390 Dr. John Kerr on a Fundamental 



the line of reference (generally downwards) by a small screw- 

 movement of the spar S ; and then, at every spark from 

 prime conductor to earth, there was a quick downward jump 

 of the fringes, the effect being as distinct as possible from the 

 irregular and slow and generally small movements that went 

 on before and after the spark. As the experiment proceeded, 

 the liquid was more thoroughly mixed, the disturbance de- 

 creased, and the effect came out much more purely. Sparks 

 were taken repeatedly at every 3rd, 5th, 10th, 15th turn of 

 the plate ; and the jump was there in every instance, and 

 beautifully distinct. The extent of the jump varied from 

 about 3 of the fringe-width at every 5th turn of the plate, to 

 about f at every 10th turn. I should add that the disturb- 

 ance-movements, though they were greatly reduced at last, 

 were still such as to prevent any good static observation of 

 the fringes. 



It is proved clearly by this set of observations, that when 

 the plane of polarization is perpendicular to the line of force, 

 the light is absolutely retarded by electric strain. The spars 

 R and S were now turned round LE through 180°, and the 

 pieces were moved across the optic bench into good position. 



Second Set. — Plane of polarization of the pencil BF (through 

 the electric field) horizontal : rise of the fringes indicates a 

 relative retardation of that pencil. The method was the same 

 as in the first set, sparks being taken from prime conductor 

 to earth at regular intervals long and short. When the 

 initial disturbance was over, movements of the fringes were 

 still seen, sometimes in one direction and sometimes in the 

 other, but not exclusively or specially at the instant of dis- 

 charge. These disturbance-movements were slow and gene- 

 rally small ; and as the experiment proceeded they became 

 very faint, and were occasionally not seen at all for a little 

 time. As to the effect specially looked for, I need only say, 

 that in several scores of observations, taken at different poten- 

 tials 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 polarization is parallel to 

 the line of force, the light is neither retarded nor accelerated 

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

 through 180°. 



Third Set, the same again as the first. Many observa- 

 tions were taken, and the former effects were obtained regu- 

 larly ; 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 improved insulation. 



