712 COSMOS. 



not on actual observations, ri vetted the attention more power- 

 fully then, as they still frequently do, than the most important 

 lesults of calculating astronomy. 



After having described the important discoveries which in 

 so small a cycle of years extended the knowledge of the 

 regions of space, it still remains for me to revert to the 

 advances in physical astronomy, which characterise the latter 

 half of this great century. The improvement in the construc- 

 tion of telescopes led to the discovery of Saturn's satellites. 

 Huygens, 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 

 astronomers 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. Domi- 

 nicus 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 primary planet in less than one day. 



Soon after Huygens' 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 deter- 

 mined until 1683 by Dominicus Cassini. The latter did not 

 regard it as a portion of the sun's atmosphere, but believed, 

 with Schubert, Laplace, and Poisson, that it was a detached 

 revolving nebulous ring.f Next to the recognition of the 

 existence of secondary planets, and of the free and concen- 

 trically divided rings of Saturn, the conjecture of the probable 

 existence of the nebulous zodiacal light belongs incontestibly 

 to the grandest enlargement of our views regarding the 

 planetary system, which had previously appeared so simple. 

 In our own time the intersecting orbits of the small planets 



* Arago, in the Annuaire for 1842, pp. 560--564; also Cosmos, p. 82, 

 j* Compare Cosmos, pp. .126 134. 



