204 A CENTURY'S PROGRESS IN ASTRONOMY. 



some of these photographs the line K was seen 

 double, while on others it was seen under its 

 normal aspect. This doubling of the lines 

 indicated that the star which we see as single 

 is in reality composed of two bodies in revolu- 

 tion round their centre of gravity, so close 

 together that even the largest telescopes cannot 

 divide them. Pickering assigned a period of 

 104 days, but in 1901 Vogel diminished this 

 to 20 days. In the same year the star 

 ft Aurigae was similarly found to be double ; 

 and in 1890 Vogel, from photographs taken 

 at Potsdam, independently inaugurated the 

 discovery of spectroscopic binaries. In the 

 spectrum of Spica he discovered the spectral 

 lines to be, not doubled, but periodically dis- 

 placed, indicating the existence of a dark or 

 nearly dark companion, both stars revolving 

 round their centre of gravity. Spica was seen 

 to belong to the same class as Algol, only that 

 in the case of Algol the plane of the satellite's 

 orbit passes through the Earth and eclipses the 

 star, while in the case of Spica the orbit is 

 inclined, and the star is constant in light. 



The line of research commenced by Vogel and 

 Pickering was soon followed up by these investi- 

 gators, as well as by Belopolsky at Pulkowa, 

 Campbell at the Lick Observatory, Slipher at 



