On the Polarization of the Atmosphere. 4''t5 



polarization of the atmosphere, and a considerable time elapsed 

 before its leading elements were determined, and its more 

 important phaenomena observed and measured. It is to M. 

 Arago, to whom this branch of science owes such deep obli- 

 gations, that we are indebted for the discovery of the first and 

 leading fact on which the law of atmospheric polarization 

 depends. In examining the region of the sky opposite to the 

 sun, he discovered a neutral point, or a point in which there 

 is no polarization whatever. This neutral point he found to 

 be 25° or 30° above the point diametrically opposite to the 

 sun, or what we may call the antisolar point ; and we shall 

 distinguish this pole of no-polarization by the name of M. 

 Arago's neutral point, or the antisolar neutral point. It is 

 best seen after sunset. 



In the year 1840, M. Babinet discovered a second neutral 

 point, situated about the same distance above the sun as the 

 neutral point of M. Arago is situated above the antisolar point. 

 This point is most distinctly seen immediately after sunset, 

 but is generally much fainter than the other, owing to the 

 discoloration of the blue sky by the yellow light of the set- 

 ting sun. 



Our readers are no doubt aware, that when light is reflected 

 from the surfaces of transparent bodies, a certain portion of 

 it, and at a particular angle the whole of it, is polarized in the 

 plane of reflexion, or positively'^', while precisely the same 

 quantity of the transmitted light is polarized in a plane at 

 right angles to the plane of reflexion or refraction, or nega- 

 tively. Now, in the part of the sky between the neutral point 

 of M. Arago and that of M. Babinet, the light is polarized 

 positively, while in the parts of the sky between the first of 

 these neutral points and the antisolar point, or between the 

 second and the sun, it is polarized negatively. Hence it 

 became obvious that the two neutral points must be produced 

 by a compensation, in which light polarized negatively neu- 

 tralized light polarized positively, and that the negative light 

 was either produced by reflexion in a plane at right angles to 

 that passing through the sun, the neutral point, and the ob- 

 server, or by refraction in a plane passing through these three 

 points, or by both these causes combined. But in whatever 

 way the negative polarization was produced, it was manifest 

 that the same cause ought to produce a neutral point beneath 

 the sun. After many fruitless attempts to discover this neutral 

 point — owing chiefly to the predominance of the sun's light 



♦ These terms are used for the purpose of abbreviation. An account of 

 the laws of the polarization of light by reflexion and refraction, will be 

 found in my papers in the Phil. Trans., 1815, p. 129, and 1830, pp. 69, 133. 



