46 THE WONDERFUL CENTURY. CHAP. vi. 



to detect and measure certain motions of the stars which 

 seemed to be wholly beyond our grasp, and also to 

 demonstrate the existence of celestial bodies which could 

 be detected in no other w r ay. 



In order to understand how this is possible we have 

 to make use of the wave-theory of light ; and the analogy 

 of other wave-motions will enable us better to grasp the 

 principle on which these calculations depend. If on a 

 nearly calm day we count the waves that pass each min- 

 ute by an anchored steamboat, and then travel in the 

 direction the waves come from, we shall find that a larger 

 number pass us in the same time. Again, if we are 

 standing near a railway, and an engine comes toward us 

 whistling, we shall notice that it changes its tone as it 

 passes us ; and as it recedes the sound will be very differ- 

 ent, although the engine is at the same distance from us 

 as when it was approaching. Yet the sound does not 

 change to the ear of the engine-driver, the cause of the 

 change being that the sound-waves reach us in quicker 

 succession as the source of the waves is approaching us 

 than when it is retreating from us. Now just as the 

 pitch of a note depends upon the rapidity with which the 

 air-vibrations reach our ear, so does the color of a par- 

 ticular part of the spectrum depend upon the rapidity 

 with which the ethereal waves which produce color reach 

 our eyes; and as this rapidity is greater when the source 

 of the light is approaching than when it is receding from 

 us, a slight shifting of the position of the dark lines will 

 occur, as compared with their position in the spectrum of 

 the sun or of any stationary source of light, if there is 

 any motion sufficient in amount to produce a perceptible 

 shift. On experimenting with a powerful spectroscope 



