays which are polarised by refraction, whenever light is polarised by 

 reflexion, and they referred these abnormal phenomena to a horizontal secondary 

 reflexion from the atmosphere itself, modifying, and, in three neutral points, 

 extinguishing the light polarised by reflexion. Whether or not such a secondary 

 reflexion exists, or is adequate, if it does, to account for these phenomena, are 

 questions which will be considered in another paper on the polarisation of the 

 atmosphere. But however ingenious may be the hypothesis, it has no support 

 either from experiment or observation. The reduction of complete to partial 

 polarisation, by the opposite action of light polarised by reflexion and light 

 polarised by refraction, and the production of neutral points where these two 

 lights are equal, whenever light is incident on surfaces, which, like the atmo- 

 sphere, disperse and polarise it, is next to an ocular proof of the true laws of 

 atmospherical polarisation. 



It is not one of the least wonders of terrestrial physics, that the blue atmo- 

 sphere which overhangs us, exhibits in the light which it polarises phenomena 

 somewhat analogous to those of crystals with two axes of double refraction. 



