M. Arago on the Light of Comets. 31 



The method which I have now described so much at length, 

 is exposed, I believe, to only one kind of objection ; it may be 

 imao-ined tliat the matter of which the comet is composed has no 

 inherent light of its own, but that it acquires it when under the 

 action of the solar rays. 



This hypothesis, in reality, would be nothing more than the 

 reproduction of that system which Euler has developed in his 

 *' Letters to a German Princess ;" and, according to which, the 

 light which enables us to perceive certain bodies, such as paper, 

 porcelain, &c. is not composed of rays which are truly reflected, 

 but rather of a particular kind of light, which these bodies en- 

 gender by being subjected to vibrations under the action of the 

 solar rays. This, it will be perceived, is a difficulty of pure 

 theory, and which might be as much applied to the light of the 

 moon, of the planets and their satellites, as to that of comets. 

 But the sole object which I proposed to myself in this paper, 

 was to discover a method which would enable us to decide 

 whether these bodies were to be ranged, as it regarded their lu- 

 minous properties, in the same category as our own globe, or as 

 Mars and Jupiter, &c. That other question, whether the light 

 which enables us to perceive coloured bodies is reflected, as 

 Newton supposed, at the surface of very fine material plates, or 

 proceeds from vibrations communicated to the air by the consti- 

 tuent particles of bodies ; this question, I say, is of a very dif- 

 ferent kind and extent, and this is not now the place to dis- 

 cuss it. 



3. On the Comet which will pass its Perihelion in November 1835. 



In the Annuaire of 1832 I published a long notice, in which 

 there is an elementary explanation of all that astronomy at the 

 present day possesses of precise and mathematical information 

 on the motion and nature of comets. To that treatise I beg to 

 refer the reader. In the present note I shall content myself 

 with mentioning some modifications made on the former results, 

 pointing out the principal circumstances which may render this 

 sixth appearance of the same comet useful to science, and indi- 

 cating the constellations in which we are to seek it. 



After having determined, by the assistance of very laborious 



