262 Dr. J. Kerr. Experiments on a [Mar. ] 5, 



of observations, taken at different potentials, high and low, there was 

 not a trace observed of a regular jump of the fringes at the instant of 

 discharge. It appears, therefore, that when the plane of polarisation 

 is parallel to the line of force, the light is neither retarded or accele- 

 rated by electric strain. The spars R and S were now turned back 

 through 180. 



Third Set. The same again as the 6rst. Many observations were 

 taken, and the former effects were obtained regularly ; but they were 

 now more striking, because of the strong contrast with the negative 

 results of the set of observations immediately preceding. The action 

 appeared also to be stronger than before, probably because of im- 

 proved insulation. The extent of the jump, taken at every 5th turn 

 of the plate, was now half the fringe-width ; and at every 10th or 

 15th turn it was clearly four-fifths. 1 find in my notes that this 

 large jump of the fringes impressed me here, again and again, as a 

 thing peculiarly beautiful. 



Fourth Set. The same again as the second. The only question in 

 this case was, whether it might still be possible, by the most careful 

 work and under the best conditions attainable, to detect a very small 

 jump of the fringes at the instant of discharge. Many observations 

 were taken at high potential, some at the highest, but without a 

 trace of effect of that kind. 



Fifth Set. The same again as the first. The results of the first and 

 third sets were recovered regularly. Sparks were then taken, some- 

 times at every turn of the plate, sometimes oftener. At every spark 

 there was a very small downward jump of the fringes, so small 

 sometimes as to be barely caught, but quite regular and beautifully 

 distinct. 



Remarks. The jump of the fringes was chosen as the principal 

 object of observation, because it was never quite concealed, nor even 

 much obscured, by the mechanical disturbance of the liquid ; but I 

 should add that the contrary motion the gradual ascent of the fringes 

 during the process of charging was generally evident enough in the 

 experiments, though not often undisturbed or quite regular in its 

 course. 



The best observations were got when the fringes happened to con- 

 tinue at rest through a sensible interval of time, including the instant 

 of discharge. The contrast between the two cases was then very 

 remarkable, especially at high potential ; in the one case, the beauti- 

 fully clear jump so often mentioned; in the other case, no trace of 

 a jump in either direction, generally not even a perceptible shiver of 

 the fringes at the instant of strongest discharge. Instances of this 

 kind occurred not very arely in the experiments ; and there could 

 be no contrast more striking than that between the phenomena in 

 the two cases. 



