Refraction of Quartz produced by Electrical Force. 105 
I pass on now to describe the electro-optic experiments. I 
employed for these experiments the square quartz plate de- 
scribed in the first paper cut parallel to the side faces, as well 
as the parallelepiped marked II. and a small quartz cylinder. 
A fuller description of the two first mentioned crystals will 
be found in my first paper. The plate was examined both 
in parallel and in convergent light. For the experiments with 
parallel light, the plate was placed in a horizontal position in 
the flask rilled with benzol; the lower electrode was the brass 
disk provided with two glass strips; the upper electrode a 
brass wire projecting into the depression in the plate (compare 
the former arrangement of apparatus). 
Sodium-light linearly polarized at an angle of 45° to a 
horizontal plane traversed the plate parallel to the principal 
axis ; and the analyzer was turned so as to give a dark field. 
When electrification took place, the centre of the field of view 
— the place beneath the depression — became markedly bright; 
the arrangement of the bright places being different according 
as the upper or lower electrode was made positive. 
This result surprised me, since electrical forces acting along 
an axis of no piezoelectricity ought to produce no compression 
or dilatation in this direction. 
The behaviour of the plate in convergent light was just as 
surprising to me. It certainly brought me nearer to an expla- 
nation of the first experiment; but a complete explanation only 
became clear after carrying out the piezoelectric experiments 
described above. 
In order to examine the plate in convergent light, glass 
tubes 0*7 centim. wide, bent upwards at right angles, were 
cemented to the square faces of the plate, and were filled with 
mercury. Then the plate was so placed under a Steeg's 
polarizing microscope that the axial image of concentric 
circles could be observed, illumination being obtained from a 
sodium-flame. 
The tubes containing mercury were severally connected 
with the electrodes of the electrical machine. At the 
moment that the electricity began to act upon the plate, the 
circles became changed into ellipses, the major axis of which, 
longer than the diameter of the corresponding circle, made an 
angle of 45° with the lines of force (which were at right 
angles to the square faces of the plate); the minor axis was 
shorter than the diameter of the circle. If the plate was so 
placed under the microscope that the side becoming positive 
under pressure was on the right hand, and consequently the 
side becoming negative under pressure was on the left, and if 
positive electricity was conducted to the side turned towards 
