and Law in Electro-optics. 159 



When the cell is charged with carbon disulphide, and well 

 closed by screw-pressure on the planks, it is almost perfectly 

 tight, the trifling leakage being due to a flaw in the workman- 

 ?• -j "? G 4 ' inc]l P late of powerfully refractive and dispersive 

 liquid thus obtained is perfectly transparent; and a beam of 

 light, incident normally on either of the limiting panes, passes 

 through the plate without sensible traces of dispersion, devia- 

 tion, or depolarization. The absence of all cement in the con- 

 struction of the cell is, I think, a most important condition. 

 liie liquid is in contact with nothing more than compact 

 masses of metal and the glass walls of the cell ; and accord- 

 ingly, with a fair start, it may be kept sufficiently clean for 

 any length of time. 



The shaded pieces in the diagram represent the two con- 

 ductors. The lower conductor is a thick plate of solid brass, 

 which rests on the floor of the cell. The upper is also a plate 

 of solid brass, of the same length and thickness as the lower, 

 but much narrower: it is supported by a small rafter of thin 

 plate glass at each end, rafter and conductor being connected 

 by a bullet-headed screw, which passes freely through a fine 

 hole drilled m the rafter, and works deeply into the conductor, 

 -bach of the rafters rests on the two trapezoidal pieces shown 

 in the diagram. These are prisms of glass taken out of one 

 thick plate, the horizontal surfaces (upper and lower) being 

 primitive plate-faces kept intact. These lateral supports rest 

 simply on the brass floor, and extend from front to back of 

 the cell. 



The opposed faces of the two conductors are perfectly flat 

 and smooth, just as they left the planing-machine : they are 

 also sensibly parallel, and generally distant from each other 

 about one twelfth of an inch. All' edges and corners on the 

 upper conductor have been carefully rounded away, as well as 

 the two adjacent edges on the lower conductor. The length 

 of the conductors (at right angles to the plane of the diagram) 

 is such as to leave the brass barely short of contact with the 

 limiting panes. 



The dotted lines in the diagram represent three borings in 

 the central slab. The vertical boring admits a glass tube, 

 within which passes a fine shaft of brass, resting below in a 

 small cavity made in the upper conductor, and terminating 

 outwards m a perforated knob and connecting-screw. The 

 smaller boring on the right gives passage to a wire from the 

 lower conductor; the larger boring on the left is used for 

 charging the cell with liquid, and for emptying it, the latter 

 ° P | ra TTr n beiDg effected b J means of a small siphon. 



S. Working of the Cell.—The slabs, the closing-panes, and 



