PROGRESS OF MODERN ASTRONOMY 



star sending such light is alternately approaching and 

 receding, and the inference that it is revolving about a 

 companion is unavoidable. From this extraordinary 

 test the orbital distance, 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 questioned by an instru- 

 ment of such necromantic power? 



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 astrono- 

 mer 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 im- 

 portant proportions. In 1880, Dr. Henry Draper, at 

 Hastings-on-the-Hudson, made the first successful 

 photograph of a nebula. Soon after, Dr. David Gill, 

 at the Cape observatory, made fine photographs of a 

 comet, and the flecks of starlight on his plates first 

 suggested the possibilities of this method in charting 

 the heavens. 



Since then star-charting with the film has come vir- 

 tually to supersede the old method. A concerted effort 

 made by astronomers in various parts of the 

 <1 to make a complete chart of the heavens, and 

 before the close of our century this work will be accom- 

 plished, some fifty or sixty millions of visible stars be- 



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