POLARISATION OF LIGHT. 57 



light of the setting sun, reflected from the windows of the Palais 

 du Luxembourg, has afforded unexpected results to science by 

 the more thorough investigation of the phenomena of double re- 

 fraction, of ordinary (Huygens's) and of chromatic polarisation, 

 of interference, and of diffraction of light. Among these results, 

 may be reckoned the means of distinguishing between direct 

 and reflected light,* the power of penetrating, as it were, into 

 the constitution of the body of the sun and of its luminous 

 envelopes, 10 of measuring the pressure of atmospheric strata, 



'Arago's investigation of cometary light may here be 

 adduced as an instance of the important difference between 

 proper and reflected light. The formation of the comple- 

 mentary colours, red and green, showed by the application of 

 his discovery (in 1811) of chromatic polarisation, that the 

 light of Halley's Comet (1835) contained reflected solar light. 

 I was myself present at the earlier experiments for comparing, 

 by means of the equal and unequal intensity of the images in 

 the polariscope, the proper light of Capella with the splendid 

 Comet, as it suddenly emerged from the rays of the sun at 

 the beginning of July, 1819. (See Annuaire du Bureau des 

 Long, pour 1836, p. 232; Cosmos, vol. i. p. 90; and Bessel in 

 Schumacher's Jahrbuchfur 1837, 169.) 



10 Lettre de M. Arago d M. Alexandre de Humboldt, 1 840, 

 p. 37 : " A 1'aide d'un polariscope de mon invention, je 

 reconnus (avant 1820) que la lumiere de tous les corps ter- 

 restres incandescents, solides ou liquides, est de la lumiere 

 naturelle, tant qu'elle emane du corps sous des incidences per- 

 pendiculaires. La lumiere, au contraire, qui sort de la surface 

 incandescente sous un angle aigu, offre des marques manifestes 

 de polarisation. Je ne nvarrete pas a te rappeler ici, comment 

 je deduisis de ce fait la consequence curieuse que la lumiere 

 ne s'engendre pas seulement a la surface des corps ; qu'une 

 portion nait dans leur substance meme, cette substance fut- 

 elle du platine. J'ai seulement besoin de dire qu'en repetant 

 la meme serie d'epreuves, et avec les memes instruments sur la 

 lumiere que lance une substance gazeuse enflammee, on ne lui 

 trouve, sous quelque inclinaison que ce soit, aucun des caracteres 

 de la lumiere polar isee ; que la lumiere des gaz, prise a la 



