PORTION OF THE COSMOS. MULTIPLE STARS. 199 



VI. 



MULTIPLE, OR DOUBLE STARS. THEIR NUMBER AND DI3 



TANCES APART. PERIOD OP REVOLUTION OF TWO 



SUNS ROUND A COMMON CENTRE OF GRAVITY. 



IF, in considerations on the subject of the fixed stars, we descend 

 from conjectural, higher and more general relations, to such as 

 are more special, we find ourselves on ground firmer and better 

 adapted for direct observation. In multiple stars, to which 

 class Unary or double stars belong, several self-luminous 

 cosmical bodies (Suns) are connected with each other by 

 mutual attraction, and this attraction necessarily calls forth 

 motion in re-entering curved lines. Previous to the recog- 

 nition, by actual observation, of the revolutions of double 

 stars, ( 329 ) our knowledge of the existence of motion in re- 

 entering curved lines was limited entirely to our own planetary 

 solar system. On this apparent analogy hasty inferences 

 were based, which led aside from the true path. As the 

 name of double-star was applied in all cases where proxi- 

 mity prevented separation by the unassisted eye (as in 

 Castor, a Lyrae, /3 Orionis, and a Centauri), the term very 

 naturally came to include two classes of multiple stars; 

 those whose proximity might be occasioned solely by their 

 accidental position in relation to the observer, while they 



