";^ [ 444 ] 



LXVIII. On the Polarization of the Atmosphere. By Sir 

 David Brewster, K,H., D.C.L., F.Ii.S.y and V.F.R.S. 



Edin.^ 



WHEN the light of the sun or of any self-himinous body 

 has been transmitted through certain crystallized sub- 

 stances, or has been reflected from, or refracted by, bodies 

 not metallic, it suffers a physical change, to which the name 

 o^ plajie polarization has been given. This physical change 

 consists in decomposing common light into two equal portions 

 of polarized light, one of which is polarized in a plane at right 

 angles to that in which the other is polarized. In doubly 

 refracting crystals, the two pencils are polarized in opposite 

 or rectangular planes ; and when common light is reflected 

 from any body not metallic, whether it is solid, or fluid, or 

 gaseous, a portion of the incident light enters the body ; and 

 of the portions thus reflected and refracted, precisely the same 

 quantity is polarized, — the light polarized by refraction being 

 polarized in a plane at right angles to that which is polarized 

 by reflexion. 



If the earth had no atmosphere the sky would appear ab- 

 solutely black ; and when the sun sets we should be left in 

 utter darkness. The existence of twilight, however, the blue 

 colour of the sky, and the refraction of the rays which emanate 

 from the stars and planets, place it beyond a doubt that the 

 pure air in which we live and breathe is capable of acting 

 upon light like all other bodies, and consequently of producing 

 that physical change which constitutes polarization. The 

 polarization of the blue sk}', or of the atmosphere, was there- 

 fore observed and studied by different philosophers, both in 

 France and England ; and it was speedily ascertained, in con- 

 formity with the laws of polarization, that the polarization was 

 a minimum in the vicinity of the sun, where his light is reflected 

 at angles approaching to 90°, or where the incident and re- 

 flected rays form an angle approaching to 1 80° ; that it was 

 also a minimum in the region opposite the sun, where his light 

 is reflected at an angle approaching to 0°, or at a perpendi- 

 cular incidence; and that it was a maximum in those interme- 

 diate parts of the sky, which are distant about 90° from the 

 sun, and where his light is reflected at an angle of about 45°, 

 the polarizing angle for air. 



Such was the first view which was naturally taken of the 



* This paper is reprinted, with the permission of Dr. Berghaus and Mr. 

 A. K. Johnston, from the Seventh Part of their vakiablePh3sical Atlas now 

 in the course of publication. A map representing the four neutral points, 

 and the system of lines of equal polarization, will be found in that work. 



