THE STATUS OF THE ETHER 109 



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 

 force, 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 



