1869.] On some Recent Spectroscopic Researches. 213 



passing through a prism is determined, it follows that any circmn- 

 stances which would alter the length of the waves relatively to the 

 observer, or in other words cause a large number of waves to enter 

 the eye in a second of time, would cause a change in the colour or 

 refrangibility of the light so far as the observer was concerned. 

 An alteration in the velocity of the propagation of light would 

 effect a change of wave-length, not, of course, intrinsically, but as 

 measured by the eye which it entered, or the prism through which 

 it passed. Less than twice the actual velocity of light would cause 

 red light to be changed into violet, and all the other colours, the 

 yellow, green, and blue, to be raised into force of that quality which 

 exists in the long invisible spectrum beyond the violet. 



Now it is obvious that the velocity of light is virtually altered 

 to an observer moving in the direction in which the light is 

 travelling. If the observer advances to meet the light, the velocity 

 with which it enters his eye is increased, a longer series of waves 

 falls upon the retina in a second of time, each wave appears 

 shorter, and he therefore ascribes to the light a different colour, a 

 higher refrangibility, than he would do if he were not advancing 

 to meet the light. There is no change in the light itself, the 

 alteration consists in a new relation of the observer to it. 



To a swimmer striking out from the shore, each wave appears 

 shorter, and he passes a greater number in a given interval in 

 proportion to the speed of his progress through the water. If the 

 distance between the men in a single file of soldiers be taken to 

 represent the length of the waves of light, the distance will be 

 diminished to a man meeting the soldiers, for he will pass a greater 

 number of them in a given time than if he had been standing 

 stiU. 



Further, since the change of refrangibility depends alone upon 

 the continually shortened distance between the observer and the 

 source of the hght, it is obviously of no moment whether the 

 motion of approach belongs to the observer or to the source of the 

 light. It requires no consideration to see that an opposite change 

 in the refrangibility of the light will be produced by a motion of 

 recession between the observer and the source of the light. 



The idea that an eftect upon the refrangibility of light would 

 be produced by a motion of the observer or of the source of the 

 light towards or from each other, is due to Dopj^ler, who stated at 

 the same time (in 1841) that an analogous change should take place 

 in sound. In 1845 Doppler's theory was put to the test by Ballot, 

 who published a series of acoustic experiments which confirm the 

 correctness of the theory in the case of sound. 



The comparatively slow velocity of sound through the air 

 makes it an easy matter to give to the sounding body or to 

 the listener a relative motion sufficiently great to produce a very 



VOL. VI. Q 



