[ 96 1 
XV. On the Change in the Double Refraction of Quartz pro- 
duced by Electrical Force. By W. C. BSntgen*. 
IN a previous paper f I have described the electro-optical 
behaviour of plates of quartz cut parallel to the optical 
axis, through which the rays of light passed at right angles 
to this axis; this I proposed to supplement by a descrip- 
tion of experiments with plates cut at right angles to the 
optical axis. I had already observed most of the optical phe- 
nomena described in the following paper ; but the explanation 
of them was not clear. This is now given by a piezoelectric 
examination of the quartz which I have carried out. The 
present paper therefore consists of two parts — the first descri- 
bing the piezoelectric, the second the electro-optical expe- 
riments. 
Two pieces of quartz were, for the most part, employed for 
the piezoelectric experiments. The one is a circular plate cut 
exactly at right angles to the optical axis, having a thickness of 
0*58 centim. and a diameter of 1*8 centim.; the other is a 
sphere of 3 centim. diameter. Both are of good quality, as 
shown by optical examination. It will be clearly seen, in 
what follows, how advantageous it is to employ pieces of quartz 
of this form, and not crystals with natural faces. 
The plate was employed to subject the piezoelectric beha- 
viour of quartz described in the first paper to a direct experi- 
mental proof. The result obtained was that there are in reality 
three directions in quartz, all at right angles to the principal 
axis, and making angles of 120° with each other, which possess 
the property that pressure exerted upon the crystal in any one 
of these directions produces no electricity at the points of 
pressure: these are the three axes of no piezoelectricity. A 
pressure exerted in any other direction at right angles to the 
principal axis produces electricity at the points of pressure, 
the greatest manifestation taking place in the lines bisecting 
the angles between the axes named. These lines I called axes 
of maximum piezoelectricity. The sign of the electricity pro- 
duced is in accordance with the previous description. 
In order to exert a pressure upon the plate in a definite 
direction a compression-apparatus of ordinary construction 
was employed, the opposing steel cheeks of which were faced 
with ebonite plates upon which pieces of silver wire 0*06 
centim. thick were cemented. These silver wires were exactly 
opposite and parallel to each other, and the quartz plate was 
* Translated from the Ber. der Oberh. Ges./iir Natur- und Ileilkunde. 
t Phil. Mag. [5] xv. p. 132. 
