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XXXVII. On Maxwell's Theory of Light. By J. J. Thom- 

 son, B. A., Scholar of Trinity College, Cambridge*. 



rriHE theory that light is an electromagnetic disturbance 

 J- propagated in the medium which transmits ordinary 

 electromagnetic action, and the supposition on which the 

 theory depends (viz. that the variation of the electric displace- 

 ment in a dielectric produces the same electromagnetic effect 

 as an ordinary current), seem first to have been stated by 

 Maxwell, in his paper on the " Electromagnetic Field" (Phil. 

 Trans. 1848). In the ' Treatise on Electricity and Magnetism' 

 two chapters are devoted to the theory, one of them being 

 entirely on the rotation of the plane of polarization. These 

 appear to constitute the literature of the subject. The object 

 of the present paper is, by taking into account the motion of 

 the medium through which light is passing, to obtain equa- 

 tions a little more general than those used by Prof. Maxwell, 

 and to develop some of the consequences of the theory. 



Let (F, G-, H), (a, b, c), (P, Q, K) be the x, y, z components 

 of the vector potential of electric induction, of magnetic in- 

 duction, and of electromotive force respectively at the point 

 (#, y, z), in a medium whose specific inductive capacity is K, 

 and whose coefficient of magnetic induction is /j, ; then (Max- 

 well's i Electricity and Magnetism/ vol. ii. § 598) 



dt dt dt dx 



f>— dz dx _^dG __d^r 



dt dt dt dy 



■r i _-fdx dy dH dyfr, 



dt dt dt dz 



~~ dy 



dG 

 -^z> 



&- — - 



■ dz 



dR 



dx 



_dG_ 

 dx 



dy ' 



da db 

 dx dy 



4:=°- 



Now the electromotive force (P, Q, R) acting on the dielectric 

 will polarize it. Let <f> be the potential due to the polariza- 



* Communicated by the Author. 



