On the Change in the Double Refraction of Quartz. 133 



at angles of 60°, into six fields, which possess the following 

 properties : — A pressure exerted upon the crystal in any 

 direction through the point, or in any parallel direction, 

 causes the crystal to become electrified at the two points of 

 pressure, the one becoming positive, the other negative. If 

 we change from one direction of pressure to another lying 

 in an adjacent field,, the sign of the electricity at the points of 

 pressure changes whenever the direction of pressure crosses 

 the boundary between adjacent fields. 



Hence it follows that a pressure exerted in the direction of 

 one of the three lines mentioned can produce no piezoelec- 

 tricity ; on this account I propose to call these three direc- 

 tions the axes of no piezoelectricity. In the three directions 

 bisecting the angles between these axes, there must be a 

 maximum of piezoelectricity produced ; these directions may 

 therefore be called the axes of maximum piezoelectricity. 

 They coincide more or less exactly with the so-called secondary 

 axes, the lines joining the two opposite edges of the quartz 

 crystal. I am not yet able to decide whether or not they 

 coincide exactly, since the experiments which I have made 

 to test the point are not sufficiently numerous ; with some 

 crystals, however, it seems to be really the case. If this 

 were so, the axes of no piezoelectricity would have the same 

 direction as the intermediate axes of the quartz. 



Let us suppose that the three axes of maximum piezo- 

 electricity give the three directions of the natural polariza- 

 tion : if we consider the ends of each axis as positive or 

 negative, corresponding to the natural distribution of elec- 

 tricity in the interior, then these ends, if we follow them 

 round in order, must be alternately positive and negative. 

 The electricity produced by pressure is correspondingly posi- 

 tive or negative; and this holds good, as already remarked, 

 for the whole field in which any axis lies. 



If now a piece of quartz be so exposed to the inductive 

 action of static electricity, that at any place the lines of 

 force run at right angles to the principal axis, and at the 

 same time do not run in the direction of an axis of no piezo- 

 electricity, these forces will produce an increase or decrease 

 of the natural polarization at this place, and with it, in accor- 

 dance with the hypothesis stated at the outset, we shall 

 have an increase or a decrease of the natural double refraction 

 of rays which traverse the crystal at right angles to the prin- 

 cipal axis and to the lines of force. The occurrence of the 

 one case or of the other will depend wholly upon which of the 

 three pairs of opposite fields the direction of the lines of 

 force lies in, and in what direction they traverse it. There 



