of a Charged Franklin's Plate. 369 



and then the desired effect was obtained by a small diminution 

 of that horizontal tension. In any case, the permanent initial 

 restoration required in this experiment, when tested by the 

 hand-compensator, was clearly strengthened by a feeble 

 horizontal compression, and clearly weakened by a feeble 

 horizontal tension. When these conditions were fulfilled, 

 the ' light was regularly and very clearly strengthened by 

 charge of the plate, the intensity rising and falling gradually, 

 — before and after discharge — as in the first experiment. 

 Under adverse conditions, the insulation bad, or the plate 

 optically imperfect, this experiment succeeded a good deal 

 better than the first, the effeGt being, as it ought to be, 

 distinctly stronger. 



Third Experiment. — The light was restored faintly from 

 extinction, as by horizontal tension of the fixed compensator ; 

 and this initial effect was seen to be properly affected under 

 the action of the hand-compensator, clearly weakened by a 

 feeble horizontal compression, and clearly strengthened by a 

 feeble horizontal tension. The electric force being applied as 

 formerly, the changes now observed in the polariscope were 

 contrary to those obtained in the second experiment, and 

 apparently of equal range. During charge the light was 

 weakened, its intensity falling regularly and very clearly for 

 a second or two, and then, after discharge, returning to the 

 initial value at much the same rate. In this experiment, as 

 in the other two, the intensity of the effects was very mode- 

 rate at the best ; but there w T as no uncertainty or indistinct- 

 ness about the results, except when the conditions were known 

 to be very unfavourable. 



I have referred already to the earliest known instance of 

 electro-optic double refraction. In that instance, the electric 

 field lay between and around the ends of two straight col- 

 linear wires, at the centre of a block of glass, the thickness 

 of glass between the terminals being (as in the present experi- 

 ments) about J of an inch, and the thickness traversed by 

 the ray f of an inch. It would be difficult to realize an elec- 

 tric field much more unlike that of the Franklin's plate ; and 

 yet the optical effects obtained in that case were of precisely 

 the same kind as those obtained in the present experiments, — 

 negative double refractions with reference to line of electric 

 force as axis. It appears thus, that the fact of electro-optic 

 double refraction in glass is not dependent on form of the elec- 

 tric field, or mode of distribution of the electric forces. 



There is one thing brought out by the preceding compari- 

 son which is at first sight not satisfactory. In the Franklin's 

 plate as compared with the old dielectric, there was a con 



Phil. Mag. S. 5. Vol. 20. No. 125. Oct. 1885. 2 D 



