Refraction of Quartz produced by Electrical Force. 99 
the sphere is exerted. It is unnecessary to explain how the 
adjustment is effected, and the means adopted to ensure that 
the pressure was actually exerted along a diameter. The 
tube of the microscope was weighted with 2 kilogrammes; 
there was no perceptible friction in lowering it, between the 
brass rod and the sphere; any such must be carefully avoided, 
since it would cause an energetic excitement of electricity. 
As I designed to test other portions of the sphere for piezo- 
electricity besides those upon which the pressure was directly 
exerted, the gold leaf of the electroscope was not always con- 
nected directly with the brass rod by means of which the 
pressure was exerted, but often with a small metallic holder 
fastened to an ebonite rod, in which could be placed at pleasure 
either a simple brass wire, or a wire provided with a small 
metallic disk*. The ebonite rod was held by a jointed stand, 
so that the wire or metal disk which served as electrode could 
be placed in contact with any point of the quartz sphere. 
The following experiments were made with this sphere: — 
(1) Besides the three axes of no piezoelectricity, the prin- 
cipal axis of the quartz also possesses the property that 
pressure exerted upon the crystal in this direction produces 
no electricity at the points of pressure. The question then 
arises whether there exist also other directions having the 
same property. 
Without having previously determined by optical means 
the position of the principal axis, I placed the sphere upon 
the previously described brass disk of the stage, connected the 
rod producing the pressure with the electroscope, and deter- 
mined by experiment the points on the surface of the sphere 
which did not become piezoelectric when pressure was 
exerted upon them in the direction of a diameter. When 
such a point had been found, it was marked upon the sphere 
by moistening the brass rod with india-ink and then gently 
lowering it upon the sphere. In this way the sphere was 
marked with 40 or 50 small dots. It is easily understood 
that in this examination a somewhat systematic method had 
to be adopted. We find, for example, very soon that certain 
portions of the surface of the sphere become only very feebly 
electric ; these, as seen afterwards, were the portions of the 
surface in the neighbourhood of the ends of a diameter parallel 
to the principal axis. It is difficult here to say exactly where 
no piezoelectricity at all results ; hence these places are not 
* These parts of the apparatus were so constructed as to have as small 
a capacity as possible. The chief advantage of Fechner's Electroscope 
over Thomson's Electrometer, apart from its better insulation, consists in 
its extremely small capacity. The instrument is thus well adapted for the 
present investigation. 
