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



Nicol, and the collimator (with very narrow vertical slit) before 

 the first Nicol, the light employed being either a sunbeam or 

 a beam from the bright sky. This arrangement served well 

 in the case of the very thin plate already mentioned, which 

 could not be managed well with the naked eye, because of the 

 confusion produced by internal reflections ; but for the larger 

 and better plates the first arrangement was preferred. 



The electric force was applied most conveniently as follows. 

 A Leyden jar stood on the table near the Franklin's plate ; 

 and the outer coating of the jar and second coating of the 

 plate were put permanently to earth. The jar was charged 

 by twenty to thirty turns of the plate-machine : a common 

 discharger, connected with the first coating of the Franklin's 

 plate, was brought up to contact with the knob of the jar, but 

 gradually, so as to avoid spark-discharge over the margin of 

 the plate; and when the contact had been maintained for about 

 three seconds, the connection was broken, and the plate was at 

 the same time discharged. In this way the optical observation 

 lasted only for the few seconds before and after discharge of 

 the plate, so that small changes of intensity of the light in the 

 polariscope were the more easily detected. 



First Experiment . — Particular attention was given to the 

 delicacy of action of the polariscope. The extinction was 

 made as pure as possible by proper strain in the fixed com- 

 pensator. The sharpness of the extinction was tested by the 

 hand-compensator, the requisite delicacy being obtained when 

 the light was restored clearly by feeble strains, horizontal 

 compression and tension successively. When these conditions 

 were fulfilled, the electric force, applied in the manner just 

 described, gave good restoration from extinction in the pola- 

 riscope, the light brightening gradually for a second or two, 

 and then, after discharge of the plate, falling back to extinction 

 at much the same rate. The intensity of the restored light 

 was never great, being equal to that given by a very moderate 

 compression in the hand-compensator. When the insulation 

 was very defective, or the plate optically bad, the effect was 

 very poor indeed, and could not always be seized with cer- 

 tainty; but when these unfavourable conditions were avoided, 

 the rise and fall of the light, from extinction and back to it, 

 were quite distinct and regular. 



Second Experiment. — The light was restored faintly from 

 extinction, as by horizontal compression of the fixed compen- 

 sator. I say as by horizontal compression, because the 

 action of the uncharged Franklin's plate in the polariscope was 

 sometimes equivalent to horizontal compression, and had to be 

 neutralized by horizontal tension of the fixed compensator ; 



