160 Dr. J. Kerr on Measurements 



all the pieces that lie within the cell are cleaned with the 

 greatest care. The lower conductor is inserted in the central 

 slab, and has its connecting-wire drawn up through the proper 

 boring; and the cell-block is completed by the addition of the 

 outer slabs. The lateral supports of glass are then put in 

 position, also the upper conductor and the external knob con- 

 nected with it. The conductors are adjusted stably, with their 

 opposed faces exactly parallel. All the pieces are now cleared 

 of dust as thoroughly as possible, and the cell is closed. 



The liquid is introduced rapidly into the large boring, 

 through filtering-paper and funnel properly placed. To pre- 

 vent the formation of air-bubbles between the two conductors 

 during the charging of the cell, it is necessary to have the 

 plane faces of the conductors inclined a little to the horizon ; 

 and this is done by the temporary insertion of a thin slip of 

 wood under one end of the cell-block. The charging is con- 

 tinued until the liquid stands well above the balls of the upper 

 conductor; but the cell is never quite filled, as that might 

 result in a very inconvenient explosion. When the borings 

 are now stoppered and the cell firmly closed by screw-pres- 

 sure, there is very little loss of the liquid by evaporation and 

 leakage. 



Sometimes, in spite of all precautions, the liquid is found to 

 be not sufficiently clean, a quantity of solid particles gathering 

 in chains between the electrified conductors, and masking the 

 optical effect in some degree, or even discharging the upper 

 conductor at comparatively low potentials. In such a case 

 the charge is drawn off (as much of it as is possible) by the 

 siphon; the cell is opened, and the faces of the conductors are 

 swept clear of dust, which is done well by friction with narrow 

 slips of clean writing-paper. The cell is then closed and 

 charged anew. 



These operations in preparation for experiment are some- 

 times very troublesome and tedious, a great deal more so than 

 in the case of the old plate cell. Partly on this account, and 

 partly because of the troublesome and expensive character of 

 the workmanship, I consider the present cell very imperfect; 

 but these and other faults are forgotten in the presence of the 

 following phenomena. 



4. Electro-optic Experiment in Chromatic Polarization. — 

 Here, as in all the following experiments, the cell is charged 

 with rather less than a pint of clean carbon disulphide. No 

 other optical pieces are employed at present than the charged 

 cell and a couple of Nicol's prisms. 



A beam of light from a bright cloud is reflected horizontally 

 into the room through a port in the window-shutter, passes 



