548 COSMOS. 



been more accurately investigated by Leverrier. Messier 

 discovered this remarkable comet as a feeble nebulous spot in 

 Sagittarius upon the 14th of June, 1770; but eight days 

 after, its nucleus shone as brightly as a star of the 2nd 

 magnitude. Before the perihelion passage, no tail was 

 visible ; afterwards it developed itself by slight emanations 

 scarcely one degree in length. Lexell found for his comet an 

 elliptic orbit, and the period of rotation of 5-585 years, which 

 Burckhardt confirmed in his excellent prize essay. According 

 to Clausen it had approached the Earth upon the 1st of July, 

 1770, to a distance of 363 times the Earth's radius (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 previous conjecture, is 

 analytically demonstrated by Laplace, in the fourth volume of 

 the Mecanique Celeste, from the perturbations occasioned by 

 the Jovial system on the occasion of the approximations in 

 the years 1767 and 1779. Leverrier finds that according to 

 one hypothesis respecting the cometary orbits, this comet 

 passed through orbits of the satellites in 1779 ; according to 

 another, that it remained at a considerable distance without 

 the fourth satellite. 19 



The molecular conditions of the head or nucleus, so seldom 

 possessing a definite outline, as well as the tail of the comets, 

 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. 90, 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 emanations of 

 the tail, and even through the centre of the nucleus itself, or 



19 Leverrier, in the Comptes Rendus, torn. xix. 1844, 

 pp. 982-993. 



