238 Dr. J. Kerr's Electro-optic 



The numbers for the first three liquids are probably exact ; 

 those for xylol and the following liquids are doubtful, though 

 certainly not far from the truth. It is worthy of notice that 

 the potentials for carbon dichloride are to the optically equi- 

 volent potentials for carbon disulphide almost exactly as 2 

 to 1. The corresponding ratios for cumol as compared with 

 carbon disulphide are also very nearly equal to each other, 

 their values being 1*41, 1*41, 1*38, 1*35. We infer that opti- 

 cally equivalent potentials in carbon disulphide, cumol, carbon 

 dichloride are very nearly as the numbers 10, 14, 20. The 

 last line of numbers contains the results of a particular expe- 

 riment (apparently a very accurate one) with benzol, in 

 which the arrangements were so made that the restoration in 

 the polariscope was in the form of a star or bright fringe on 

 the contour of the positive ball. By a comparison of the 

 numbers with those above them, it will be seen how r great are 

 the variations that may be induced in the optical effect by a 

 small change of conditions, and how great is the care that 

 must be taken to secure any thing like consistency of results 

 in this line of work. 



Optical Effect of Electric Discharge through Nitrobenzol. 



51. Experiment. — The diagram shows all the pieces in ho- 

 rizontal section through the source of light L and the obser- 

 ver's eye E. 



E- 



M 



H" 



Earth -Qjg» 



The cell D F is charged with nitrobenzol; and its outer balls 

 D and F are connected by wires, one with the prime conduc- 

 tor and the other with earth. One of the wires is interrupted, 

 by an air-interval about half an inch long, limited by two brass 

 balls. A and B, which .are supported upon movable pillars of 

 varnished glass. The light from a paraffin flame L passes 

 through a horizontal slit (about an eighth inch wide) in a 

 screen P Q, then through the first Nicol M, which is fixed 

 with its principal section at 45° to the horizon, then through 

 the plate of nitrobenzol, then through the second Kicol 1ST. 

 The object seen in the polariscope is a narrow and very well 

 outlined luminous oblong, lying along the axis of the electric 

 field, and extending nearly from ball to ball. 



