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



the incident polarized pencil. The insertion of the plate 

 restored the light always from extinction ; and the fainter this 

 restoration, the more promising was the plate. The fixed 

 compensator, which stood between the Nicols, and immediately 

 after the plate, was then strained by trial, with or without 

 small rotation of the second Nicol, till good extinction was 

 obtained permanently ; and the feebler the strain thus required, 

 and the purer and sharper the extinction obtained, the more 

 promising was the plate. The hand-compensator was then 

 inserted between the fixed compensator and the second Nicol, 

 and supplied the last part of the test, which consisted in 

 observing the degree of strain (horizontal tension or com- 

 pression) which had to be applied, to give good restoration 

 from extinction. When the requisite strain was small or very 

 moderate, the plate was accepted as good, otherwise not. 



The most of the plates that I have had in hand were pure 

 failures under this test, and were therefore thrown aside as 

 evidently quite unsuited for the electro-optic experiment. Of 

 the set of six plates last started with, four were rejected with- 

 out hesitation, the fifth was very imperfect, but worthy of 

 trial, the sixth was good, though not perfect. Of another set 

 of plates, which w r ere only half as thick as the former, there 

 was none even moderately good ; but one of them was re- 

 tained, as in a small degree promising. The serious failure 

 was generally in the last step of the test, that with the hand- 

 compensator. When a good or very fair permanent extinc- 

 tion had been obtained by the joint action of the plate under 

 trial and the fixed compensator, I found often that it required 

 the exertion of almost all my strength on the hand-compensator, 

 to get anything like a good restoration in the polariscope. 



The light should have been well restored by a barely sensible 

 effort ; but, in its passage through such a length of irregularly 

 strained glass, it had lost ever so much of that sensitiveness 

 under bi-refringent action which constitutes the whole value 

 of the plane-polarized ray in any really delicate electro-optic 

 work. No clear optical effect of moderate electric stress 

 could be reasonably looked for in a plate such as that now 

 mentioned, which allowed only a very small effect, or a mere 

 trace of effect, to be given by a comparatively very strong 

 bi-refringent action of the hand-compensator. 



The plate chosen was coated with tinfoil in the usual way, 

 the margin left on each face all round being rather less than 

 a quarter inch. Projecting from the centre of each coating 

 there was a small conducting tongue, made of a bit of tinfoil, 

 which was folded several times on itself, opened out at one 

 end, and pasted securely by that end to the coating. When 



