RECENT PEOGRESS IN ASTROPHYSICS. 



By C. G. Abbot. 



[With 3 plates.] 



According to the definition of the word by the late Prof. Newcomb 

 in the last edition of the Encyclopaedia Britannica, "Astrophysics is 

 that branch of astronomical science which treats of the physical 

 constitution of the heavenly bodies." Interpreting this definition in 

 a manner somewhat narrower than that which is generally accepted 

 in astronomical circles, Prof. Newcomb, in his article on astrophysics, 

 mentioned the principal conclusions of the science to be that the 

 heavenly bodies are composed of like matter with that which we 

 find to make up our globe ; that as a rule the incandescent heavenly 

 bodies are mainly composed of gas, or of substances gaseous in their 

 nature; and that the temperature of the great heavenly bodies is 

 extremely high. He thus omitted from the province of astrophysics 

 the study of the motions of the celestial objects and their parts by 

 aid of the spectroscope, although this certainly has a bearing on the 

 physical constitution of these objects. Information of fundamental 

 importance in relation to the nature of the heavenly bodies and the 

 evolution of the universe has resulted from investigations of the 

 radial velocity of stars by the spectroscope ; and this is supplemented 

 and confirmed by observations with the telescope alone. Hence I 

 shall not confine myself strictly in what follows to Prof. Newcomb's 

 definition of astrophysics, but shall include the discussion of several 

 subjects which have at least an astrophysical bearing, though not 

 strictly, perhaps, astrophysical in themselves. 



THE WAVE LENGTHS OF LIGHT. 



All modern spectroscopic progress depends upon the exact knowl- 

 edge of the wave lengths of the lines of absorption or emission of the 

 chemical elements. Long ago it was discovered that sodium and its 

 compounls, when heated to incandescense, gave out a yellow light, 

 which when examined by the spectroscope, resolved itself into two 

 lines of wave lengths 5,890 and 5,896 Angstrom units. It was also 

 found that when sodium vapor was interposed between a source of 

 white light, like the electric arc, and the slit of the spectroscope, 

 there would be found in the place of the bright yellow lines of sodium 



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