A HISTORY OF SCIENCE 



miniature are furnished by the earth and the smaller of 

 its companion planets. But there are larger bodies of 

 the same type out in stellar space veritable "dark 

 stars" invisible, of course, yet nowadays clearly rec- 

 ognized. 



The opening up of this " astronomy of the invisible" 

 is another of the great achievements of the nineteenth 

 century, and again it is Bessel to whom the honor of 

 discovery is due. While testing his stars for parallax, 

 that astute observer was led to infer, from certain 

 unexplained aberrations of motion, that various stars, 

 Sirius himself among the number, are accompanied by 

 invisible companions, and in 1840 he definitely predi- 

 cated the existence of such "dark stars." The cor- 

 rectness of the inference was shown twenty years 

 later, when Alvan Clark, Jr., the American optician, 

 while testing a new lens, discovered 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, not 

 merely by renewed telescopic observations, but by the 

 curious testimony of the ubiquitous spectroscope. 



One of the most surprising accomplishments of that 

 instrument is the power to record the flight of a lumi- 

 nous object directly in the line of vision. If the lumi- 

 nous body approaches swiftly, its Fraunhofer lines are 

 shifted from their normal position towards the violet 

 end of the spectrum ; if it recedes, the lines shift in the 

 opposite direction. The actual motion of stars whose 

 distance is unknown may be measured in this way. 

 But in certain cases the light lines are seen to oscillate 

 on the spectrum at regular intervals. Obviously the 



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