324 DISCOVERY OH. 



period, we are justified in concluding that there is an 

 invisible body in the neighbourhood swaying the bright 

 one round an orbit by the controlling influence of 

 gravitation. 



A new field of study has thus been opened, and it 

 promises to add very greatly to our knowledge of the 

 universe. Measurements of the movements of stars 

 towards or away from the earth have been made for a 

 third of a century, but it is only in recent years that the 

 method employed has been brought to sufficient per- 

 fection to enable the observations to be made to a 

 high degree of accuracy. Now, however, that the rate 

 of approach or recession can be determined accurately, 

 the exact analysis of the motions has assumed a com- 

 pletely new aspect. Hitherto it has been of little 

 importance to know whether a star was increasing or 

 decreasing its distance from the earth, but now, by 

 examining the details of the movements, we find in 

 many cases secondary impulses backward or forward, 

 and these are unmistakable signs of bodies to be con- 

 sidered in addition to those of which we have ocular 

 proof. 



The star Spica is an example of a bright body which 

 has a dark partner like that belonging to Algol, but does 

 not undergo a periodic loss of light by eclipse. Spectro- 

 scope observations show that the bright star alternately 

 swings towards the earth and away from it in a period of 

 four days, and they can be completely explained by 

 assuming that Spica has a companion that cannot be 

 seen, but is massive enough to make the visible star 

 move in an orbit. This attendant may be a dark globe 

 or a faint one so close to the star Spica that the most 

 perfect telescope available is powerless to show the two 

 bodies separately. But, whether lucid or obscure, the 



