214 On some Recent Spectroscopic Researches. [April, 



sensible alteration in the pitch of the sound. In the case of light, 

 however, which travels with a velocity of about 185,000 miles per 

 second, it would be necessary to give to the luminous body and the 

 observer a relative motion of 20 to 30 miles per second, even for 

 such a minute change as could be just detected by a powerful 

 spectroscope.* 



The change in sound can be observed under many circumstances. 

 The pitch of a railway-whistle when a train in rapid motion has 

 passed the station of the listener is no longer the same as it was 

 when the train was approaching. The sound of the whistle re- 

 flected from a wall is different from the sound which comes directly 

 to the observer's ear. If two tuning-forks sounding in unison be 

 held in the hands, and if at the same moment one hand be thrust 

 suddenly forward and the other hand be drawn quickly backward, 

 immediately to a listener standing in front of the experimentalist 

 the imison will be interrupted by beats which tell of a difference of 

 pitch produced on his ear by the opposite motions of the forks. 

 Doppler in 1841 proposed by a similar consideration to account for 

 the remarkable difference of colour observed in some double stars. 

 But here he went wrong, for he overlooked the important circum- 

 stance, that if a star could be conceived to be moving with a velocity 

 sufficient to alter its colour sensibly to the eye, still no change of 

 colour would be perceived, for the reason that beyond the visible 

 spectrum, at both extremities, there exists a store of invisible waves 

 which would be at the same time exalted or degraded into visibility, 

 to take the place of the waves which had been raised or lowerol in 

 refraugibility by the star's motion. No change of colour, therefore, 

 could take place until the whole of these invisible waves of force 

 had been used up ; which would not be the case until the relative 

 motion of the source of light and the observer was several times 

 greater than that of light. 



These considerations bring into prominence the conditions that 

 must be fulfilled before the alteration of the refraugibihty of light 

 by the relative radial motion of the source of hght and the observer 

 can be employed to detect the motions in the line of sight of the 

 heavenly bodies. It is essential, first, that we can ascertain the 

 original colour or refrangibility of some part of the light at the 

 moment when it left the star ; and, secondly, that we can recognize 



* The writer thought it night be possible to obtain an experimental illustra- 

 tion of the principle discussed in this article by observing an induction sp;irk 

 taken butweon electrodes of sodium placed at a little distance apart. It seemed 

 possible that by the force of the spark the sodium, in a state of vapour, might be 

 carried from one electrode towards tlie other with a velocity suthcient to alter tlie 

 refraugibility of tlie lines of sodium wlien the spark was viewed in the direction in 

 whicii it was proceeding. Several expcrimeids, witli a large induction coil, were 

 made with the necessary care, but it was found that tlie vapour of the sodium 

 remainetl very close abuut the electrodes, and no cliang(! of pn.sition of the lines 

 in the spectrum could be detected. 



