oi3covi:rie3 in the celestial spaces. 323 



^atiou of the theory of gravitation, have immortaUzed the 

 name of Kepler.* Cosmical considerations, even whon based 

 merely on feeble analogies and not on actual observations, 

 riveted the attention more powerfully then, as they still fre- 

 quently do, than the most important results of calculating 

 astronomy. 



After having described the important discoveries vv'hich in 

 so small a cycle of years extended the knowledge of the re- 

 gions of space, it still remains for me to revert to the advances 

 in physical astronomy which characterize the latter half of 

 this great century. The improvement in the construction of 

 telescopes led to the discovery of Saturn's satellites. Huy- 

 gens, on the 25th of March, 1655, forty-five years after the 

 discovery of Jupiter's satellites, discovered the sixth of these 

 bodies through an object-glass which he had himself polished. 

 Owing to a prejudice, which he shared with other astrono- 

 mers of his time, that the number of the secondary planetary 

 bodies could not exceed that of the primary planets,! he did 

 not seek to discover other satellites of Saturn. Dominicus 

 Cassini discovered four of these bodies, the Sidera Lodivicea, 

 viz., the seventh and outermost in 1671, which exhibits 

 great alternation of light, the fifth in 1672, and the fourth 

 and third in 1684, through Campani's object-glass, having a 

 focal length of 100-136 feet; the two innermost, the first and 

 second, were discovered more than a century later (1788 and 

 1789) by William Herschel, through his colossal telescope. 

 The last-named of these satellites presents the remarkable 

 phenomenon of accomplishing its revolution round the prima- 

 ry planet in less than one day. 



Soon after Huygens's discovery of a satellite of Saturn, 

 Childrey first observed the zodiacal light, between the years 

 1658. and 1661, although its relations in space were not de- 

 termined until 1683 by Dominicus Cassini. The latter aid 

 not regard it as a portion of the sun's atmosphere, but believ- 

 ed, with Schubert, Laplace, and Poisson, that it was a de- 

 tached revolving nebulous ring,| Next to the recognition of 

 the existence of secondary planets, and of the free and con 

 centrically divided rings of Saturn, the conjecture of the prob- 

 able existence of the nebulous zodiacal light belongs incon- 

 testably to the grandest enlargement cf our views regarding 

 the planetary system, which had previously appeared so sim 



* Delarabre, Hist, de V A?tro%omie Moderne. ^- i., p. 3G0. 

 t Arago, in the Annuaire for 1842, p. 560-5^>4; also Cosmos, vol. i. 

 p. 97. ± Compare Cosmr-,, vol. i., p. 137-144. 



