162 Dr. J. Kerr on Measurements 



a uniaxal crystal, as a plate of quartz with optic axis parallel 

 to the lines of force, the plate increasing in thickness continu- 

 ously and very rapidly as the potential rises. 



I may mention here, that with the old plate cell, under 

 favourable conditions, I obtain one step of the scale of colours, 

 that from white to yellow, but not easily nor very distinctly. 



5. By simple observation in connexion with this experi- 

 ment, the electro-optic field is seen to be sensibly uniform 

 through the greater part of the space between the two con- 

 ductors, and for potentials high or low. 



Beginning at very low potentials, for which the optical 

 effect lies below intense white, and applying a hand compen- 

 sator of glass, I obtain, as the first effect of stress in the glass 

 against electric force in the liquid, a couple of extinction- 

 bands in the outer borders of the field, just as in my old expe- 

 riments with the plate cell. As the stress of the glass, hori- 

 zontal tension or vertical compression, is increased, the bands 

 move inwards, till at last, as they pass through the borders of 

 the field, and enter the space between the parallel faces of the 

 conductors, they widen inwards and meet each other, coales- 

 cing in one wide and uniformly black band. 



At higher potentials, and using only the two Nicols, I ob- 

 tain equally distinct results. Thus, at one stage of the expe- 

 riment the band of light between the two conductors is of a 

 deep red, and sensibly uniform from end to end ; but at each 

 end, in the borders of the field, where the upper conductor is 

 rounded off, the band is tipped with a narrow fringe of bril- 

 liant yellow. Similarly, another stage of the experiment 

 gives a violet or blue band, sensibly of the same tint from end 

 almost to end, and tipped at each end with a narrow fringe of 

 dense red. 



As an important practical effect of the characteristics of the 

 new electro-optic field, its intensity and uniformity, I have 

 been able to discard the old and imperfect compensator of 

 strained glass. One such compensator that had been elabo- 

 rately prepared was found quite inadequate, the action of a 

 fine slip of glass (not an inch wide), under a compressing 

 weight of 50 pounds, being neutralized and far exceeded by 

 the action of the plate of CS 2 at low potentials. 



6. Rate of Disappearance or Degradation of the optical effect 

 after the stoppage of the machine. — Some observations were 

 made upon this subject at different times ; and the results are 

 worthy of brief notice in passing. The arrangements were the 

 same as in the preceding experiment. 



The light is restored from extinction up to one of the tints ; 

 and the effect is kept pretty constant for several seconds. 



