MIRACLES OF SCIENCE 



were, crowded together and thus its spectral lines are 

 shifted toward the violet end of the spectrum; if it is 

 receding, they are shifted toward the red end. 



By measuring the shift of the spectral lines, the 

 astronomer tests the speed of a star flight with mar- 

 velous accuracy. With the newest instruments, his 

 range of error is scarcely greater than three-fifths of 

 a mile per second in the case of bodies moving ten 

 miles or fifty miles per second. He can even measure 

 the speed of the earth in its orbit by noting the seem- 

 ing approach and regression of a given star at half 

 yearly intervals. 



Or again by observing that the spectral lines of 

 a certain star shift periodically back and forth regard- 

 less of the earth's movement, he is informed that the 

 star in question is revolving in a mutual orbit with 

 some other star. The spectroscope may thus "re- 

 solve" double stars that are far too close to be sep- 

 arated visibly in the most powerful telescope. In- 

 deed, the star whose presence is thus demonstrated 

 the very size and weight of which may in some cases 

 be estimated may be a dark body that must forever 

 remain invisible to all telescopic powers. 



The spectroscope is, then, a chief instrument of 

 the astronomer's equipment. The new science of 

 astro-physics is the record of its revelations. The 

 spectroscope as used by the astronomer has found 

 its fullest development perhaps in the Mills spectro- 

 graph of the Lick Observatory, and the spectro- 

 graphs in connection with the Snow telescope and 

 the Tower telescopes of Mt. Wilson. An interesting 

 and important modification of the instrument is Pro- 



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