270 PHYSICAL SCIENCE 



number of waves which reach him in a given 

 time will be less than before. The same effects 

 will be produced if the observer be stationary 

 and the source of light move. Doppler's 

 principle, as this change in periodic time is 

 called, is well illustrated in the case of sound. 

 Here the frequency of wave impulse on the ear 

 determines the pitch of the note heard, and it 

 is easy to detect a distinct flattening by a semi- 

 tone or more, as the whistling engine of an 

 express train passes the observer. The source of 

 the waves of sound still vibrates with the same 

 frequency, the change is only in the number of 

 impulses reaching the observer per second. 



The frequency with which waves of light are 

 received by the optic nerve determines the colour 

 perceived by the brain, and also the amount of 

 refraction in passing through a prism. Thus 

 the colour of a ray of a single definite wave- 

 length, as well as its position in the prismatic 

 spectrum, will be different from the normal value 

 when the source of light and the observer are 

 moving relatively to each other. An approach 

 will result in a shifting towards the blue end of 

 the spectrum owing to the increase in frequency ; 

 a recession will involve a reddening of the light, 

 or a movement of the spectral lines towards the 

 red end of the spectrum. Owing to the great 

 velocity of light, the change will relatively be 

 much less than in the case of sound. Light 

 travels about 186,000 miles in one second, and, 

 great though the speeds of the stars may be, 

 they fall far short of such tremendous values. 

 A velocity of eighteen miles a second, for 

 example, the velocity of the earth in her orbit. 



