192 COSMOS. 



dius (1.244,000 geographical miles, or six times the Moon's 

 distance). That the comet was not seen 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 of 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 aliter 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,! 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 medium^ itself, which gradually contracts the orbit 



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



t Newton considered that the most brilliant comets shone ouly with 

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

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

 p. 577.) J Bessel, in Schum. Jahrbuch for 1837, p. 169. 



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



