Refraction of Quartz produced by Electric Forces. 143 



respects in complete accordance with the results obtained from 

 the experiments previously described. 



It is necessary to mention that the phenomena described 

 above may be explained by means of two known facts. One 

 of these facts has been recently demonstrated by MM. J. 

 and P. Curie*, as follows: — If electricities of opposite sign 

 be communicated to the ends of a secondary axis of a quartz 

 crystal, the crystal contracts or expands in the direction 

 of this axis, according as the signs of the electricities com- 

 municated to the ends of the axis are opposed to, or the same 

 as the signs of the piezoelectricities produced at these ends 

 by a pressure exerted in the direction of the axis. It seems 

 to me very probable that this result, found in the first instance 

 for the direction of a secondary axis, would be found to hold 

 good also for every direction at right angles to the principal axis, 

 and that consequently the direction of the intermediate axis 

 or the axis of no piezoelectricity possesses the property that 

 electrical forces acting in this direction produce no perceptible 

 change in the form of the quartz. So far I have had no 

 opportunity to test the accuracy of Curie's experiment, and to 

 extend it iu the direction indicated; but inasmuch as those 

 experiments interest me very much because of the close rela- 

 tionship in which they stand to my former experiments on the 

 so-called electrical expansionf, I shall undertake this inves- 

 tigation at the earliest possible opportunity. 



The second fact, admitting of easy confirmation, is that a 

 mechanical compression of quartz. at right angles to the prin- 

 cipal axis exerts the same qualitative effect upon the rays of 

 light which traverse the crystal at right angles to the prin- 

 cipal axis and to the direction of compression as a compression 

 exerted in the same direction upon an interposed plate of 

 glass. 



We shall find without difficulty that the phenomena de 

 scribed are even in detail completely in agreement with the 

 properties of quartz just now described. 



I hope shortly to give an account of the remarkable pheno- 

 mena which I have observed when the rays of light traverse 

 the crystal parallel to the principal axis. 



Giessen, November 25, 1882. 



* Compt. Bend. vol. xciii. p. 1137 (1881). 

 t Be?\ d. Oberh. Ges. vol. xx. p. 1 (1881). 



