and Law in Electro-optic*. 161 



through the first Nicol, then perpendicularly through the plate 

 of liquid, then through the ocular Nicol. The pieces are so 

 levelled and directed, that the observer at the polariscopo 

 looks fairly through the deep slit that separates the two con- 

 ductors in the cell. I may mention again that the dimensions 

 of the slit are about one twelfth of an inch, one inch, and four 

 inches — the first dimension lying vertically as the lines of 

 force, and the last lying horizontally along the line of sight. 

 The two Nicols are fixed, the first with its principal section at 

 45° to the vertical, and the second at extinction, which is here 

 quite pure. Wires are led permanently from the two conduc- 

 tors — from the lower to earth, and from the external knob of the 

 upper to the prime conductor. To give greater steadiness and 

 distinctness to the progress of the optical effect, the wire from 

 prime conductor to cell is put in permanent contact with the 

 knob of a Leyden jar, whose outer coating is uninsulated. 



When the machine is set in motion at a moderate rate, the 

 potential of the upper conductor rises slowly, and the black 

 space between the two conductors is illuminated, the light 

 passing gradually through impure black, faintly bluish grey, 

 faint white, and so forward, up to a sensibly pure and brilliant 

 white. Thus far there is nothing new, except that the highest 

 potential yet reached is comparatively low, while the optical 

 effect is very large, and already far beyond neutralization by 

 the action of any hand compensator of strained glass. 



As the potential of the prime conductor still rises, the pola- 

 riscope gives a fine progression of chromatic effects, which 

 descend regularly and continuously through a certain range of 

 Newton's scale. The luminous band between the conductors 

 passes first from white to a bright straw-colour, which deepens 

 gradually to a rich yellow, then passes through orange to a 

 deep brown, then to a pure and dense red, then to purple 

 and very deep violet, then to a rich and full blue, then to 

 green. All the colours are beautifully dense and pure, cer- 

 tainly as fine as any that I have ever seen in experiments with 

 crystals in the polariscope. 



Generally about the point last named of the scale of colours, 

 at or near the green of the second order, the process termi- 

 nates in spark-discharge through the liquid. Sometimes, but 

 not frequently in my observations, it terminates at an earlier 

 stage, to run its regular course at the next trial. The irregu- 

 larity appears to be due to an accidental precipitation of dis- 

 charge by the action of solid particles, impurities in the liquid. 



Through this whole range of effect, from the pale blue or 

 impure black of the first order to the green of the second 

 order, the plate of electrically-charged liquid acts regularly as 



