THE STATUS OF THE ETHER | 10g 
in the propagation of light might also be the agent in electromagnetic 
phenomena. Faraday says: 
For my own part, considering the relation of a vacuum to the magnetic 
foree, and the general character of magnetic phenomena external to the magnet, 
I am much more inclined to the notion that in the transmission of the force 
there is such an action, external to the magnet, than that the effects are merely 
attraction and repulsion at a distance. Such an action may be a function of 
the ether, for if there be an ether, it should have other uses than simply the 
conveyance of radiation. 
This expression of Faraday is the key-note of Maxwell’s theory. In 
examining the properties of the medium necessary to transmit electric 
and magnetic forces, he concentrates his attention on two quantities 
having direction, namely, the magnetic and electric polarization of the 
medium at every point. He shows that these states of polarization are . 
propagated in waves, and that these waves have all the properties of 
light-waves. They are transverse, no longitudinal wave occurs, and 
moreover for the first time the conditions at the surface of separation 
of two media are exactly sufficient to give the proper explanation of 
reflection and refraction. Everything accomplished by any undulatory 
theory was accomplished by the electromagnetic theory, with this in 
addition, so that it is perhaps surprising that it remained for the ex- 
perimental production in 1888 by Hertz of undoubtedly electromag- 
netic waves having all the properties predicted by Maxwell to give this 
theory the overwhelming preponderance that it has since maintained. 
We may now touch upon the question, what is a mechanical theory. 
A mechanical theory is one that can be stated in terms of the principles 
of mechanics. ‘The laws of mechanics, as they have been held since 
their exact statement by Newton, are all embraced in the single 
mathematical principle of least action, best comprised in the enuncia- 
tion of Hamilton. In this enunciation occur two functions repre- 
senting the two forms of energy, kinetic and potential. If these de- 
pend in a certain simple manner on two quantities having direction, or 
vectors, irrespective of their physical nature, the differential equations 
follow, which lead to wave propagation. Maxwell’s field vectors have 
this property, and consequently Maxwell’s theory is a mechanical theory. 
I will now define the properties of the ether, as they seem to me to be 
required by our present-day notions. The ether connotes those proper- 
ties of space in virtue of which a change in either of two field vectors 
at any point gives rise to a field of the other sort, the lines of which tend 
to symmetrically surround the lines of the original and varying vector 
in circles. In addition the direction of these surrounding lines is con- 
trary according to the field that we begin with. This is a qualitative 
statement in plain English of what is quantitatively stated in the six 
differential equations of Maxwell’s theory, and it avoids the use of the 
