108 Electrical Effect on the Double Refraction of Quartz. 
I come now to the experiments with the small quartz cy- 
linder. The axis of the cylinder is parallel to the principal 
axis. The cylinder is Oo centim. in height and 0*45 centim. 
in diameter, and has an exactly centrical perforation in the 
direction of the axis O08 centim. wide ; the end surfaces are 
polished. I examined first the piezoelectric behaviour of 
the cylinder, and marked the direction of the three axes of no 
piezoelectricity. Then the cylinder was cemented by its end 
faces with Canada balsam to two glass plates, each 4*5 centim. 
long and 1*5 centim. wide, each of which was provided with a 
perforation also 0*08 centim. wide. The perforation in the 
one plate coincided exactly with the perforation in the cylinder, 1 
the perforation in the other was at a distance of 2 centim. from 
that in the cylinder. The edges of the glass plates, which were 
parallel two and two, were then cemented together with strips 
of glass so as to form a glass box, which was filled with mer- 
cury through the hole in the one plate. This mercury, com- 
pletely surrounding the curved surface of the cylinder, formed 
the outer coating of the cylinder, and during the experiments 
was connected by means of a wire with the one electrode of 
the Holtz machine. The inner coating was formed by a thin 
wire passing through the hole in the other glass plate into the 
perforation in the cylinder, and connected with the other elec- 
trode, which was put to earth. 
The apparatus described was placed under the Steeg's 
polarizing microscope, so that the first circle of the system of 
rings was concentric with the perforation in the cylinder, and 
could be distinctly observed with sodium-light; it is necessary 
that the upper system of lenses of the apparatus should be 
adjusted at a certain distance, easily found, from the cylinder. 
The electrification of the two coatings of the quartz cylinder 
gave rise to the following phenomena. Only the six portions 
of the circle which lay in the direction of the three axes of no 
piezoelectricity passing through the centre retained their 
position ; at all other points there was a displacement either 
towards the centre or away from it : this was greatest in the di- 
rections of the three axes of maximum piezo-electricity passing 
through the centre. In each of these directions there was upon 
one side a displacement inwards, and on the opposite side a 
displacement outwards, corresponding in each case to the piezo- 
electric and to the previously observed electro-optic behaviour 
of quartz in these three directions. These displacements give 
to the ring a shape resembling an equilateral triangle with 
rounded corners. Exchange of electricities changes the posi- 
tion of the triangle, so that the new position would result by 
turning the triangle round from its former position through 
