PROGEESS IN ASTRONOMY. 129 



Now that in astronomical work eye observations are indispensably 

 supplemented by the employment of photography, an important modi- 

 fication of the refracting telescope has become necessary. This was 

 first suggested by Rutherfurd. 



The ordinary achromatic object glass consists, as a rule, of two 

 lenses, one made of flint and the other of crown glass; ))ut in this form 

 the photographic ra3^s are not brought to the same focus as the visual 

 rays. This, however, can be achieved by employing three lenses 

 instead of two, each of ditJ'erent kinds of glass. The most modern 

 improvement in the telescope is due to Mr. Dennis Taylor, of Cooke & 

 Sons, and to Dr. Schott and Professor Abbe, whose researches in the 

 manufacture of old and new varieties of optical glass have rendered 

 Mr. Taylor's results feasible. By the Taylor lens outstanding color 

 is abolished, all the rays being brought absolutely to the same focus. 

 Such lenses can therefore be used either for visual observations or for 

 photography for spectroscopy. 



SPECTROSCOPIC ASTRONOMY. 



The branch of physics which at the present day has assumed such 

 mighty and far-reaching proportions in astronomical work is that deal- 

 ing with spectrum analysis, which, although suggested as early as the 

 time of Kepler, did not receive any impetus as regards its application 

 to celestial bodies until the beginning of the present century at the 

 hands of WoUaston and Fraunhofer. Then, however, it still lacked 

 the chemical touch supplied afterward by Kirchhofl^' and Bunsen. 

 They showed us that the spectrum observed when the light from any 

 heated body is passed through a pi'ism is an index to the chemical com- 

 position of the light source. The constitution of a vapor when in a 

 condition to absorb light can be determined by an extension of the 

 same principle first demonstrated by Stokes, Angstrom, and Balfour 

 Stewart when the centur}^ was about half completed. 



The first celestial body toward which the spectroscope was turned 

 was our central luminary, the sun. 



WoUaston first discovered that its spectrum was crossed by a few 

 dark lines. We learned next from Fraunhofer, who in 1814 worked 

 with instruments of greater power, that the solar spectrum was crossed 

 not only by a few dark lines, but by some hundreds. Not content 

 with examining the light of the sun, Fraunhofer turned his instru- 

 ment toward the stars, the light of which he also examined, so that he 

 may be justly called the inventor of stellar spectrum analysis. It is 

 not to the credit of modern science that from this time forward spec- 

 trum analysis did not become a recognized branch of scientific inquir}", 

 but as a matter of fact Fraunhofers observations were buried in obliv- 

 ion for nearly half a century. The importance of them was not rec- 

 ognized till the origin of the dark lines, both in sun and stars, had been 



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