THE LIGHT OF THE STABS. 183 



83 Ursae Majoris, Piscis Australia, fi Leonis, 

 a Ophiuchi, 77 Crateris, and others. The secular 

 variations of some of these stars have been 

 detected by Gore himself during the past thirty 

 years, while in other cases he has detected them 

 by comparison of the most important star-cata- 

 logues, from Hipparchus and Al-Sufi down to 

 our own time. In some cases the star in ques- 

 tion seems to be slowly gaining in brilliance, 

 in others slowly diminishing. 



Thanks to the application of the spectroscope, 

 much is now known of the cause of the light 

 changes in variable stars. Goodricke's theory of 

 the variations of Algol was theoretically con- 

 firmed by the researches of E. C. Pickering in 

 1880. In 1889 Vogel proved beyond a doubt 

 that the variation in the light of Algol is due to 

 the partial eclipse of its light by a dark satellite. 

 It was obvious to Vogel that, as both Algol 

 and its companion are in revolution round their 

 common centre of gravity, the motion of Algol 

 in the line of sight might be detected by the 

 spectroscopic method of observation. Vogel 

 found that before each eclipse Algol was retreat- 

 ing from our system, while on recovering it gave 

 signs of rapid approach, proving conclusively that 

 both the star and its dark satellite were in 

 revolution round their centre of gravity, Algol 



