BIOT AND J. HERSCHEL 



9 



which he made of the laws of this rotation, remarked 

 that certain quartz crystals, of a definite thickness, rotate 

 the plane of polarization as much to the right as other 

 quartz crystals of the same thickness turn it to the left. 

 He summed up the whole matter briefly by saying that 

 there are right-handed and left-handed quartz crystals. 

 But here a curious circumstance presented itself. 

 Haiiy had observed at the angles of his prismatic quartz 

 crystals some hemihedral facets (x, x' Fig. 2) different 

 from those in the simple example which we have just 



Quartz plagihedral left. 



FIG. 2. 



Quartz plagihedral right. 



been considering, but which when prolonged would 

 still give a rhombohedron. He had also remarked that 

 these facets, which, in pursuance of symmetry should 

 have been doubled for each of the angles which they cut, 

 were in the majority of cases single, that is to say that 

 only one of them was preserved and this facet inclined 

 according to the crystal, sometimes in one direction 

 sometimes in the other, to the edge which bore it. 

 When the inclination was in one direction with respect 

 to one edge of the prism it was in the same direction 

 with respect to the five other edges. He called plagi- 

 hedrons all crystals which had these hemihedral facets; 

 right-handed plagihedrons, those in which these facets 



