101 



the son of the celebrated William Herschel, and his labours 

 were remarkable from the fact that he carried his rese- 

 arches into the southern hemisphere, and, working at the 

 Cape of Good Hope, observed and studied there 2100 

 double stars. 



The observations of double stars is attended with a 

 mass of difficulties owing to the slow rate of their visible 

 movement. Whole decades are sometimes required to 

 distinguish an optical from a physical pair. If however 

 stars which seem in close proximity to one another 

 appear so only owing to a perspective effect, their rate 

 of visible movement will differ, and in the long run they 

 will obviously part company as separate spheres. Of 

 000 pair of double stars observed in 1843, 800 noticeably 

 changed their relative position in the sky. William Stroove 

 divides double stars into 8 groups, according to their 

 mutual distance, as follows: 



1-st group: with interval from 0'' to 1'' 

 2-nd 1" 2" 



3-rd n n 2 4 



4-th 4" 8" 



5-th 8'' 12" 



6-th 12" 16" 



7-th 16" .-24" 



8-th 24" 32" 



From his researches in the year 1837* he came to the 

 conclusion that, of the 653 double stars of various groups 

 which he had examined, 605 were genuine physical pairs and 

 only 48 optical, giving a general proportion of 13 to 1. 

 The 178 pair included in the first and second groups 

 proved without exception physical double stars; of the 

 263 pair belonging to the third and fourth groups 3 were 

 optical; out of 106 pair with a distance of from 8" to 

 16" 9 were optical; finally from 106 pair of the two last 

 groups 36 were optical. The parts of the northern 

 hemisphere most crowded with these stars were the 

 constellations of Andromeda, Taurus, Orion and the Great 



