COMETS. 97 



influence on the zenith distances of the heavenly bodies ? 

 In the passage of a comet over a star, there has often 

 been noticed a greater or less diminution of the light of 

 the star, but this has been justly ascribed to the bright- 

 ness of the ground, on which, during the coincidence, the 

 star is seen. 



We are indebted to Arago's polarisation experiments, for 

 the most important and decisive observations on the nature 

 of the light of comets. His polariscope instructs us con- 

 cerning the physical constitution of the sun, as well 

 as that of the comets; it informs us whether a lumi- 

 nous ray, which reaches us from a distance of many 

 millions of miles, is a direct, or a reflected or refracted 

 ray; and, if direct, whether the source of light is a 

 solid, a liquid, or a gaseous body. The light of Capella, 

 and that of the great comet of 1819, were examined at the 

 Paris Observatory with the same apparatus. The comet 

 showed polarised, and therefore reflected light ; whilst, as was 

 to be expected, the fixed star was proved to be a self-lumi- 

 nous sun ( 51 ). The existence of polarised cometary light 

 announced itself not only by the inequality of the images, but 

 was shown with still greater certainty, at the reappearance 

 of Halley's comet in 1835, by the more striking contrast of 

 complementary colours, in accordance with the laws of chro- 

 matic polarisation discovered by Arago in 1811. These 

 fine experiments leave it however still undecided, whether, 

 besides this reflected solar light, comets may not have a proper 

 light of their own. Even in planets, in Venus for example, 

 an evolution of independent light appears very probable. 



The variable intensity of the light of comets is not 

 always to be explained by their place in their orbit, and 



