166 A CENTURY'S PROGRESS IN ASTRONOMY. 



subject was taken up by Maunder at Greenwich 

 and by Vogel at Bothkamp ; but the delicacy 

 of the measurements prevented satisfactory re- 

 sults from being reached through visual observa- 

 tions, and accordingly the measurements were 

 very discordant. 



In 1887 H. C. Vogel, working at Potsdam 

 Astrophysical Observatory, applied photography 

 to the measurement of radial motion. Assisted 

 by Julius Scheiner (born 1858), he determined 

 the radial motions of fifty-one bright stars by 

 photographing the stellar spectra and measur- 

 ing the photographs. Vogel found 10 miles a 

 second to be the average velocity of stars in 

 the line of sight, the tendency of the eye being 

 to exaggerate the displacements. The swiftest 

 of the stars measured by Vogel proved to be 

 Aldebaran, with a velocity of recession of 30 

 miles a second. Since 1892 the subject has 

 been pursued by Vogel himself with the new 

 30 -inch refractor at Potsdam, by Campbell at 

 the Lick Observatory, B^lopolsky at Pulkowa, 

 and other observers. Towards the end of 1896 

 Campbell undertook, with the 36 - inch Lick 

 refractor, a series of measures on radial motion, 

 and many important discoveries were made. 

 These, however, must be reserved for the chapter 

 dealing with double stars. 



