308, 



Double Stars 



399 



probability turn out to be genuine binaries (chapter xn., 

 264). 



In addition to a number of minor papers Struve published 

 three separate books on the subject in 1827, 1837, and 1852.* 

 A comparison of his own earlier and later observations, and 

 of both with Herschel's earlier ones, shewed about 100 cases 

 of change of relative positions of two members of a pair, 

 which indicated more or less clearly a motion of revolution, 

 and further results of a like character have been obtained 



Scal 



1876 



FIG. 101. The orbit of Ursae, shewing the relative positions of 

 the two components at various times between 1781 and 1897. 

 (The observations of 1781 and 1802 were only enough to 

 determine the direction of the line joining the two components, 

 not its length.) 



from a comparison of Struve's observations with those of 

 later observers. 



William Herschel's observations of binary systems 

 (chapter XIL, 264) only sufficed to shew that a motion of 

 revolution of some kind appeared to be taking place ; it 

 was an obvious conjecture that the two members of a pair 



* Catalogus novus stellarum duplicium^ Stellarum duplicium et 

 multiplicium mensurac micrometricae, and Stellarum fixarum imprimis 

 duplicium et multiplicium positiones mediae pro epocha 1830. 



