370 Dr. J. Kerr on the Electro-optic Action 



siderable increase of thickness of the dielectric in the direction 

 of the ray, and yet there was a large decrease of intensity of 

 effect. But if we assume, as we may fairly do, that the 

 quantity of optical effect varies as the square of the difference 

 of potential of the two coatings, we see that a moderate in- 

 crease of ray-length in the dielectric may be much more than 

 counterbalanced by a moderate decrease of difference of 

 potentials. Now in the old experiment and the new, the 

 ratio of ray-lengths in the dielectrics was about I to 6, while 

 the change of spark-length was from an average of 6 inches 

 to a maximum of \ inch, implying a large fall of potential. 

 I may even observe that the increase of ray-length in the 

 present experiments appears to have told clearly on the in- 

 tensity of the effect, because, in the old experiment, and in 

 the most favourable circumstances, hardly any effect could be 

 obtained with as little as an inch of spark-length. 



But, further, we must remember those permanent and irre- 

 gular strains which were sought to be eliminated in the pre- 

 liminary testing of the plates. The disturbing influence of 

 these strains may be expected to increase with every increase 

 of ray-length in the dielectric, and with every decrease of the 

 difference of potential. When the ray-length in glass 

 amounts to several inches, I believe that it is not possible to 

 get rid of this disturbance perfectly. In the best plates that 

 I have worked with, not only was the disturbance present, 

 but it always told seriously, weakening if not masking the 

 effect of any very small birefringent action to which the 

 transmitted ray could be subjected. Taking these things 

 into account, and viewing the new phenomena and the old 

 as cases of the same effect of electric force, I consider the 

 small effects now obtained with the Franklin's plate to be as 

 good as could fairly be expected, and better than could be 

 expected confidently ; and I therefore hold it proved by ex- 

 periment, that electrostatically strained glass acts in the polari- 

 scope as if compressed along the lines of electric force, and thus 

 always, whether the electric field is uniform or not. 



I conclude briefly with a simple and important argument, 

 which appears to be secured now against all reasonable ob- 

 jection. With reference always to resultant effect in the 

 polariscope, as characterized by (say) the hand compensator: — 

 Electrically charged glass acts as if compressed along the lines 

 of force ; electrically charged resin acts as if extended along the 

 lines of force ; electrically charged oil of colza, a negative di- 

 electric, acts precisely as glass compressed in a direction parallel 

 to the lines of force ; electrically charged carbon disulphide, a 

 positive dielectric, acts precisely as glass extended in a direc- 



