132 M. H. Brongersma on Double Refraction 



Two black tails issue from the sphere, at points the radii be- 

 longing to which make an angle of 90°, which is bisected by 

 the axis of the disk. Two other black tails issue from the disk, 

 whose directions at the beginning likewise form an angle of 

 45° with the horizon. A glass plate placed before the analy- 

 zing nicol, upon the centre of which the rays fell perpendicu- 

 larly, had, with an extremely gentle horizontal pressure, the 

 following influence upon the phenomenon: — With slowly in- 

 creasing difference of potential, first a small black bow, nearly 

 in the form of a semicircle, with its centre in the point of the 

 surface of the spherical electrode where the prolongation of the 

 disk-axis cut that surface, was observed. Further, this bow 

 divided into two branches connecting two points of the sphere 

 with two points of the disk. These branches receded con- 

 stantly further from one another till at length fig. 5 e again 

 came into view. If the glass plate was pressed somewhat 

 more forcibly, the two first-mentioned phases of the pheno- 

 menon, and with still stronger pressure the first only, namely 

 the black bow, were observed. If the pressure was still further 

 enhanced, the entire field remained uniformly illuminated, 

 even when the potential-difference was the greatest possible. 

 If, with a great potential-difference, the horizontal pressure 

 upon the glass plate is slowly increased, the same phenomena 

 follow in reverse order, from fig. be to the above-described 

 black bow, which is seen to become slowly smaller till at last it 

 vanishes. 



If on the repetition of the experiment without the com- 

 pressed glass plate the potential-difference is slowly increased, 

 the same transitions can be remarked, as it appeared to me, as 

 were observed when a gently compressed plate was employed; 

 only the first transitions were slight and less distinctly marked, 

 so that already with a proportionally slight difference of poten- 

 tial the phenomenon is seen as in fig. he, at first faint, but 

 gradually coming out sharp and clear. If the origin of the 

 black tails at the sphere be joined to its centre, the angle which 

 the lines thereby formed make with the horizon, on the poten- 

 tial-difference diminishing and without a horizontally com- 

 pressed glass plate, or with the potential-difference constant 

 and with increasing horizontal pressure upon the glass plate, 

 becomes smaller; but in no case, not even with the greatest 

 possible potential-difference, does this angle become greater 

 than 45°. 



If the glass plate was exposed to a vertical pressure, with 

 increasing pressure the tails issuing from the sphere and those 

 issuing from the disk removed further from one another, so 

 that the illuminated intervals at the sides of the electrodes 



