most fundamental fact of stellar cosmo- 

 gony, the existence of physically-con- 

 nected stellar groups. In the case of the 

 conspicuous pairs of adjacent stairs (the 

 so-called double stars) he anticipated that 

 orbital revolution round each other, 

 owing to their mutual gravitation, would 

 in time be detected, a prediction after- 

 wards brilliantly realised on a grand scale 

 by Sir William Herschel. He even 

 pointed out that knowledge of the period 

 of their orbital revolution, combined 

 with their distance from the solar sys- 

 tem, would provide means of determining 

 the mass of such a stellar pair in com- 

 parison with the mass of the Sun 1 , a 

 problem which is being worked out 

 into exact knowledge by aid of refined 

 determinations of parallax in our own 

 time. 



" These considerations occur in the 

 course of discussion of a plan for esti- 

 mating the distances of the stars by com- 



1 Phil. Tram. 1784, p. 36, et $eq. 



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