271 



VI. 



MULTIPLE OH DOUBLE STARS. THEIR NUMBERS AND 



RECIPROCAL DISTANCES. PERIOD OF REVOLUTION OF 



TWO SUNS ROUND A COMMON CENTRE OF GRAVITY. 



WHEN, in contemplating the systems of the fixed stars, we 

 descend from hypothetical, higher, and more general con- 

 siderations to those of a special and restricted nature, we 

 enter a domain more clearly determined, and better calculated 

 for direct observation. Among the multiple stars, to which 

 belong the Unary or double stars, several self-luminous cosmical 

 bodies (suns) are connected by mutual attraction, which 

 necessarily gives rise to motions in closed curved lines. 

 Before actual observation had established the fact of the revolu- 

 tion of the double stars, such movements in closed curves were 

 only known to exist in our own planetary solar system. On 

 this apparent analogy inferences were hastily drawn, which 

 for a long time gave rise to many errors. As the term 

 " double stars " was indiscriminately applied to every pair of 

 stars, the close proximity of which precluded their separation 

 by the naked eye (as, in the case of Castor, a. Lyrae, /3 Orionis, 

 and a Centauri) this designation naturally comprised two 

 classes of multiple stars : firstly, those which, from their in- 

 cidental position in reference to the observer, appear in close 

 proximity, though in reality widely distant and belonging to 

 totally different strata; and, secondly, those which, from their 

 actual proximity, are mutually dependent upon each other 



1 Compare Cosmos, vol. i. pp. 136-139. (Struve, ilber 

 Doppelsterne nach Dorpater Micrometer -Mesmngen von 1824 

 Us 1837, s. 11.) 



