THE STORY OF NINETEENTH-CENTURY SCIENCE 



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 luminous 

 object directly in the line of vision. If the luminous 

 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 star sending such 

 light is alternately approaching and receding, and the 

 inference that it is revolving about a companion is una- 

 voidable. From this extraordinary test the orbital dis- 

 tance, relative mass, and actual speed of revolution of 

 the absolutely invisible body may be determined. Thus 

 the spectroscope, which deals only with light, makes 

 paradoxical excursions into the realm of the invisible. 

 What secrets may the stars hope to conceal when ques- 

 tioned by an instrument of such necromantic power? 



IV 



But the spectroscope is not alone in this audacious 

 assault upon the strongholds of nature. It has a worthy 

 companion and assistant in the photographic film, whose 

 efficient aid has been invoked by the astronomer even 

 more recently. Pioneer work in celestial photography 

 was, indeed, done by Arago in France and by the elder 

 Draper in America in 1839, but the results then achieved 

 were only tentative, and it was not till forty years later 

 that the method assumed really important proportions. 



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