268 IN STARRY REALMS. 



having to deduce that speed from a tardy comparison 

 between the places of the stars obtained after the lapse of 

 a sufficient interval between them. This the spectroscope 

 supplies, it tells us while we are looking at the star the 

 speed at which it is winging its flight. 



To determine the rate at which a star is moving towards 

 or from us requires no nice comparison and discussion 

 of observations made after an interval of a century by 

 different observers using different methods and obtaining 

 determinations which are entitled to very different degrees 

 of confidence. The modern spectroscopic observer takes 

 his seat at his instrument, adjusts his spectroscope on 

 the star. He fixes his attention on a certain line in 

 the spectrum, a line, for instance, known to belong to 

 the element hydrogen ; he sees it shifted to the right 

 or to the left of a standard hydrogen line, introduced 

 for the purpose of comparison. If the shift be one 

 way, the star is urging its course towards us, if the 

 shift be the other way the star is retreating from us, 

 while by measuring the amount of the shift the actual 

 velocity of the motion is determined. There is a certain 

 indicator sometimes put on a locomotive engine which 

 shows by a dial the speed at which the engine is running. 

 By a glance at this dial the engine-driver learns at once 

 the pace he is making. The spectroscope is like this little 

 indicator ; it shows at once the pace of the body to or from 

 the observer without the necessity for awaiting the time 

 necessary to pass over a certain interval. Thus the 

 spectroscope offers to us a means of obtaining promptly 

 valuable information with regard to the proper motions of 

 the stars. 



