A HISTORY OF SCIENCE 



earth's orbit, from which the parallax might be esti- 

 mated, is so extremely minute that it proved utterly 

 inappreciable, even to the almost preternaturally acute 

 vision of Herschel, with the aid of any instrumental 

 means then at command. So the problem of star dis- 

 tance allured and eluded him to the end, and he died 

 in 1822 without seeing 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 ap- 

 parent brilliancy, was a singularly sagacious one, but it 

 was at best a scientific guess, not a scientific measure- 

 ment. 



The Distance of the Stars 



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 per- 

 fected the reflector, and invented a wonderfully accu- 

 rate "heliometer," or sun-measurer. With the aid of 

 these instruments the old and almost infinitely difficult 

 problem of star distance was solved. In 1838 Bessel 

 announced from the Konigsberg observatory that he 

 had succeeded, after months of effort, in detecting and 

 measuring the parallax of a star. Similar claims had 

 been made often enough before, always to prove falla- 

 cious when put to further test; but this time the an- 

 nouncement carried the authority of one of the greatest 

 astronomers of the age, and scepticism was silenced. 



Nor did Bessel' s achievement long await corrobora- 

 tion. Indeed, as so often happens in fields of discov- 

 ery, two other workers had almost simultaneously 

 solved the same problem Struve at Pulkowa, where 



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