xii TOWARDS INFINITY 319 



panion of Procyon has a mass more than half as great 

 as that of our sun, and is, therefore, capable of exerting 

 appreciable gravitational influence upon a body near it. 

 But though it is such a ponderous globe, its light is much 

 less than that which the sun would give if placed in the 

 same position among the stellar host. It is therefore 

 not surprising that the gravitational influence was 

 detected before the faint glimmer of the rays from the 

 star was seen and understood. 



Three instances have now been mentioned in which 

 invisible celestial bodies were found by indirect evidence 

 before they were looked for with telescopic aid we 

 refer to the planet Neptune, and the companions of 

 Sirius and Procyon. Predictions fulfilled in this way 

 should encourage confidence in conclusions based upon 

 similar premises. That is to say, if observations show 

 that a star is not moving through space in a straight line, 

 they afford presumptive evidence of the existence of one 

 or more bodies near it. The problem thus resolves itself 

 into one of studying stellar motions. 



A comparison of the exact positions of stars year by 

 year shows that every one has a motion of its own across 

 the blue background of infinity. The amount of move- 

 ment as seen from the earth is very minute, and can only 

 be detected by accurate determinations of position, but 

 it is none the less real, and has to be taken into account 

 in precise astronomy. Every movement has a cause, and 

 when deviations from a direct course are found, it is 

 certain that the star showing them is being disturbed by 

 a massive body which may or may not be visible. 



But what of stars which are moving straight towards 

 the earth or away from it ? Such movements cannot 

 produce any change of position upon the background 

 upon which they are projected. This is true enough, 



