448 Dr. J. Kerr on a new Relation between 



forated as in prolongation of the tunnel, or rather a little more 

 widely, so as to allow distinct vision through the whole cell 

 along the length of the tunnel. When the press is closed, the 

 cell is perfectly tight. The surface of pillars and block is well 

 varnished with lac, except the spaces covered by the closing- 

 plates, which are kept clean. The outer ends of the borings, 

 where the wires leave the block, are well closed with sealing-wax. 

 The cell is filled with liquid either through the central 

 boring, already mentioned, or otherwise — more easily in many 

 cases through a partially closed mouth of the tunnel ; it is 

 easily emptied by unscrewing the press, and allowing the liquid 

 to run off upon one of the glass plates, loosened and properly 

 directed. The only inconvenience (and a very serious one) which 

 I have noticed in the working of this piece, is the difficulty 

 of cleaning the cell perfectly when charging it with a new 

 liquid. 



26. General Form of the Experiments. — The plate- cell is 

 charged with clean liquid. The electric force is obtained some- 

 times from the inductorium, but generally from an electric ma- 

 chine. In the former case the wires from the cell are connected 

 with the terminals of the secondary coil ; in the latter case one 

 of the wires is inserted in the prime conductor, and the other is 

 connected with a good discharging train. A horizontal beam 

 of light, emitted by a narrow paraffin-flame, passes first 

 through a polarizing Nicol, then through the plate of dielec- 

 trified liquid, and then through a second Nicol, which acts as an 

 analyzing eyepiece. The eye is generally so placed as to see the 

 flame through the centre of the cell, midway between the induc- 

 tion-terminals. The light traverses the liquid in a direction 

 perpendicular to the closing-plates of the cell, and therefore per- 

 pendicular to the lines of electric force. 



It will be observed that the arrangements and procedure are 

 much the same as they were in the former experiments, with the 

 dielectric of plate glass. The transparent dielectric is now a 

 plate of liquid ; and the electric forces applied are much less in- 

 tense ; but there is no other material change. 



Bisulphide of Carbon. 



27. The plate-cell is charged with bisulphide of carbon; the 

 wires from the cell are connected with the knobs of the secon- 

 dary coil ; and all the other arrangements are as indicated 

 above. The compound plate of thin glasses and bisulphide 

 (placed in almost any light and towards any object) is as purely 

 transparent to the eye as so much air ; and when it is intro- 

 duced properly between the two Nicols at extinction, it gives not 



