220 HERSCHEL AND HIS WORK 



them both at the same distance. A little 

 however, soon convinces him that the one star, though 

 shining at a vast distance from the other, may be so 

 placed in a line drawn from our eye to the latter as; 

 to be nearly or altogether eclipsed by it Sometimes 

 these stars are so close that the two pass for on* till 

 .in improvement in the telescope separates the com- 

 panions, and shows them to be distinct. Herschel had 

 this experience, and one of the most singular inst 

 of it is not yet thirty years old. The dog-star Sirius 

 is among the best known stars in our southern skies. 

 Its brightness is forty- to sixty-fold that of the sun, 

 its distance is such that a flash of light from it 

 takes perhaps ten years to reach our eyes, and its 

 weight exceeds that of two of our suns. This vast and 

 brilliant sun was found to indulge in vagaries which 

 were, and some of which still are, the puzzle of 

 astronomers. They could not see, and therefore did 

 not know. But although they could not see, they 

 could imagine what the unseen cause of these vagaries 

 was : for " the eyes of the mind can supply the want of 

 the most powerful telescopes, and lead to astronomical 

 discoveries of the highest importance." 1 Another star 

 in the neighbourhood of Sirius, the mathemati 

 said, is moving round him. They calculated its orbit, 

 they told observers where to apprehend the disturber, 

 but in vain. At last the eighteen-inch object-glass, 

 made for the Chicago Observatory in the United States, 

 was turned on Sirius by way of trial. Great was the 

 surprise of the manufacturers when they saw that the 

 mighty sun had a fainter but a very bulky companion 

 in his company, and was seen in the direction j>iv- 



1 Arago, Biographies, etc., p. 224. 



