318 THE BINARY STARS. 



example of our second class), if taken by itself, affords an example 

 of this fourth class, as it is found to be a splendid spectroscopic 

 double. 



It is with stars of our first class, or binaries proper, in which 

 revolution of the pair is visible, and of the fourth class, or spectro- 

 scopic binaries, in which the spectroscope declares revolution to 

 exist, that we are now principally concerned. It was, I believe, 

 Sir William Herschel who first discovered this movement of revo- 

 lution more than a hundred years ago, and since that time some 

 of these pairs have been watched through more than one complete 

 revolution. It is not my intention to attempt the description of 

 many of the binary stars separately, for their name is legion. I 

 shall only single out one or two, and use them as examples to 

 show the main principles of the methods of research which have 

 been adopted, and the results which these researches have led to. 



In the first place, when by observation we have learnt the time 

 of revolution of the smaller star around the larger — or, rather, of 

 the pair round their common centre of gravity — we can, if we also 

 know the distance at which they are situated from us, tell the mass 

 of the pair as compared with the mass of the sun. We will take 

 Sirius as an example of this. The presence of a large and massive 

 companion revolving round Sirius was suspected long before this 

 companion was ever actually seen. It had been observed that the 

 proper motion of Sirius was somewhat irregular, and it was confi- 

 dently predicted by the illustrious astronomer, Bessel. that this 

 irregularity of movement would be found to be caused by the 

 disturbing influence of a large but comparatively faint companion^ 

 and this has since proved to be the fact. This body, which (like 

 the planet Neptune) was known to exist before it was ever seen, 

 owing to its perturbing influence upon the movements of Sirius, 

 was first observed in 1862 by Alvan Clark, jun., one of the cele- 

 brated firm of American opticians, when testing a new and power- 

 ful refracting telescope which they had been making. I believe 

 the object-glass of this telescope which first showed the companion 

 to Sirius was eighteen inches in diameter, but it has since been seen 

 with telescopes of much less size, for it is always found to be the 

 case that when an object has once been discovered, observers pro- 

 vided with much smaller telescopes than the original discoverer 



