Refraction of Quartz produced by Electric Forces. 139 



axis gave the same result, that, on the contrary, a pressure 

 parallel to the secondary axis produced considerable quantities 

 of electricity. 



Next the plate was perforated in the centre of a square 

 end face so that a nearly hemispherical depression was formed 

 there (depth 01 centim). A further examination for piezo- 

 electricity gave the same result as before. 



The plate so prepared was placed upon the brass plate 

 employed in my former electro-optic experiments between 

 two thin strips of glass cemented to it, and introduced into 

 the little flask filled with benzol. The end of a well-rounded 

 brass wire projected into the depression in the plate, and 

 formed the upper electrode, the plate being the lower elec- 

 trode. The rays of light traversed the crystal parallel to the 

 secondary axis, and consequently at right angles to the prin- 

 cipal axis and to a secondary axis. The Nicols were placed 

 in position I. 



In order to compensate the natural double refraction, I 

 employed the same means as in the experiments with the 

 quartz parallelepiped; a second square plate of quartz, which 

 was cut at right angles to the principal axis and had nearly 

 the same dimensions as the first, was placed between the 

 analyzer and glass flask, upon a stand movable about three 

 axes at right angles to each other, and so placed that its 

 principal axis was at right angles to that of the first plate. 

 The double refraction could not be exactly compensated; but 

 it was very easy to decide whether any such change of double 

 refraction as took place with the first plate occurred here, 

 by observing one of the vertical dark bands which crossed 

 the field of view whem the compensating plate was turned a 

 little about a vertical axis. Any change in the double refrac- 

 tion would have been perceived by a displacement of the band 

 to the right or to the left. 



The experiment was made by adjusting a band in the 

 middle of the field of view, consequently exactly under the 

 bulb-shaped depression, which was to see if any displace- 

 ment occurred when the difference of potential between the 

 electrodes was subjected to rapid change. I have never been 

 able to observe any such displacement, however varied the con- 

 ditions under which the experiment has been made. Hence 

 it follows that a change in the double refraction is not produced 

 in any perceptible degree by electric forces acting in the 

 direction of the axis of no piezoelectricity*. 



* It is obvious that I cannot assert that no trace of electro-optic effect 

 could have been observed in the direction of an axis of no piezoelectricity 

 if much greater differences of potential than those in my experiments, 



L2 



