u/V SPECTROSCOPY. n 



end of the spectrum, then we know that the star is approaching the earth. 

 If the lines of the star are shifted towards the red, we learn that the 

 star and the earth are receding from each other. Further, an exact 

 determination of the amount of alteration of wave-length of the lines, 

 will tell us the velocity with which the star is moving in approach or 

 recession relatively to the earth. 



The line F in Sirius compared with the line of hydrogen shows that 

 this star is receding with a velocity of 25 miles per second. 



Solar Spectrum 

 line F 



Spectrum oj 

 Sirius 



Hydrogen in 

 Vacuum tube 



In the case of Arcturus independent comparisons were made with 

 lines of magnesium, hydrogen and sodium. The lines in the star cor- 

 responding to "B," "F" and "D" of the solar spectrum were all found 

 to have a small shift towards the blue end of the spectrum as compared 

 with the bright lines of the substances to which they are due. This 

 shift of the spectrum shows that the star has a motion of approach. 

 These comparisons are represented in the accompanying diagram. 



F 6 E D 



~~T~ ~~TT~ 



Solar Spectrum 



Spectrum of 

 Arcturus 



Hydrogen line 

 Magnesium line. 

 Sodium line 



H M<7 



In a similar \vay, the motions in the line of sight of about 21 stars 

 were ascertained. It is obvious that by combining these motions with 

 those at right angles to the line of sight obtained, by ordinary astro- 

 nomical observation, the true motions of the stars may be discovered. 

 The latest contribution to this part of science was made by Mr. 



