Appendix. Ixxv 



the result of vibrations or waves in ether, as sound is the result 

 of vibrations in air, and that just as slow and rapid vibrations of 

 air produce respectively low- and high-pitched notes, so do slow 

 and rapid vibrations of ether produce red and yellow or blue and 

 violet light respectively. If now one imagines oneself standing 

 beside a railway track and that an engine comes along sounding its 

 whistle, it is clear that as the engine approaches the bystander more 

 waves of sound of the whistle would reach the ear in a second of 

 time than if the engine were at rest. As a consequence of this if 

 the engine is travelling at a rate in any way comparable with the 

 velocity of sound a sharper note will be heard than if the engine 

 were at rest — on the other hand, if the engine is running away from 

 the bystander the pitch of the whistle will for a like reason be 

 lowered. The matter is easily put to the test by any one who 

 chances to be beside a railway track when an engine blowing its 

 whistle is approaching at high speed ; the instant that the engine 

 passes a sudden lowering of the whistle-note will be perceived. If 

 one had a tuning-fork, emitting the exact note given by the railway 

 whistle when at rest, it would be possible, with the aid of another 

 suitable whistle that could be tuned to the note of the moving 

 whistle, to determine the velocity of approach or recession of the 

 train from the difference of the number of vibrations per second 

 between the two forks. Just in the same way, if we knew the exact 

 wave-length of a particular line in the spectrum of a star, and if we 

 observe the wave-length of the same ray as it reaches the earth, we 

 have a means of determining the velocity of approach or recession of 

 the star, provided that the velocity of the star's motion has a measur- 

 able relation to the velocity of light. Doppler pointed out this possi- 

 bility in 1841, but it was not until Huggins had begun stellar 

 spectroscopy that, about 1865, he turned his attention to this possi- 

 bility of the new astronomy, and in 1866 made the first attempts to 

 determine motions in the line of sight. Such a task was, of course, 

 impossible until the lines of the star spectra had been identified 

 with those of known terrestrial substances, just as it would have 

 been impossible for an observer to determine the velocity of a railway 

 train at any moment by means of the note of the whistle that 

 reached his ear, unless the observer also had a tuning-fork emitting 

 the same note as the whistle of the engine when at rest. But 

 Huggins had already identified many star lines with those of terres- 

 trial spectra, and, so far, was in a position to attempt the task. He 

 showed in 1866 that such work was possible, but it required the 

 application of photography (first used for this purpose by Vogel) and 

 exhaustive study of the theory of the spectroscope, and the greatest 



