Earth and the Lunniniferous Ether. 341 



displacement should be 2D™=2DxlO -8 . The distance D was 



about eleven meters, or 2xl0 7 wave-lengths of yellow light; 

 hence the displacement to be expected was 0*4 fringe. The 

 actual displacement was certainly less than the twentieth part 

 of this, and probably less than the fortieth part. But since the 

 displacement is proportional to the square of the velocity, the 

 relative velocity of the earth and the ether is probably less than 

 one sixth the earth's orbital velocity, and certainly less than 

 one-fourth. 



In what precedes, only the orbital motion of the earth is con-' 

 sidered. If this is combined with the motion of the solar sys- 

 tem, concerning which but little is known with certainty, the 

 result would have to be modified ; and it is just possible that 

 the resultant velocity at the time of the observations was small 

 though the chances are much against it. The experiment will 

 therefore be repeated at intervals of three months, and thus all 

 uncertainty will be avoided. 



It appears, from all that precedes, reasonably certain that if 

 there be any relative motion between the earth and the luminif- 

 erous ether, it must be small ; quite small enough entirely to 

 refute Fresnel's explanation of aberration. Stokes has given a 

 theory of aberration which assumes the ether at the earth's sur- 

 face to be at rest with regard to the latter, and only requires 

 in addition that the relative velocity have a potential ; but 

 Lorentz shows that these conditions are incompatible. Lorentz 

 then proposes a modification which combines some ideas of 

 Stokes and Fresnel, and assumes the existence of a potential, 

 together with Fresnel's coefficient. If now it were legitimate 

 to conclude from the present work that the ether is at rest with 

 regard to the earth's surface, according to Lorentz there could 

 not be a velocity potential, and his own theory also fails. 



Supplement. 



It is obvious from what has gone before that it would be 

 hopeless to attempt to solve the question of the motion of the 

 solar system by observations of optical phenomena at the surface 

 of the earth. But it is not impossible that at even moderate dis- 

 tances above the level of the sea, at the top of an isolated moun- 

 tain peak, for instance, the relative motion might be percepti- 

 ble in an apparatus like that used in these experiments. Per- 

 haps if the experiment should ever be tried in these circum- 

 stances, the cover should be of glass, or should be removed. 



It may be worth while to notice another method for multi- 

 plying the square of the aberration sufficiently to bring it within 

 the range of observation, which has presented itself during the 



