.92 COSMOS. 



dius (1,244,000 geographical miles, or six times the Moon's 

 distance). That the comet was not se^^n before March, 1776, 

 and not later than October, 1781, according to Lexell's pre- 

 vious conjecture, is analytically demonstrated by Laplace, in 

 the fourth volume of the Mecanique Celeste, from the per- 

 turbations occasioned by the Jovial system on the occasion 

 of the approximations in the years 1767 and 1779. Lever- 

 rier finds that, according to one hypothesis respecting the 

 cometary orbits, this comet passed through orbits of the sat- 

 ellites in 1779 ; according to another, that it remained at a 

 considerable distance without the fourth satellite.^ 



The molecular conditions of the head or nucleus, so seldom 

 possessing a definite outline, as well as the tail of the com- 

 ets, is rendered so much the more mysterious from the fact 

 that it causes no refraction, and, as was proved by Arago's 

 important discovery {Cosmos, vol. i., p. 105, and note), that 

 the cometary light contains a portion oi polarized light, and 

 consequently reflected sun-light. Although the smallest stars 

 are seen in undiminished brilliancy through the vaporous em- 

 anations of the tail, and even through the center of the nu- 

 cleus itself, or at least in very great proximity to the center, 

 (per centrum non alitor quam per nubem ulteriora cernatur : 

 Seneca, Nat. Qucest., vii., 18) ; so, on the contrary, the an- 

 alysis of the cometary light in Arago's experiments, during 

 which I was present, shows that the vaporous envelopes are 

 capable! of reflecting light, notwithstanding their extremely 

 slight density, and that these bodies have " an imperfect 

 transparency, t since light does not pass through them unim- 

 peded." In this group of vaporous bodies, the solitary in- 

 stances of great luminous intensity, as in the Comet of 1843, 

 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 reflection of the solar rays. Is there not, how- 

 ever, 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 vapory matter of millions of miles in length, 

 diffuse themselves in space, and form, perhaps, either the re- 

 sisting mediums itself, which gradually contracts the orbit 



* Leverrier, in the Comptes Rendvs, torn, xix., 1844, p. 982-993. 



t Newton considered that the most brilliant comets shone only with 

 reflected sun-light. " Splendent cometa;," says he, " luce Solis a se 

 reflexa." (Princ. Mathem., ed. Le Seur et Jaquier, 1760, torn, iii,, 

 p. 5r7.) X Bessel, in Schum. Jahrbuch for 1837, p. 169. 



$ Cosmos, vol. i., p. 106, and vol. iii., p. 39. 



