202 A CENTURY'S PROGRESS IN ASTRONOMY. 



chief assistant at the Lick Observatory, which 

 position he resigned in 1892. Some years later 

 he became astronomer in the Yerkes Observa- 

 tory. Altogether he has discovered 1308 double 

 stars, with telescopes ranging from a 6 - inch 

 refractor to the gigantic 40 -inch of the Yerkes 

 Observatory. 



The computation of double-star orbits has been 

 undertaken by various astronomers, among them 

 Madler, Klinkerfues, Duner, Flammarion, Seeliger, 

 See, Gore, Burnham, Robert Grant Aitken (born 

 1864) of the Lick Observatory, and Giovanni 

 Celoria (born 1842), who was, from 1866 to 

 1900, assistant in the Brera Observatory of 

 Milan, and since 1900 director of that institu- 

 tion. On June 9, 1890, Gore presented to the 

 Royal Irish Academy a catalogue of computed 

 binaries containing reference to fifty-nine stars. 



In 1844 Bessel discovered a remarkable irreg- 

 ularity in the proper motion of Sirius. He 

 ascribed this to the gravitational influence of 

 some obscure body, probably a large satellite. 

 In 1857 Peters calculated an orbit for the 

 supposed satellite with a period of 50 years. 

 In 1861 an orbit was computed by Truman 

 Henry Safford (1836-1901), which indicated the 

 position of the satellite. Close to this position 

 it was accidentally discovered by Alvan Clark 



