THE CENTURY'S PROGRESS IN ASTRONOMY 



in 1840 he definitely predicated the existence of such 

 "dark stars." The correctness of the inference was 

 shown twenty years later, when Alvan Clark, Jun., the 

 American optician, while testing a new lens, discovered 





STAlt SPECTRA 



the companion of Sirius, which proved thus to be faintly 

 luminous. Since then the existence of other and quite 

 invisible star companions has been proved incontestably, 



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