A—MATHEMATICAL AND PHYSICAL SCIENCES 23 
a source in such a medium emitting waves of one frequency is evaluated, 
it will be found that it is oscillatory, and this is also true when the potential 
energy function is of the most general type for an elastic medium. It 
should be observed that, just as in the case of waves of sound from a 
source or of waves in water, there is an actual displacement of the 
medium itself, e.g. in the case of waves of sound air must be supposed 
to be pumped in and out at the source, and this accounts for the 
fact that the rate of transfer of energy is oscillatory. This suggests 
that it should be possible to pump out portions of such a medium, 
and raises the question whether a medium which is subject to the 
laws of dynamics and which possesses a kinetic energy of this type can 
be an ultimate medium which will account for the phenomena of 
light. 
The next important stage in the development of theories of light is the 
discovery by Faraday in 1845 that when polarised light passed through 
a transparent medium its plane of polarisation was rotated by the im- 
position of a magnetic field. In the introduction to his account of these 
experiments Faraday says : ‘ I have long held an opinion, almost amount- 
ing to conviction, in common I believe with many other lovers of natural 
knowledge, that the various forms under which the forces of matter are 
made manifest have one common origin ; or, in other words, are so directly 
related and mutually dependent, that they are convertible, as it were, one 
into another, and possess equivalents of power in their action. ‘This 
strong persuasion extended to the powers of light, and led, on a former 
occasion, to many exertions, having for their object the discovery of the 
direct relation of light and electricity, and their mutual action in bodies 
subject jointly to their power; but the results were negative. ‘These 
ineffectual exertions, and many others which were never published, could 
not remove my strong persuasion derived from philosophical considera- 
tions ; and, therefore, I recently resumed the inquiry by experiment in 
a most strict and searching manner, and have at last succeeded in mag- 
netizing and electrifying a ray of light.’ Ina footnote added subsequently 
Faraday says: ‘ Neither accepting nor rejecting the hypothesis of an 
aether, or the corpuscular, or any other view that may be entertained of 
the nature of light ; and, as far as I can see, nothing being really known 
_ of a ray of light more than of a line of magnetic or electric force, or even 
of a line of gravitating force, except as it and they are manifest in and by 
substances ; I believe that, in the experiments I describe in the paper, 
light has been magnetically affected.’ 
Almost twenty years later, in 1865, Maxwell propounded a theory of 
light in his memoir, A Dynamical Theory of the Electromagnetic Field.* 
In the introduction Maxwell states : ‘ We have therefore some reason to 
believe, from the phenomena of light and heat, that there is an aethereal_ 
medium filling space and permeating bodies, capable of being set in 
motion and of transmitting that motion from one part to another and of 
2 What might be termed an electric theory of light was propounded by 
Oersted ; in this theory light was regarded as a succession of electric sparks. 
