172 M. Arago on the Light of Comets. 



It has just been stated, that the difference of the two images 

 in the comet of 1819, was very slight ; but although M. Arago's 

 observations were confirmed by those of MM. Humboldt, Bou- 

 Vc.''d and Mathieu, it was still desirable that the result, fraught 

 with such important consequences to astronomy, should not de- 

 pend solely on a fugitive inequality of splendour. Errors of 

 this nature, as every one is aware, are found in the works of th^ 

 most celebrated natural philosophers. 



M. Arago therefore modified his first apparatus in such a 

 manner as to change the inequality of the images into a dissimi- 

 larity of colour. Thus, instead of a strong or sveak image, he 

 produced, in certain positions, a red and a green one ; and in 

 other positions, a yellow or a violet image, and all the other 

 prismatic colours in succession. We shall not now speak of ex- 

 periments, bv means of which it has been ascertained that very 

 slight differences in intensity are less easily distinguished than 

 similar differences in colour; but every one will admit the jus- 

 tice of the assertion, that difference in colour is a phenomenon 

 quite unequivocal, and leaving no doubt on the mind, while a 

 slight inequality of splendour is very far from deserving that 

 character. 



On the 23d October, M. Arago, having applied his new ap- 

 paratus to the observation of Halley's comet, immediately saw 

 two images, presenting the complementary colours, one of them 

 red, the other green. By turning the instrument half round, 

 the red image become green, and vice versa. The light of the 

 star, therefore, at least the whole of it, is not composed of rays 

 possessing the property of direct light, but consists of that which 

 is polarised or reflected specularly, that is, to speak definitively, 

 of light derived Ji'om the sun. 



MM. Bouvard, Mathieu, and Eugene Bouvard, an astrono- 

 mical pupil in the observatory, have repeated the experiment just 

 described, with precisely the same result. " I should be over 

 scrupulous," adds M. Arago in the close of his verbal commu- 

 nication, *' not to be gratified with the testimonies I have cited, 

 and I trust that they will contribute to cause the result of my 

 observations to be adopted ; viz. that comets, as well as planets, 

 derive their light from the sun."*^ 



