in Glass and Bisulphide of Carbon. 129 



In the first experiment the planes of polarization of the 

 crossed nicols made an angle of 45° with the horizon (first 

 position). Without a glass plate there was, in the middle 

 of the field, a dark spot ; the borders of the field were not 

 completely dark. When the glass plate was inserted, there 

 appeared, principally in consecpience of the drilling, pheno- 

 mena of double refraction as in fig. 5 c. 



Soon after the Holtz machine was brought into action the 

 field changed : first the dark portion between the electrodes 

 divided into two parts; and two dark tails made their appear- 

 ance, which joined the electrodes to one another above and 

 below, leaving a strongly illuminated space of elliptical form 

 between them. Gradually these tails receded from each other 

 at one of the electrodes, the ellipse approaching more to a 

 parabola. Fig. 5a shows the phenomenon as it appeared 

 when the difference of potential was the greatest possible. 

 After about three minutes' powerful working of the machine, 

 the field underwent no further perceptible change ; the maxi- 

 mum action had therefore been attained. After a sudden 

 discharge the phenomenon vanished, at first rapidly, and then 

 more slowly; and within three minutes every thing was again 

 as before the experiment. 



Having placed before the analyzing nicol a plate of calc 

 spar polished perpendicularly to the axis, I saw, on repeating 

 the experiment, the black cross in the plate change into a hy- 

 perbola; so that the light must have been elliptically polarized. 



When, instead of the calc-spar plate, I took a vertically 

 compressed glass plate, and slowly increased the pressure, the 

 tails of fig. 5 a approached one another, at last they assumed 

 the above-mentioned elliptic shape, and with still stronger 

 pressure the phenomenon disappeared. 



If the calc-spar plate was now again placed before the ana- 

 lyzing nicol without any pressure taking place upon the 

 compensating glass plate, with increasing difference of poten- 

 tial the black cross, as remarked above, was converted into a 

 hyperbola. By gentle vertical pressure of the compensating 

 glass plate the hyperbola was again converted into the black 

 cross. 



If the polarization-planes of the nicols were directed hori- 

 zontally and vertically (second position), the phenomenon 

 appeared as in fig. 5 b. 



The phenomena of double refraction with the first position 

 were seen by Dr. Kerr incompletely ; and he did not succeed 

 in observing those which accompany the second position, 

 perhaps because his field of view was too small to take in the 

 whole. By placing the glass lens in front of the analyzing 



Phil. Mag. S. 5. Yol. 14. No. 86. Aug. 1882. K 



