386 Dr. John Kerr on a Fundamental 



shown in the diagram, which is about three fourths of the 

 fringe-width, is not much below the highest that can be got 

 with the apparatus. The direction of the dislocation is con- 

 stant, and indicates a relative retardation of that vibration in 

 the electric field which is parallel to the line of force ; and 

 this agrees with the known character of the medium CS 3 as 

 a positive dielectric. 



It is very interesting to watch the two sets of fringes (8) 

 and (e), and to see them sometimes moving rapidly and 

 very fitfully, but moving always as one system, with its two 

 parts dislocated unchangingly, except so far as the extent of 

 the dislocation varies with varying potential. It is equally 

 interesting to see the effect of spark-discharge of the prime 

 conductor, especially from high potential. At the instant of 

 the spark there is a sudden disappearance of the dislocation, 

 an extremely quick jump of the fringes into line with each 

 other, and this without perceptible check or sudden change of 

 any kind in the disturbance — motion common to the two sets 

 at the time. The best way of observing the effect is to take 

 sparks from prime conductor to earth at stated intervals, 

 while the machine is kept working at some constant rate. 

 The dislocation then 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 (8) and (e) is due to the interference of two 

 such pencils as BF and CG reunited in G-E, the vibrations 

 being horizontal in one pair of interfering 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 

 vibration : both pairs of pencils are therefore similarly and 

 equally affected at each instant, and the corresponding dis- 

 placements 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 birefringent action of 

 the medium ; for here the two pairs of pencils are differently 

 affected at each instant, and the difference is determined 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. 



