364 Dr. J. Kerr on the Electro-optic Action 



required ; but on looking into M. Wiedemann's treatise, now 

 in course of publication, I have been led to believe that a 

 short paper on the subject may be of some use. 



In his opening remarks on Electro-Optics*, M. Wiedemann 

 draws attention to the experiments tried on Franklin's plates 

 by MM. Quincke, Gordon, Mackenzie, and Grassman ; and 

 he seems to accept the absence of effect in these experiments 

 as decisive of the question of action or no action, in a uni- 

 formly charged plate of glass. Accordingly, when he comes 

 to the earliest known instance of electro -optic double refrac- 

 tion (the effect of intense electric stress at the centre of a 

 block of glass, which I discovered in 1875), he introduces mv 

 experiment as an example of a non-uniform and optically 

 active electnc field, in contrast with those other experiments f , 

 as examples of a field sensibly uniform and optically inactive. 

 Afterwards, at the end of his exhaustive account of the whole 

 subject, M. Wiedemann returns to the same point, and makes, 

 in substance, the three following statements : — 



(1) When the dielectric is a solid (a block of glass), and 

 the distribution of electric force is not uniform, there are 

 certain strains known to be produced (simple expansions, 

 which are greatest at the place of shortest lines of force) ; 

 and these known strains may account for the observed 

 effects. 



(2) There appears to be no electro-optic double refraction 

 in the case of a uniformly charged Franklhrs plate. 



(3) But if one of the coatings be much reduced in size, 

 or if we use, instead of it, a little mercury, contained within a 

 small ring of glass, which is cemented to the upper surface 

 of the plate at its centre, then the charging of the plate gives 

 rise to a strain (a simple expansion, such as that produced by 

 local rise of temperature) in the part of the plate immediately 

 between the coatings, and a ray which passes through the 

 plate and between the coatings is doubly refracted of course. 



For some additional matter on the subject, not required 

 here, I must refer to M. Wiedemann's treatise. The prece- 

 ding statements give a correct view of a theory of the pheno- 

 mena, which was advanced some years ago by M. Quincke ; 

 ihe first statement giving his explanation of my experiment 

 already referred to, and the third giving an explanatory 

 account of a subsequent experiment of his own. My principal 

 objection to this theory has always been, that it ignores 



* Die Lehre von der Electricitiit, vol. ii. p. 125. 



t All published subsequently to mine, and all, I believe, with explicit 

 reference to it ; though Prof. Quincke's experiment was performed some 

 ten years earlier. 



