2^6 PHYSICAL SCIENCE 



ellipsoidal luminous bodies, which revolve round 

 each other very near together, and send to the 

 earth more light when they lie side by side than 

 when one lies behind the other and to a certain 

 extent obscures it. 



It is evident that the double nature of such 

 systems can be demonstrated by variations in 

 luminosity only in the few cases where the 

 motion is in such a plane that one of the 

 partners is periodically interposed between the 

 other and the earth. Only a limited number 

 of Algol variables are known. When this eclipse 

 does not happen, the dark companion could never 

 be detected without the aid of the spectroscope. 

 By continuous records of the spectra of many 

 stars, however, periodic changes in the lines 

 have been observed, and the times of the orbital 

 movements determined. Binary systems have 

 been discovered with periods varying from a 

 few hours to many years. In some cases the 

 spectral changes merely consist in periodic shift- 

 ings of the lines. Here we probably have a 

 luminous body and a dark companion revolving 

 round their common centre of gravity. In other 

 cases, a periodic doubling of the lines indicates 

 two bodies, both luminous, but too near together 

 and too far from us to be separated by the 

 telescope. The number of both classes seems 

 to be considerable, and our visible universe must 

 be studded pretty closely with dark stars, the 

 existence of which is only to be detected 

 when they are associated with some luminous 

 companion. 



Triple and multiple stars are also known. 

 For instance, the Pole Star is a spectroscopic 



