178 Sir David Brewster on the Polarization 



analogous to neutral points, that is, the coexistence in the 

 transmitted light of rays polarized by reflexion and rays polar- 

 ized by refraction; but I did not observe the effect where 

 the intensities of these rays were such as to neutralize each 

 other. 



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

 points in the atmosphere, and the partial polarization of the 

 light which it reflects, are produced by the opposite action of 

 lights polarized by reflexion and refraction ; and, regarding the 

 congeries of separate molecules which constitutes the atmosphere 

 as a rough or dispersing surface, I made a series of experiments 

 on such surfaces as were likely to act upon light in the same 

 manner as the aerial molecules. In these experiments, which 

 have been already communicated to the Society*, I found that 

 such surfaces not only polarized partially the incident light, but 

 produced neutral points, like those of the atmosphere, by the 

 opposite action of the rays which they polarized by reflexion and 

 refraction. 



Appendix. 



The partial polarization of the light reflected from the blue 

 sky was first discovered by M. Arago in 1811 f. In 1812, with- 

 out knowing what had been done by M. Arago, I discovered 

 " that the light reflected from the clouds — the blue light of the 

 sky and the light which forms the rainbow — had experienced a 

 partial polarization " J. In 1825 M. Delezenne§ of Lille, and 

 M. Quetelet|| of Brussels, made the same observation without 

 knowing what had been previously published. At a later date 

 the polarization of the atmosphere was studied by A. B. Fran- 

 cesco Zantedeschi, who with a simple polariscope made several 

 observations at Venice on the distribution of the polarization in 

 different parts of the atmosphere ^[. 



In 1854 M. Felix Bernard made several observations at Bor- 

 deaux on the intensity of the maximum polarization at different 

 hours of the day. Though made only on four days in the month 

 of September, he found "that in proportion as the sun ap- 



* See Phil. Mag. S. 4. vol. xxv. p. 344, May 1863. 



t (Euvres, vol. vii. p. 394. 



X Treatise on New Philosophical Instruments. Edinburgh, March 1813, 



§ Recueil des Travaux de la Societe de Lille, 1825, 



|| Correspondance Mathtmatiqueet Physique. Gand, 1825, vol. i. No. 5, 

 p. 275. 



H " Delle leggi della polarizzazione della luce solare nelle atmosphaera 

 serena," comunicato con lettera al David Brewster, LL.D., in the Raceolta 

 Physico-Chemica Italiana, vol, i, fascic, 10, 1846, p, 429, 



