Refraction of Quartz produced by Electric Forces. 141 



as the band was situated to the right or to the left of the 

 centre. Vertical or horizontal compression of the ntercalated 

 glass plate produced a displacement of the whole band 

 parallel to itself to the left or to the right. 



The explanation of these phenomena is easy to find if we 

 consider that as the experiments described show the direc- 

 tion on the right, so on the left; that is, the horizontal direc- 

 tion at right angles to the rays of light in the crystal employed 

 does not exactly coincide with an axis of no piezoelectricity. 

 The phenomena are thus easily deduced from the first-described 

 experiments: — The lines of force in the upper part of the plate 

 run nearly horizontally in the neighbourhood of the depres- 

 sion; some of them therefore coincide with directions in which 

 the double refraction is capable of being altered : in the lower 

 part of the plate, on the other hand, the lines of force are 

 vertical; they consequently run in the direction of the princi- 

 pal axis, and therefore produce no change of double refraction. 

 Consequently it is only the upper end of the band which is 

 displaced, and not the lower. The observation that the direc- 

 tion of displacement changes when the electrification or the 

 position of the band changes is in complete agreement with 

 the observed fact that the increase of the double refraction 

 of quartz is changed into a decrease if the direction of the 

 lines of force is reversed. We have seen above that from the 

 distribution of piezoelectricity we can predict with certainty, 

 for a given direction of the lines of force, whether an increase 

 or a decrease of the double refraction will take place; and the 

 question consequently arises whether with the new crystal the 

 established rule will be found confirmed or not. 



The plate was examined for piezoelectricity. A pressure 

 upon the square surface in the direction of the principal axis 

 produced no distinctly perceptible quantity of electricity at 

 the points of pressure. The four narrow side faces, however, 

 behaved differently; they may be designated a, b y c, d in 

 order. An increase of pressure in the direction parallel to b 

 and d produced positive electricity at a, and negative electri- 

 city at c; a decrease of pressure, electricities of opposite sign. 

 An increase of pressure in the direction parallel to a and c 

 gave negative electricity at b, positive electricity at d; a de- 

 crease of pressure, the opposite. In both cases I obtained 

 vigorous deflection of the electroscope*. The plate was again 



* Between the two directions, parallel to b and d and parallel to a and 

 c, there must be a field which in piezoelectric relation would behave op- 

 positely to the two fields to which these directions belong. In fact, an 

 increase of pressure in the direction of the diagonal of the square which 

 joins the corner a d with the corner b c, gave negative electricity at ad 

 and positive electricity at b c ; a decrease of pressure, the opposite. 



