128 M. H. Brongersma on Double Refraction 



In a copious memoir*, Quincke briefly touches on this part 

 of Kerr's investigation. He says : — " In fact, plate glass and 

 bisulphide of carbon show, according to Dr. Kerr's observa- 

 tions, opposite electric double refractions; in my experiments 

 I found this confirmed for flint glass and bisulphide of carbon." 

 After the many negative results attained by others, it is be 

 regretted that Quincke has not described in detail his method 

 of experiment with glass, as in the other parts of his investi- 

 gation. 



Lastly, Grrunmachf did not succeed in observing the phe- 

 nomenon. 



After this I think I ought to communicate my own results, 

 and the more so since, according to them, Kerr himself has 

 not seen these phenomena in their whole extent. 



In a piece of common plate glass 14 centini. high, (3 centim. 

 broad, and about 1 centim. thick, two holes about 3 millim. in 

 diameter were drilled coaxially, parallel with the largest lateral 

 surface, so that their ends remained 5 millim. distant from one 

 another. Into each of these openings a small quantity of 

 mercury was introduced, and then thin-drawn-out glass tubes 

 were fixed therein with a mixture of shellac and wax, through 

 which fine copper wires 15 millim. in length were carried till 

 they reached into the mercury. They were then thickly 

 coated to their ends, together with a portion of the glass plate, 

 with the mixture above mentioned. Moreover the plate was 

 varnished, except at the two places which bounded the field of 

 view. 



Much care was necessary in doing this, in order to ensure 

 the possibility of obtaining a satisfactory difference of poten- 

 tial. The investigation had to be interrupted several times, 

 partly because the insulation was not sufficient, and partly 

 because the plate was perforated by electric sparks. 



The glass plate was set up midway between two nicols 15 

 and 11 millim. in diameter respectively, so that the incidence 

 of the light -rays was perpendicular, and the space between the 

 wires or electrodes occupied the centre of the field. The light 

 of an albo-carbon lamp (which was placed in a sort of Duboscq 

 lantern), before passing through the polarizing nicol, fell upon 

 a lens, so that the real image of the round aperture of the lan- 

 tern coincided with the centre of the plate. Afterwards, with 

 the aid of a second lens, mounted in front of the analyzing 

 nicol, a magnified image was observed by the eye: and it was 

 soon found that by this alteration the method was improved. 

 The electric charge was obtained by means of a Holtz machine, 

 with the conductors of which the copper wires were connected, 



* Wied. Ann. x. p. 537 (1880;. t Ibid. xiv. p. 110 (1881). 





