Septemeek 18, 1896.] 



SCIENCE. 



401 



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 

 e£fect of a varying electro-magnetic field in 

 setting the ether in motion. I do not re- 

 member 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 vary- 

 ing electric polarization, which on the the- 

 ory 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 mag- 

 netic 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 me- 

 chanical 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 disturb- 

 ances requires a knowledge of the consti- 

 tution of the ether which we do not pos- 

 sess. We now assume that the ether is not 

 set in motion by an electro-ma.gnetic wave. 

 If we do not make this assumption we 

 must introduce into our equation quantities 

 representing the components 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 act- 

 ing on it, there will be in the equations of 

 the electro-magnetic field more unknown 

 quantities than we have equations to deter- 

 mine. It is, therefore, a very essential 

 point in electro-magnetic theory to investi- 

 gate whether or not there is any motion of 

 the ether in a varying electro-magnetic 

 field. We have at the Cavendish Labora- 

 tory, using Prof. Lodge's arrangement 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 vi- 

 brator the average velocity would be zero ; 

 we used, therefore, a heavily damped vibra- 

 tor, 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 

 neighborhood of a vacuum tube emitting 

 Eontgen 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 conclu- 

 sion. 



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 trans- 

 mission of electrical vibrations, and there- 

 fore of light, will be affected by a steady 

 magnetic field. Such a field, even if con- 

 taining nothing but ether, will behave to- 

 wards light like a crystal, and the velocity 

 of propagation will depend upon the direc- 

 tion of the rays. A similar result would 

 also hold in a steady electric field. We 



