706. REPORT—1896. 
cessfully the ether resisted them. It seems to be conclusively proved that a solid 
body in motion does not set in motion the ether at an appreciable distance outside 
it; so that if the ether is disturbed at all in such a case, the disturbance is not com- 
parable with that produced by a solid moving through an incompressible fluid, but 
must be more analogous to that which would be produced by the motion through: 
the liquid of a body of very open structure, such as a piece of wire netting, where 
the motion of the’ fluid only extends to a distance comparable with the diameter of 
the wire, and not with that of the piece of netting. There is another class of 
phenomena relating to the movement of the ether which is, I think, deserving of 
consideration, and that is the effect of a varying electro-magnetic field in setting 
the ether in motion. I do not remember to have seen it pointed out that the 
electro-magnetic 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 travelling. 
through the ether. We have parallel to the wave front a varying electric polari- 
sation, which on the theory is equivalent to a current; 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 con- 
stitution of the ether, which we do not possess. We now assume that the ether is 
not set in motion by an electro-magnetic wave. If wedo not make this assump- 
tion we must introduce into our equation quantities representing the components 
of the velocity of the ether, and unless we know the constitution 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, therefore, 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 
Professor Lodge’s arrangement of interference fringes, made some experiments to 
see if we could detect any movement of the ether in the neighbourhood 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 velocity 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 far 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 neighbourhood of a vacuum-tube emitting Réntgen rays, but could not find any 
trace of such a movement. Professor Threlfall, who independently tried the same 
experiment, has, I believe, arrived at the same conclusion. 
Unless the ether is immovable under the mechanical forces in a varying electro- 
magnetic field, there are a multitude of phenomena awaiting discovery. If the 
ether does move, then the velocity of transmission of electrical vibrations, and 
therefore of light, will be affected by a steady magnetic field. Such a field, even if 
containing nothing but ether, will behave towards light like a crystal, and the 
velocity of propagation will depend upon the direction of the rays. A similar 
result would also hold in a steady electric field. We may hope that experiments 
on these and similar points may throw some light on the properties of that medium 
which is universal, which plays so large.a part in our explanation of physical 
phenomena, and of which we know so little. 
