238 



SIR DAVID BREWSTER 



In addition to the vertical polarisation produced by the direct illumination 

 of the aerial particles, there must be an opposite polarisation by which the 

 neutral points are produced. M. Arago, M. Babinet, and, we believe, every 

 other writer on the subject, have sought for this counter-polarisation " in the 

 secondary illumination which the same aerial particles receive from the reflexion 

 of the rest of the atmosphere, which sends to them light polarised horizontally,"* 

 or oppositely to the light polarised vertically. That is, all the phenomena of 

 atmospherical polarisation are produced by the opposite action of two lights 

 polarised by reflexion, the one vertical, arising from the direct illumination of 

 the aerial particles, and the other horizontal, produced by a secondary illumina- 

 tion of the same particles b}' the rest of the atmosphere. 



This theory of atmospherical polarisation, omitting all consideration of the 

 light polarised by refraction, never appeared to me satisfactory. There is no 

 evidence whatever that such a secondary reflexion exists, even in a perfectly 

 cloudless sky, and still less evidence that, if it did exist, it would be capable of 

 neutralising the light polarised by reflexion at considerable distances from the 

 antisolar point. It must be very feeble when the neutral point is about to dis- 

 appear at the close of twilight ; and as the polarisation by direct reflexion must 

 be visible when the secondary reflection ceases to be visible, this cessation ought 

 to be marked by a return of the neutral point to the antisolar point, the place 

 which it M'ould occupy were there no secondary reflexion. 



Were the neutral points produced by a secondary reflexion, their distances 

 from the antisolar point and from the sun ought to be affected when the sk}^ is 

 more or less covered with clouds ; but though I have observed the neutral point 

 of Arago in a clear part of the sk}', I never observed that its distance from the 

 antisolar point was changed when the rest of the atmosphere was obscured by 

 clouds. ^ 



On these grounds I was led to the opinion that the neutral points must be 

 produced by the opposite action of two polarised lights which had nearly the 

 same relative intensit}^ and this opinion was strengthened by observations which 

 I had made on the polarisation of light by refraction and transmission through 

 piles of glass plates. In these experiments, published in the " Philosophical 

 Transactions" for 1814, I observed phenomena analogous to neutral points, that 

 is, the co-existence in the transmitted light of rays polarised by reflexion, and 

 rays polarised by refraction, but I did not observe the effect where the intensities 

 of these rays were such as to neutralise each other. 



Guided by these views, I never doubted that the three neutral points in the 

 atmosphere, and the partial polarisation of the light which it reflects, are produced 

 by the opposite action of lights polarised by reflexion and refraction ; and con- 



* Babinet, Comptes Rendus, &c. 1846. Tom. xxiii. p. 233. 



