1896.] and Rontgen's X Rays. 57 



which represent the breaking of a sea wave as it rushes into 

 shallow water, would tend, if there were any inequalities in the 

 distribution of electrification in the tube to begin with, to accen- 

 tuate these inequalities, and may possibly show itself in the 

 sharply defined striations in the positive column. 



Normal Waves in a Moving Ether. 



We have seen that the ether in a varying electromagnetic field 

 must in general be moving. Let us consider the differences 

 between this case and the ether at rest. In the case of insu- 

 lators with a quiescent ether the dielectric current is proportional 

 to the rate of increase of the polarization with the time, in this 

 case there is no ambiguity, — but when the ether is in motion 

 the rate of increase to which the current is proportional may 

 either be estimated at a point fixed in space or at a point moving 

 along with the ether. The phenomena connected with the effect 

 of the motion of a refracting substance on the velocity of light 

 passing through it seem on the whole to be in favour of the latter 

 view. But if this is the case then the motion of the ether 

 through an electric field in which the electric intensity varies 

 from point must produce forces which tend to keep the intensity 

 constant at a point moving with the ether ; for example, when the 

 ether is moving through an electrified plate, forces must be 

 generated tending to make the electricity leave the plate and 

 travel along with the ether. 



When the ether is at rest the variation of the electric in- 

 tensity at any point is opposed by all the inertia of the electro- 

 magnetic field, while when the ether is moving all the influence of 

 the inertia is on the side of making the electric intensity at a fixed 

 point vary, the variation being such that the intensity remains 

 constant in a given portion of the moving ether. 



Let us take the simple case when the electric intensity, X, 

 and the velocity of the ether, p, are each parallel to the axis of X. 

 Then the dielectric current parallel to x is 



K_(dX dX) 

 4>7r\dt +P dx) 



Thus if the ether were moving in a field in which the electric 

 intensity was constant with respect to the time, but which varied 

 from place to place, i.e. a field in which dXjdt vanishes, but where 

 dX/dx is finite, the motion of the ether would produce a system of 



