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XIL A Dijnamical Theory of the Electric and Luminiferous Medium. 
By Joseph Larmor, F.R.S., Felloiv of St. Johns College, Cambridge. 
Received November 15,—Read December 7, 1893. 
Revised June 14, 1894. 
1 . The object of this paper is to attempt to develope a method of evolving the 
dynamical properties of the sether from a single analytical basis. One advantage of 
such a procedure is that by building up everything ab initio from a consistent and 
definite foundation, we are certain of the congruity of the different parts of the 
structure, and are not liable to arrive at mutually contradictory conclusions. The 
data for such a treatment lie of course in the properties of the mathematical function 
which represents the distribution of energy in the medium, when it is disturbed. The 
consequences which should result from the disturbance are all deducible by dynamical 
analysis from the expression for this function ; and it is the province of physical 
interpretation to endeavour to identify in them the various actual phenomena, and in 
so far to establish or disprove the explanation offered. A method of this kind has 
been employed by Clerk Maxwell with most brilliant results in the discovery and 
elucidation of the laws of electricity ; he has also been led by its development into 
the domain of optics, and has thus arrived at the electric theory of light. His 
expression for the energy of the active medium has been constructed fi’om reasoning 
on the phenomena of electrification and electric currents ; this procedure oilers 
perhaps difficulties greater than might be, owing to the intangible character of the 
electric co-ordinates, and their totally undefined connexion vuth the co-ordinates of 
the material system which is the seat of the electric manifestations. In the following 
discussion, the order of development began with the optical problem, and was found 
to lead on naturally to the electric one. We shall show that an energy-function can 
be assigned for the sether which .will give a complete account of what the sether has 
to do in order to satisfy the ordinary demands of Physical Optics ; and it will then 
be our aim to examine how far the phenomena of electricity can be explained as non¬ 
vibrational manifestations of the activity of the same medium. The credit of applying 
with success the pure analytical method of energy to the elucidation of optical 
phenomena belongs to MacCullagh ; he was however unable to discover a mechanical 
illustration such as would bring home to the mind by analogy the properties of his 
medium, and so his theory has fallen rather into neglect from supposed incompatibility 
17.12.94 
