COMETS. 549 



at least in very great proximity to the centre (per centrum 

 non aliter quam per nubem ulteriora cernatur : Seneca, Nat. 

 Oldest, vii. 18) ; so, on the contrary, the analysis of the 

 cometary light in Arago's experiments, during which I was 

 present, shows that the vaporous envelopes are capable 20 of 

 reflecting light notwithstanding their extremely slight den- 

 sity, and that these bodies have " an imperfect transparency, 21 

 since light does not pass through them unimpeded." In this 

 group of vaporous bodies, the solitary instances of great 

 luminous intensity, as in the Comet of 1343, or the star-like 

 shining of a nucleus, excite so much the more astonishment 

 when it is assumed that their light proceeds solely from a reflec- 

 tion of the solar ravs. Is there not, however, in addition to 

 this, a peculiar light-producing process going on in the comets? 

 The brush-like membered tails emanating from the comets, 

 and consisting of vapoury matter of millions of miles in 

 length, diffuse themselves in space; and form, perhaps, either 

 the resisting medium ** itself, which gradually contracts the 

 orbit of Encke's Comet, or they mix with the old cosmical 

 matter which has not aggregated into spheres, or condensed 

 into the rings, and which appears to us as the zodiacal light. 

 We see, as it were, before our eyes, material particles dis- 

 appear, and can scarcely conjecture where they again collect. 

 However probable may be the condensation, in the neighbour- 

 hood of the central body of our system, of a gaseous fluid 

 filling space, still in the case of the comets whose nuclei, 

 according to Valz, diminish in the perihelion, this fluid con- 



20 Newton considered that the most brilliant comets shone 

 only with reflected sun-light. " Splendent comets," says he, 

 " luce Solis a se reflexa." (Princ. Matliem. ed. Le Seur et 

 Jaquier, 1760, torn. iii. p. 577. 



21 Bessel, in Schum. Jalirlucli for 1837, p. 169. 



22 Cosmos, vol. i. p. 92, and vol. iii. p. 47. 



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