A.—_MATHEMATICAL AND PHYSICAL SCIENCES 25 
was published in the Philosophical Magazine in 1846. In this letter he 
states : ‘ The view which I am so bold as to put forth considers, therefore, 
radiation as a high species of vibration in the lines of force which are 
known to connect particles and also masses of matter together. It 
endeavours to dismiss the aether, but not the vibration. The kind of 
vibration which, I believe, can alone account for the wonderful, varied, 
and beautiful phenomena of polarization, is not the same as that which 
occurs on the surface of disturbed water, or the waves of sound in gases 
or liquids, for the vibrations in these cases are direct, or to and from the 
centre of action, whereas the former are lateral. It seems to me, that the 
resultant of two or more lines of force is an apt condition for that action 
which may be considered as equivalent to a Jateral vibration ; whereas 
a uniform medium like the aether does not appear apt, or more apt than 
air or water. 
* The occurrence of a change at one end of a line of force easily suggests 
a consequent change at the other. The propagation of light, and there- 
fore probably of all radiant action, occupies time; and that a vibration 
of the line of force should account for the phenomena of radiation, it 
is necessary that such vibration should occupy time also.’ 
And again: ‘ The aether is assumed as pervading all bodies as well as 
space: in the view now set forth, it is the forces of the atomic centres 
which pervade (and make) all bodies, and also penetrate all space. As 
regards space, the difference is, that the aether presents successive parts 
or centres of action, and the present supposition only lines of action ; as 
regards matter, the difference is, that the aether lies between the particles 
and so carries on the vibrations, whilst as respects the supposition, it is 
by the lines of force between the centres of the particles that the vibration 
is continued.’ 
Faraday, like Fresnel, appears to be thinking in terms of geometrical 
relations, while Maxwell is seeking to construct a mechanical model 
whose motions will resemble those which constitute light. 
Starting from Faraday’s ideas, the problem of the propagation of 
a magnetic disturbance in free space can be approached in a direct manner. 
There are three vectors involved—the electric current at a point in the 
space, the magnetic force at the point, and the electric force at the point. 
The relation between the electric current and the magnetic force is given 
by Ampere’s law,® and the relation between the magnetic force and the 
electric force is given by Faraday’s law. Assuming, with Faraday, that 
the phenomena of light and of electricity have a common origin, Fresnel’s 
law of transversality, that the vectors which specify the disturbance are 
perpendicular to the direction of propagation, will hold for the propaga- 
tion of an electric or a magnetic disturbance as well as for light. These 
three laws are sufficient to determine the circumstances of the propagation 
of a magnetic disturbance in free space. It follows that for plane waves 
$ It should be noted that Ampére’s law was established initially for steady 
electric currents ; its extension to the case where the electric currents are varying 
is a result of Faraday’s work. 
