of a Charged Franklin'* s Plate. 3G7 



the paste had dried thoroughly, the plate was covered with a 

 coat of thick lac-varnish all over, with exception of the polished 

 ends and the conducting tongues. Another coat of the varnish 

 was applied afterwards ; and the operation was repeated more 

 than a dozen times, proper intervals of time being allowed 

 for the drying of the varnish. When all was done, the tin 

 coating was dimly visible in a good light, and the envelope of 

 lac appeared to be perfectly continuous. 



The plate was placed edgeways on the top of a fixed pillar 

 of glass, and was attached to the pillar by a few drops of 

 melted lac. Two perforated balls of brass, supported on 

 separate pillars, were brought up to the plate at the centres of 

 the two coatings; and the conducting tongues, passing into 

 the perforations of the balls, were in good permanent contact 

 with the brass. Through these balls the two coatings could 

 be put into and out of connection with prime conductor and 

 earth, and without any pressure on the plate. Two small 

 screens of thin vulcanite were attached lightly to the plate on 

 opposite sides of it, so as to protect the eye from all useless 

 light. 



Final State of the Best Plate. — The insulating power was 

 not great. When one coating of the plate was connected w^ith 

 prime conductor and the other with earth, the spark-length 

 between prime conductor and knuckle was at first about one 

 sixth of an inch ; but afterwards, when the lac had hardened 

 better on the plate, and in favourable weather, the spark- 

 length was fully a quarter inch. Spark and shock were of 

 course a good deal heavier than from the prime conductor 

 unconnected. 



The optical action of the best plate (still uncharged) was 

 not sensibly different from what had been shown already in 

 the process of testing. The sensibility of the ray under the 

 action of the hand-compensator was indeed very satisfactory, 

 considering that the light had passed through more than four 

 inches of glass. 



Method of Experiment. — In the simplest mode of working, 

 the pieces were placed on the table in horizontal line and in 

 this order : — paraffin flame, screen with narrow and short 

 vertical slit, first Nicol with principal section at 45° to the 

 vertical, the Franklin's plate with coated faces vertical, and 

 terminal faces perpendicular to the incident pencil, fixed com- 

 pensator, a little space for the hand-compensator, second Nicol 

 at extinction. The axis of the small transmitted pencil of 

 light was equidistant from the upper and lower edges of the 

 coatings. In another arrangement, the collimator and lunette 

 of a spectroscope were used, the lunette behind the second 



