and Law in Electro-optics. 163 



When the machine is stopped suddenly, the tints disappear at 

 once and the optical effect falls very rapidly to faint white. 

 The subsequent rate of degradation, through faint white to 

 pure extinction, varies very much with circumstances. In 

 damp weather the light disappears in one or two seconds, or 

 less ; but on several occasions, the weather being favourable, 

 and the external conductors very well insulated, I have seen the 

 light sustained clearly for five and even for six minutes. This 

 continuously sustained effect was certainly due to the present 

 action of electric force at each instant ; for it disappeared 

 sharply and totally at the instant of spark-discharge of the 

 prime conductor, and then, a few seconds after the spark, it 

 was restored quite distinctly by the action of the residual 

 charge of the jar. 



The Leyden jar is now withdrawn, and the prime conductor 

 is connected simply by a wire with the upper conductor in the 

 cell, the procedure and circumstances of the observation being 

 otherwise unaltered. There is nothing new observed in this 

 case, except a more rapid rate of degradation of the optical 

 effect. In favourable circumstances I have seen the effect 

 still sustained clearly for a good many seconds — 40 to 60. 



The permanent wire from cell to prime conductor is now 

 detached from the cell, and its extremity is fixed in a small 

 ball of brass, which is held by an insulating handle of glass. 

 The polariscope being at extinction, and the machine in mo- 

 tion, the ball connected with the prime conductor is brought 

 up towards the knob of the upper conductor in the cell, till 

 one small spark passes. At the instant of the spark there is 

 a strong restoration in the polariscope. The effect degrades 

 at first very rapidly like a flash, but afterwards more slowly, 

 so that it is still seen distinctly, though at last very faintly, 

 for 12 or 15 seconds. And here, as in the former cases, the 

 sustained effect is certainly due to the present action of electric 

 force at each instant ; for it vanishes instantaneously when the 

 knob of the upper conductor is touched by the finger. 



There are several points proved or illustrated by these ob- 

 servations : — that the birefringent action of the liquid is due to 

 electrostatic force, and not to electric discharge in any form ; 

 that the dissipation of the charge of electricity in damp 

 weather is principally from the external parts of the appa- 

 ratus, as was to be expected; that within the cell there is 

 always a sensible discharge of electricity as long as the two 

 conductors are at different potentials ; and that the rate of this 

 discharge or dissipation through the liquid is very great at high 

 tensions. I proceed now to the measurements. 



7. Instruments of Measurement. — To measure potential and 



