134 Prof. W. C. Rontgen on the Change in the Double 



would be no change of the natural double refraction to be 

 observed under the conditions assumed, if the lines of force 

 run in the direction of one of the three axes of no piezo- 

 electricity. 



These conclusions, that the double refraction of the quartz 

 may be increased or diminished at pleasure by the action of 

 statical electricity, and that under certain defined conditions 

 the double refraction is incapable of any such change, have 

 been found to be confirmed by experiment. 



The preliminary experiments were made with two rect- 

 angular parallelepipeds of pure Brazilian quartz, in which 

 optical experiment showed no deformation. The pieces of 

 quartz, which were obtained from Messrs. Steeg and Reuter, 

 were 2'0 centim. long, and were made exactly of a uniform 

 w T idth and breadth of 1*2 centim. According to my instruc- 

 tions, the longest axis of the parallelepipeds should have 

 coincided with a secondary axis ; but, owing to a misunder- 

 standing on the part of the workman, little weight was 

 attached to this condition. Subsequent inquiries, as well as 

 determinations made by means of Leydolt's * etched figures, 

 showed that in both pieces this direction deviated but little 

 from that of a secondary axis. It may further be remarked 

 that it is sufficient for the investigations in hand that the 

 direction of length should not coincide with an axis of no 

 piezoelectricity : the piezoelectric experiments have shown 

 that this was the case. Two of the lateral surfaces are 

 exactly at right angles to the principal axis ; and the other 

 two lateral surfaces are therefore strictly parallel to the prin- 

 cipal axis and nearly parallel to a secondary axis. 



Each piece had a perforation in the direction of its 

 length of about 0'2 centim. width, starting from the centre 

 of the end faces ; the coaxial perforations do not quite reach 

 each other in the middle of the crystal, but leave a thickness 

 of 0*2 centim., which forms the portion of the crystal whose 

 electro-optical properties are to be investigated. 



Both crystals were examined ; but in each experiment only 

 one crystal was placed in the electric field ; the other was used 

 to compensate the natural double refraction of the first : for 

 this purpose the pieces were cemented together with a little 

 isinglass, so as to have their principal axes at right angles to 

 each other. Light polarized in a plane making an angle of 

 45° with the principal axes traversed the crystal at right 



* Berichte der Wiener Akademie, vol. xv. p. 59 (1855). 



