I 

 THE CENTURY'S PROGRESS IN ASTRONOMY 



ters. The two Herschels between them discovered 

 some thousands of these close multiple systems; Struve 

 and others increased the list to above ten thousand ; 

 and Mr. S. W. Burnham, of late years the most enthusi- 

 astic and successful of double -star pursuers, added a 

 thousand new discoveries while he was still an amateur 

 in astronomy, and by profession the stenographer of a 

 Chicago court. Clearly the actual number of multiple 

 stars is beyond all present estimate. 



The elder Herschel's early studies of double stars 

 were undertaken in the hope that these objects might 

 aid him in ascertaining the actual distance of a star, 

 through measurement of its annual parallax ; that is to 

 say, of the angle which the diameter of the earth's orbit 

 would subtend as seen from the star. The expectation 

 was not fulfilled. The apparent shift of position of a 

 star as viewed from opposite sides of the earth's orbit, 

 from which the parallax might be estimated, is so ex- 

 tremely minute that it proved utterly inappreciable, 

 even to the almost preternaturally acute vision of Her- 

 schel, with the aid of any instrumental means then at 

 command. So the problem of star distance allured and 

 eluded him to the end, and he died in 1822 without see- 

 ing it even in prospect of solution. His estimate of the 

 minimum distance of the nearest star, based though it 

 was on the fallacious test of apparent brilliancy, was a 

 singularly sagacious one, but it was at best a scientific 

 guess, not a scientific measurement. 



Just about this time, however, a great optician came 

 to the aid of the astronomers. Joseph Fraunhofer per- 

 fected the refracting telescope, as Herschel had perfected 

 the reflector, and invented a wonderfully accurate " he- 

 liometer," or sun-measurer. With the aid of these in- 



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