Electricity and Light. 341 



soon, which exhibit this increase of effect as a reappearance from 

 extinction. 



10. Fourth experiment. — The same as the first, except that the 

 primary current, instead of being constantly in one direction, 

 is regularly reversed by the commutator at successive equal in- 

 tervals of time (say, every second). The optical effect is as good 

 as in the first experiment, if not better. 



11. Summary. — When plate glass is intensely dielectrified, and 

 traversed by polarized light in a direction perpendicular to the 

 lines of force, it exerts a partially depolarizing action] upon the 

 light, giving an effect which is much more than merely sen- 

 sible in a common polariscope. There is a good regular effect 

 when the plane of polarization is at 45° to the lines of force, no 

 regular effect when the plane of polarization is parallel or per- 

 pendicular to the lines of force. Electric force and optical effect 

 increase together. The optical effect of a constant electric action 

 takes a certain time (apparently about 30 seconds in my obser- 

 vations) to reach its full intensity, which it does by continuous 

 increase from zero ; and it falls again slowly to zero after the 

 electric force has vanished. There is as good an effect with a 

 rapid succession of contrary (Ruhmkorffian) electrizations as 

 with a continued (Ruhmkorffian) electrization in one direction. 



12. Optical Compensator. — Not having a regular instrument 

 of this kind, I supply its place by a simple slip of glass held in 

 the hands and subjected to varying stress. The action of strained 

 glass upon transmitted light has been exactly determined by ex- 

 periment. Compressed glass acts as a negative uniaxal crystal 

 with its axis parallel to the line of compression; stretched glass 

 acts as a positive uniaxal with its axis along the line of tension. 



Illustrative optical experiment. — All the pieces placed as in the 

 first experiment (2, 3, 4), the plane of polarization at 45° to the 

 horizon, the extinction in the polariscope perfect, and the dielec- 

 tric always unexcited. Two additional pieces are introduced into 

 the course of the beam — say, between the dielectric and the neu- 

 tralizing plate. The first is a small square of thin plate glass, 

 held edgeways in a vice with its surfaces perpendicular to the 

 beam, and feebly compressed in the direction of its length, which 

 is horizontal. When this piece is inserted, the light is well re- 

 stored from extinction. The second piece is the compensator — a 

 rectangular slip of plate plass, shaped like a common microscopic 

 slide, but generally larger. It is held by the two hands in 

 front of the neutralizing plate, with its surfaces perpendicular to 

 the beam, and its long edges horizontal ; it is gently bent by the 

 hands, the axes of the couples applied being perpendicular to the 

 plate-faces, so that (say) the upper parts of the slip are extended 

 horizontally, and the lower parts compressed; and it is lowered 



