CONCLUSION. 599 



the degree of its transparency, and the resisting medium 

 which fills it, I have passed on to natural and telescopic 

 vision, the limits of visibility, the velocity of light, according 

 to the difference of its sources, the imperfect measurements 

 of luminous intensity,' and the new optical means of dis- 

 tinguishing direct from reflected light. Then follows the 

 heaven of fixed stars; the numerical statement of its self- 

 luminous suns so far as their position is determined; their 

 probable distribution ; the changeable stars which reappear at 

 well-defined periods \ the proper motion of the fixed stars ; 

 the assumption of the existence of dark cosmical bodies, and 

 their influence upon the motion of the binary stars ; the 

 nebulous spots, in so far as these are not remote and very 

 dense swarms of stars. 



The transition from the sidereal part of uranology from the 

 heaven of the fixed stars to our solar system, is merely a tran- 

 sition from the universal to the particular. In the class of 

 binary stars, self-luminous cosmical bodies move about a 

 common centre of gravity. In our solar system, which is 

 constituted of very heterogeneous elements, dark cosmical 

 bodies revolve round a self-lumiuous one, or much rather 

 again round a common centre of gravity, which at different 

 times is situated within and without the central body. The 

 individual members of the solar system are of dissimilar 

 nature more dissimilar than for many centuries astrono- 

 mers were justified in supposing. They are principal and 

 secondary planets; among the principal planets a group whose 

 orbits intersect each other ; an innumerable host of comets ; 

 the ring of the zodiacal light ; and, with much probability, 

 the periodic meteor-asteroids. 



It still remains to state here fully, as actual relations, the 

 three great laws of planetary motion, discovered by Kepler. 

 First law : each orbit of a planetary body is an ellipse, in 

 one of whose foci the Sun is situated. Second law: each 



