ASTRO-PHYSICS 275 



stellar movement has led to other results quite 

 as remarkable as those already described. Our 

 sun is a single system, but many of his fellows 

 among the stars are accompanied by partners ; 

 the two existing in more or less close conjunc- 

 tion, and showing all the signs of a common 

 origin. Some of these double stars can be 

 examined by telescopic means, but the majority 

 of them lie too close together to be separated 

 thus. Often, too, one of the pair is not luminous, 

 and therefore would never be visible. In this 

 class are probably to be placed variable stars, 

 such as the star known as Algol or /3 Persei, the 

 light from which undergoes periodical fluctuations 

 in intensity. The light keeps constant for the 

 greater part of the cycle, and then diminishes for 

 a short time before again rising to its normal 

 value. This behaviour was long suspected to 

 be due to the partial concealment of the star by 

 a dark companion or satellite, and the surmise 

 was confirmed by the spectroscope, which shows 

 that the star is always receding from us before 

 the loss of light and approaching after it. This 

 result is exactly what we should expect on the 

 eclipse theory, the dark companion being so 

 nearly of the same size as the visible Algol that 

 the joint motion is similar to that of two partners 

 waltzing round each other rather than like the 

 revolution of a small satellite round a large 

 central body, which remains nearly stationary. 

 In other cases, such as that of (3 Lyrae, the 

 intensity of the light is never constant, but 

 undergoes continual variation, accompanied by 

 complicated changes in the spectrum. An 

 explanation has been given by imagining two 



