238 Dr. J. Kerr. Experiments on a [Mar. 1,' 



reappears immediately after each of the sparks, increasing regularly 

 from zero as the potential rises, and then increasing and decreasing 

 quickly or slowly as the potential rises and falls quickly or slowly. 

 Even when the potential falls most rapidly, as in spark-discharge, 

 the direction of the backward jump is always evident to the eye; 

 otherwise the disappearance of the dislocation in that case is so very 

 quick that one would call it instantaneous. 



Very little need be said upon the optical theory of these phenomena. 

 What must be remembered is, that each of the sets of fringes (0) and 

 (e) is due to the interference of two such pencils as BP and CG re- 

 united in GE, the vibrations being horizontal in one pair of interfer- 

 ing pencils, and vertical in the other pair. With regard to changes 

 of refringent power which are due to mechanical disturbance, it may 

 be assumed that these are independent of the direction of the vibra- 

 tion : both pairs of pencils are therefore similarly and equally affected 

 at each instant, and the corresponding displacements of the two sets 

 of fringes are at each instant similar and equal, however irregularly 

 they may vary from one instant to another. It is otherwise with the 

 bi-refringent action of the medium ; for here the two pairs of pencils 

 are differently affected at each instant, and the difference is deter- 

 mined solely by value of potential, so that the corresponding effect 

 comes out steadily in the midst of all the irregular changes which are 

 produced by mechanical disturbance of the dielectric. 



I think it must be admitted that in this regular dislocation of the 

 fringes there is a new and clear presentment of the double refraction 

 which is produced by electric strain. I think also that the new effect 

 is made all the more suggestive by the regularity and perfect steadi- 

 ness with which it comes out in the midst of the disturbance. 



First Appearance of the Law. Before leaving the present experi- 

 ments I must notice one or two facts observed, but not yet mentioned, 

 that go towards a solution of the question with which we started. 

 The phenomena to which I refer were seen clearly enough in some of 

 the earlier experiments ; but it was only at a later stage that they 

 were well understood, and they were then obtained more regularly. 



Beginning with the last form of the experiment that with the 

 rhomb of Iceland spar as eye-piece. The spar, I should mention, 

 was always so placed that the plane of polarisation in the set of 

 fringes (e) was vertical. What I have to notice now is a peculiar 

 feature of the jump of the fringes at the instant of discharge. To a 

 carefully strained, as to an unstrained, attention, this jump appeared 

 as a movement of the set of fringes (e) down to the level of the set 

 (6), never as a movement of the set (8) up to the level of the set (e). 

 I must say, however, that the accuracy of this perception or judgment 

 was to myself in some degree doubtful, not because of any expectation 

 that could have led to it, but because of the very fugitive character 



