162 



THE EVOLUTION OF 



detection and measurement form one of the most striking 

 chapters in the history of astronomy. 



It is well known that the apparent pitch of a note is 

 affected by the approach or recession of the body emit- 

 ting it, and the effect is capable of simple explanation. 

 A body emitting a musical note does so by generating a 

 regular succession of waves in the air, which travel out- 

 wards from it in every direction. These waves enter the 

 ear, and the sensation of pitch that is developed depends 

 solely upon the rapidity with which the individual waves 



FIG. 12. Orbits of Algol and Companion. 



follow one another. This again clearly depends upon their 

 length, the shorter waves, following in more rapid succes- 

 sion than longer ones, giving rise to the sensation of a 

 higher pitch. Now, if the sounding body were to 

 approach the observer, it would, by its motion, shorten 

 the waves in front of it that it had already generated, 

 by crowding them together, and the pitch should 

 consequently be raised. Similarly, by a motion of 

 recession, the pitch should be lowered, by the waves 

 in the rear of the moving source of sound being 

 drawn out and lengthened. These effects are fully con- 



