Chemistry and Physics. 389 



think, deserving of consideration, and that is the effect of a vary- 

 ing electro-magnetic field in setting the ether in motion. I do 

 not remember to have seen it pointed out that the electro-mag- 

 netic theory of light implicitly assumes that the ether is not set 

 in motion even when acted on by mechanical forces. On the 

 electro-magnetic theory of light such forces do exist, and the 

 equations used are only applicable when the ether is at rest. 

 Consider, for example, the case of a plane electric wave traveling 

 through the ether. We have parallel to the wave-front a varying 

 electric polarization, which on the theory is equivalent to a cur- 

 rent ; at right-angles to this, and also in the wave-front, we have 

 a magnetic force. Now, when a current flows through a medium 

 in a magnetic field there is a force acting on the medium at right- 

 angles to the plane, which is parallel both to the current and to 

 the magnetic force ; there will thus be a mechanical force acting 

 on each unit volume of the ether when transmitting an electric 

 wave, and since this force is at right-angles to the current and to 

 the magnetic force, it will be in the direction in which the wave 

 is propagated. In the electro-magnetic theory of light, however, 

 we assume that this force does not set the ether in motion, as 

 unless we made this assumption we should have to modify our 

 equations, as the electro-magnetic equations are not the same in a 

 moving field as in a field at rest. In fact, a complete discussion 

 of the transmission of electro-magnetic disturbances requires a 

 knowledge of the constitution of the ether which we do not pos- 

 sess. We now assume that the ether is not set in motion by an 

 electro-magnetic wave. If we do not make this assumption, we 

 must introduce into our equation quantities representing the com- 

 ponents of the velocity of the ether, and unless we know the con- 

 stitution of the ether, so as to be able to deduce these velocities 

 from the forces acting on it, there will be in the equations of the 

 electro-magnetic field more unknown quantities than we have 

 equations to determine. It is, therefere, a very essential point in 

 electro-magnetic theory to investigate whether or not there is any 

 motion of the ether in a varying electro-magnetic field. We 

 have at the Cavendish Laboratory, using Prof. Lodge's arrange- 

 ment of interference fringes, made some experiments to see if we 

 could detect any movement of the ether in the neighborhood of 

 an electric vibrator, using the spark which starts the vibrations as 

 the source of light. The movement of the ether, if it exists, will 

 be oscillatory, and with an undamped vibrator the average veloc- 

 ity would be zero ; we used, therefore, a heavily damped vibrator, 

 with which the average velocity might be expected to be finite. 

 The experiments are not complete, but so lar the results are 

 entirely negative. We also tried by the same method to see if 

 we could detect any movement of the ether in the neighborhood 

 of a vacuum-tube emitting Rontgen rays, but could not find any 

 trace of such a movement. Prof. Threlfall, who independently 

 tried the same experiment, has, I believe, arrived at the same con- 

 clusion. 



