Vlll 



discovery of the rotation of the plane of polarisation of polarised 

 light transmitted along the lines of magnetic force. To the qnestion, 

 how will the vibrations be affected by passing through a medium in a 

 state of rotation, Maxwell's results, as deduced from the theory, are 

 in close accordance with the experimental laws given by Yerdet. 



The deductions drawn by Maxwell from his theory of Molecular 

 Yortices were of so striking a character that it was impossible his mind 

 could rest till he had still further examined their truth. Accordingly 

 we find that in less than three years after their publication he presented 

 to the Royal Society a memoir, entitled " A Dynamical Theory of the 

 Electromagnetic Field." This, the most splendid of all his papers* 

 places the theory of action through a medium on broader dynamical 

 principles, and traces the connexions between the various charac- 

 teristic quantities of the subject, twenty in all, more simply and more 

 directly than he had formerly done. No adequate idea of this memoir 

 can be given in a short notice, for it contains matter enough, and on so 

 many different branches of the subject, to furnish the texts of many 

 ordinary papers. The electromagnetic field is treated as a dynamical 

 system possessed of different kinds of energy and under the action of 

 various forces, the connexions between which are laid down with 

 admirable clearness. General mechanical principles are employed to 

 illustrate the significance of the laws discovered by Faraday, and from 

 this aspect and without the hypothesis of any particular mechanism, 

 the equations of the electromagnetic field are simply deduced. But 

 besides this there are many interesting side issues and expositions. 

 Among these may be mentioned an experimental method of deter- 

 mining coefficients of induction by the electric balance, a theory of 

 condensers, and the calculation of the self-induction and mutual induc- 

 tion of circular coils of wire. The part of the memoir, however, which 

 excites the liveliest interest, is the investigation of the electromagnetic 

 theory of light. The two grand results previously established by the 

 theory of vortices are again proved, but without the aid of somewhat 

 doubtful hypotheses, while in addition a very substantial confirmation 

 of Maxwell's views is obtained by the deduction of an equation for the 

 velocity of wave propagation of electromagnetic disturbance in crystal- 

 lised media identical with that found by Fresnel for light. Maxwell 

 published a few years later the results of some experiments he had 

 made to determine the ratio of the electric units. His number is 

 288,000,000. Sir William Thomson found 282,000,000. When we 

 compare these numbers with that of Weber and Kohlrausch, it cannot 

 be said that this quantity has yet been definitely determined. The 

 same is true of the velocity of light, the lowest value of which is 

 298,000,000 metres per second. We cannot, therefore, affirm with 

 certainty that Maxwell's inference is correct, but there is a strong 

 prima facie presumption in its favour. 



