The Evolution of the Sciences 



when the two trains are approaching, and of a 

 lower when they are receding. The reason for 

 this fact has been given by Doppler in 1842. 

 When the observer moves towards the waves of 

 vibration radiating from the whistle he receives 

 a greater number per second than if he were 

 stationary; on the other hand he receives fewer 

 when he moves away from the source of the 

 waves. This proposition is equally applicable 

 to luminous vibrations. When a luminous body 

 emitting a simple radiation is approaching or 

 receding from the observer at a high speed the 

 light appears to correspond in the first case to 

 more rapid vibrations, and in the second 

 to slower vibrations. Now, as each radia- 

 tion is defined by its position in the spec- 

 trum, extending from red for slow vibrations to 

 violet for rapid vibrations, the position occupied 

 by the radiation will move towards the violet, 

 when its source is approaching, and towards 

 the red when it is receding. Therefore, if with 

 the spectroscope we first sight the sun's eastern 

 and then its western edge, and note the position 

 in the spectrum of a given radiation emitted by 

 each of the edges, we shall find a slight but per- 



166 



