372 THE WORLD MACHINE 



was long invisible to the eye. In each case its presence had 

 been divined, its position predicted, estimations of its mass 

 been made, before actual visual observations had confirmed 

 the fact. 



The earliest of these predictions was that of Bessel re- 

 garding the companion of great Sirius. It could have been only 

 a keen, and in some sense poetic, imagination which inspired 

 this great astronomer to his daring prophecy. " The astro- 

 nomy of the future," he said, " will be the astronomy of 

 the invisible." The embracing mind of Laplace had caught an 

 inkling of the same truth. Bessel was bolder. It was he who 

 first determined absolutely the monstrous distance of the stars, 

 the first who showed that Huyghens' estimate, vast as it was, 

 might be multiplied ten and twenty times before we approach 

 reality. For aught that any human eye could see, the space 

 between was void. 



But in 1844 his observations of Sirius led him to announce 

 the probable existence of a dark companion. He came to the 

 same conclusion regarding the very brilliant star Procyon, in 

 the neighbouring constellation of the Little Dog. Their times 

 of revolution he set at about half a century. Twenty years 

 after, his predictions were verified in a curious way. 



The American firm of telescope builders, Alvan Clark & Sons, 

 were finishing a wonderful new refractor, and the younger 

 member of the firm turned it upon Sirius to test its powers. 

 He exclaimed to his father that the star appeared to have a 

 companion. He knew nothing then of Bessel's work, nor the 

 later and more precise calculations of other investigators, who 

 had to all intents demonstrated that the companion must be 

 there. But repeated observation left no doubt. 



The companion was most remarkable for its dimness ; its 

 giant partner gives perhaps ten thousand times as much light. 

 It appears to be about half as big as Sirius that is, perhaps 

 twelve or fifteen times the bulk of our sun. It is evident, then, 

 that the companion does not shine merely by the reflected light 

 of the glowing Dog-star, for they appear to be separated as 

 widely as Uranus and our sun. The diameter of the com- 

 panion, therefore, would have to be something enormous, ap- 

 proaching the size of Canopus perhaps, did it possess no lumi- 

 nosity of its own. The inference, therefore, is that it is a dark 

 sun, one of the dying embers of the flaming universe. 



