and Law in Electro-optics. 173 



It will be noticed that the differences between observed and 

 calculated values of Q do not extend, in any case, beyond the 

 range ± 1 of probable error of observation. With this Table 

 I close the experimental proof of the Law of Squares in the 

 actual case of dielectric CS 2 . 



In proceeding now to my last experiment, I may mention 

 that the rubber of my machine is insulated on pillars of glass, 

 as well as the prime conductor. The object in view is to com- 

 pare contrary electric forces. 



22. Experiment. — The arrangements are as described in (8), 

 all the optical pieces being placed as in the diagram. 



(1) As in (8), the prime conductor is connected by wires 

 with the cell, the electrometer, and the Leyden jar, while the 

 rubber is connected by a wire with earth. A convenient po- 

 tential is chosen ; the electrometer is set accordingly ; and the 

 screw of the compensator is worked as in the former experi- 

 ments, till the optical action of the dielectric is exactly neu- 

 tralized at the instant of the critical signal. In the final state 

 of the instruments, the black band lies exactly midway be- 

 tween the reference-wires of the compensator when the index 

 of the electrometer is exactly in the sighted position. From 

 now till the end of the experiment, the electrometer and the 

 compensator are left untouched. 



(2) The wires connecting prime conductor with cell and 

 electrometer and Leyden jar are detached from the prime con- 

 ductor, and fixed in the rubber ; and the wire connecting 

 rubber with earth is detached from the rubber, and inserted 

 in the prime conductor. The electro-optic observation is re- 

 peated, and the effects are found to be the same in kind and 

 quantity as before. As the electric force increases, the black 

 band moves gradually to the right ; and when the index of the 

 electrometer is exactly in the sighted position, the band lies 

 exactly midway between the wires. 



It appears thus, as might have been confidently expected, 

 that the optical effect of electric force is independent of the 

 sense of the force, being of exactly the same kind after reversal 

 of the force as before, equal also in amount for equal intensi- 

 ties of the force. 



23. In conclusion, I observe that the principal result of the 

 experiments, what I have called the Law of Squares, may be 

 correctly stated in several very different forms. The quantity 

 of optical effect, per unit of thickness of the dielectric, varies 

 either 



(1) Directly as the square of the resultant electric force, or 



(2) Directly as the energy of the electric field per unit of 

 volume, or 



Phil Mag. S. 5. Vol. 9. No. 55. March 1880. 



