x.] DOPPLER'S PRINCIPLE. 133 



by Secchi and Huggins, a few months earlier in the same year, 

 to somewhat similar effects observed in the spectra of stars. The 

 explanation depends upon a view first advanced by Doppler in 

 1842, that the light from a moving light-source is not the same 

 in all its qualities as light from a fixed one. 



Doppler 's idea will be gathered from the following analogies 

 from sound : 



The colours which we see in the spectrum are exactly analo- 

 gous to the notes which we hear in a piano when we go from 

 one end of the scale to the other. Doppler imagined the equi- 

 valent of a piano going away from or coming towards the listener 

 with considerable velocity a velocity comparable, in fact, to 

 the velocity of sound in air. It is clear that under these 

 circumstances we should no longer get true concert pitch 

 for each note, since the note which gives us a certain tone, 

 because it produces in the air so many waves per second, will 

 change its tone if the source of the note is coming to us. Take, 

 for instance, a tuning-fork giving concert C, and imagine it 

 rapidly coming to us : the waves of sound will be crushed to- 

 gether, we shall have more waves in a second falling on the ear, 

 and we shall get a higher note. If we imagine, on the other 

 hand, the tuning-fork to be going away from us, the notes will 

 be paid out at longer intervals, so to speak, and we shall get 

 a lower note. In neither case shall we continue to have 

 concert C. 



A very familiar instance in which we do get this change of 

 pitch due to change of motion, is produced in these days of very 

 rapid railway travelling. Any of us who have been at a country 

 railway station when an express is coming by will know that as 

 the train approaches us the note of its whistle is at one pitch, 

 and as it goes from us after passing, it changes and gives a lower 

 note according to the velocity of the train. An experimental 

 illustration of this principle is to attach a whistle to the end of 

 a long india-rubber tube. If then a person sounds the whistle 



