of a Charged Franklin's Plate. 371 



tion parallel to the lines of force ; and so forward uniformly 

 (with variations of intensity and sign, and with occasional 

 disturbance, mostly from known causes) through a list of 

 more than 120 different dielectrics, as I have proved by ex- 

 periments already published. The similarity of all these 

 optical actions implies a similarity of condition in all the 

 acting media. We infer that glass, resin, oil of colza, carbon 

 disulphide, and all those other dielectrics, when electrically 

 charged, are also specially and similarly strained. And it 

 seems to be quite consistent wdth proper caution, in the ab- 

 sence of opposing evidence, to extend this conclusion to all 

 dielectrics whatsoever. 



We are led thus by the facts of electro-optics, and inde- 

 pendently of all theory, to the conception and realization of 

 electrostatic strain, — a strain maintained by the action of elec- 

 tric force, or (at least) in exclusive and indissoluble connection 

 with that action, and sustained by the dielectric as an elastic 

 medium, — a strain of one type in all dielectrics, — a strain 

 essentially directional in liquids as in solids, — a strain sui 

 generis. And in all this, or rather in the facts themselves, 

 we have a remarkable confirmation of the theoretical views of 

 Faraday and Maxwell. 



Glasgow, 21st August, 1885. 



Postscript. — In the charged Franklin's plate, the glass is 

 compressed by the electric attraction of the tw r o coatings; 

 and it may be inquired how far this mechanical strain contri- 

 butes to the effect obtained in the preceding experiments. I 

 have had this question suggested to me by Sir William 

 Thomson, as too important and too obvious to be neglected, 

 and admitting also of a simple method of solution ; but I am 

 sorry that the best answer obtainable with my present means 

 is very vague and uncertain. 



When the old Franklin's plate is laid flat, compressed by a 

 weight uniformly distributed over it, and submitted to the old 

 test in the polariscope, I find that it requires a total weight of 

 1^ pound to give an effect equal to the old effect of electric 

 charge. This compressing force of 1\ pound must be much 

 greater than the attraction of the electrically charged coatings, 

 whose dimensions are about 2x3^ inches, their distance 

 \ inch, and their difference of potentials at the moment of 

 observation probably less than that corresponding to a spark 

 of £ inch in air. It seems, therefore, that the part of the 

 optical effect of electric charge which is due to compression of 

 the glass must be very small, if not insensible. I should men- 

 tion, however, that by a simple calculation, on the assumption 



2D2 



