462 Motion of the Earth and Luminiferous ^Either. 



velocity of light without returning the ray to its starting 

 point, the problem of measuring the first power of the relative 

 velocity of the earth with respect to the aether would be 

 solved. This may not be as hopeless as might appear at first 

 sight, since the difficulties are entirely mechanical and may 

 possibly be surmounted in the course of time. 



For example, suppose m and m / (fig. 3) two mirrors re- 

 volving with equal velocity in opposite directions. It is 

 evident that light from s will form a stationary image at s, 

 and similarly light from s 7 will form a stationary image at s. 

 If now the velocity of the mirrors be increased sufficiently, 

 their phases still being exactly the same, both images will be 

 deflected from s and s t in inverse proportion to the velocities 

 of light in the two directions ; or, if the two deflections are 

 made equal, and the difference of phase of the mirrors be 

 simultaneously measured, this will evidently be proportional 

 to the difference of velocity in the two directions. The only 

 real difficulty lies in this measurement. The following is 

 perhaps a possible solution. 



99/ (fig* 4) are two gratings on which sunlight is concen- 

 trated. These are placed so that after falling on the revolv- 

 ing mirrors m and m n the light forms images of the gratings 

 at s and s n two very sensitive selenium cells in circuit with a 

 battery and telephone. If everything be symmetrical, the 

 sound in the telephone will be a maximum. If now one of 

 the slits s be displaced through half the distance between 

 the image of the grating bars, there will be silence. Suppose 

 now that the two deflections having been made exactly equal, 

 the slit is adjusted for silence. Then if the experiment be 

 repeated when the earth's rotation has turned the whole 

 apparatus through 180°, and the deflections are again made 

 equal, there will no longer be silence, and the angular dis- 

 tance through which s must be moved to restore silence will 

 measure the required difference in phase. 



There remain three other methods, all astronomical, for 

 attacking the problem of the motion of the solar system 

 through space. 



1. The telescopic observation of the proper motions of 

 the stars. This has given us a highly probably determina- 

 tion of the direction of this motion, but only a guess as to its 

 amount. 



2. The spectroscopic observation of the motion of stars in 

 the line of sight. This could furnish data for the relative 

 motions only, though it seems likely that by the immense im- 

 provements in the photography of stellar spectra, the informa- 

 tion thus obtained will be far more accurate than any other. 



