in Glass and Bisulphide of Carbon. 131 



II. 



In his subsequent memoirs* Kerr treats of double refrac- 

 tion of liquids produced by electrical influence. Rontgen has 

 repeated Kerr's experiments!- He made use of a larger nicol 

 than Kerr employed, and by so doing gained the important 

 advantage of being able to have a better view over the whole. 

 The results of the two investigations agreed in the main. 

 Only, with the horizontal and vertical position of the polari- 

 zation-planes of the nicol, Kerr observed no or only an 

 irregular light-phenomenon; while according to Rontgen's 

 observations the phenomenon with this position of the nicol 

 was complementary to that accompanying the first position. 

 Rontgen, however, appears not to have seen this phenomenon 

 in its entirety, as will be shown subsequently. 



In my experiments a square glass jar 5 centim. broad and 

 9 centim. high, filled with bisulphide of carbon, was employed. 

 In the centres of two parallel sides, apertures of about 3 cen- 

 tim. diameter were made, which were again closed by glass 

 plates 0"2 centim. thick. The jar was set up midway between 

 the nicols, upon an ebonite stand, so that the light-rays fell 

 perpendicularly onto the thin glass plates. Apertures were 

 also made in the centres of the two other sides, to admit the 

 electrodes. The latter were tightly screwed on copper wires, 

 which, inclosed in thin glass tubes, were fixed in the apertures 

 with sealing-wax. Each of these copper wires was connected 

 with one of the conductors of the Holtz machine. 



In order to avoid particles of dust, the bisulphide of carbon 

 had to be repeatedly filtered from a bottle into the experiment- 

 vessel and vice versa. Finally the vessel was filled with bisul- 

 phide of carbon from a third bottle. 



In the first experiment one electrode was a brass disk of 

 12-7 millim. diameter and 7*8 millim. thickness, the other a 

 sphere of 8'5 millim. diameter. The axis of the disk was hori- 

 zontal ; and its prolongation passed through the centre of the 

 sphere. The polarization-planes of the nicols were constantly 

 perpendicular to one another, and made an angle of 45° with 

 the horizon, and consequently also with the direction of the 

 lines of force in the centre of the field of view. As soon as 

 the machine worked, the phenomenon represented in fig. 5 e, 

 as nearly as possible, was observed. The middle of the field 

 was brightly illuminated, and most brightly in the immediate 

 vicinity of the electrodes, which is not given in the figure. 



* Phil. Mag. [4] 1. p. 446 (1875) ; [5] viii. pp. 85, 229 (1879) ; [5] ix. 

 p. 157 (1880). 



t Wied. Ann. x. p. 80 (1880). 



K2 



