gg TRANSACTIONS OF WAGNER 



ETHER WAVES AND THE MESSAGES THEY BRING 



A and B will be occupied by these ten waves and one wave length will be one- 

 tenth of the distance A B. But if the body be moving away from the observer 

 so that, having started from B of the second line, it reaches B' at the end of one 

 second, the ten waves will occupy the longer distance A B'. Each wave length 

 will be one-tenth of A B', and therefore greater than when the body was sta- 

 tionary at B. Again, if in the lower line the body move toward the observer 

 while producing the waves, going from B to B' in one second, the wave length 

 must be shorter than in either of the other cases. Thus, in sound, if a loco- 

 motive be moving toward one as it whistles, the sound waves must be shorter 

 and therefore the sound be higher in pitch than if the locomotive were sta- 

 tionary. Likewise if the locomotive be moving away from the observer the 

 sound must obviously be of lower pitch. 



"^\/\A/V\/\/\Ay\/\/" ^ 



Fig. 3. — Diagram illustrating Doppler's principle. 



To understand the application of Doppler's principle to spectroscopy it is 

 only necessary to remember that the position of the lines in the spectrum de- 

 pends upon the wave lengths represented. Therefore if there is a change in 

 the wave lengths there will be a shift in the position of the lines concerned. 

 Consequently, if a luminous body is moving av/ay from us, the length of the 

 waves will be increased and the spectrum with its lines will be shifted in the 

 spectroscope toward the red end, while if the body be moving toward us the 

 shift will be toward the end of the shorter waves, or the violet. Consequently 

 a shift from the usual position of any characteristic set of lines may be used for 

 the determination of the speed of the source of the light toward us or away from 

 us. Many interesting facts have been established as a result of this method of 

 study. It has been used to calculate the speed of different portions of the sun, 

 such as the gases in the neighborhood of sun spots, or the tongues of flame — 

 the prominences — which shoot out from the central body sometimes at a speed 



