X THE STARRY HEAVENS 421 



therefore invisible, companion. The spectro- 

 scope, in fact, makes known to us the 

 presence of many stars which no telescope 

 could reveal. 



Thus the floor of heaven is not only 

 " thick inlaid with patines of bright gold," 

 but studded also with extinct stars, once prob- 

 ably as brilliant as our own Sun, but now 

 dead and cold, as Helmholtz tells us that our 

 Sun itself will be some seventeen millions of 

 years hence. 



Such dark bodies cannot of course be seen, 

 and their existence, though we cannot doubt 

 it, is a matter of calculation. In one case, 

 however, the conclusion has received a most 

 interesting confirmation. The movements of 

 Sirius led mathematicians to conclude that it 

 had also a mighty and massive neighbour, the 

 relative position of which they calculated, 

 though no such body had ever been seen. In 

 February 1862, however, the Messrs. Alvan 

 Clark of Cambridgeport were completing 

 their 18-inch glass for the Chicago Observa- 

 tory. " ' Why, father,' " exclaimed the younger 

 Clark, "'the star has a companion.' The 



