844 Michelson and Morley — Motion of the Earth, etc. 



would be about one-millionth of the original intensity, so that 

 if sunlight or the electric arc were used it could still be readily 

 seen. The mirrors bb / and cc, would differ from parallelism 

 sufficiently to separate the successive images. Finally, the 

 apparatus need not be mounted so as to revolve, as the earth's 

 rotation would be sufficient. 



If it were possible to measure with sufficient accuracy the 

 velocity of light without returning the ra} r to its starting point, 

 the problem of measuring the first power of the relative velocity 

 of the earth with respect to the ether 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 sur- 

 mounted in the course of time. 



For example, suppose (fig. 3) m and m, two mirrors revolving 

 with equal velocity in opposite directions. It is evident that light 

 from 5 will form a stationary image at s / and similarly light 

 from s / 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, 

 in inverse proportion to the velocities of light in the two direc- 

 tions ; or, if the two deflections are made equal, and the differ- 

 ence 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 measure- 

 ment. The following is perhaps a possible solution : gg, (fig. 

 4) are two gratings on which sunlight is concentrated. These 

 are placed so that after falling on the revolving mirrors m and 

 m n the light forms images of the gratings at s and s,, two very 

 sensitive selenium cells in circuit with a battery and a 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 distance through which s must be moved to restore 

 silence will measure the required difference in phase. 



There remain three other methods, all astronomical, for at- 

 tacking 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 determination 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 



