1099 



For the measurements of the angles enabled us onl}^ to state, that 

 a hexagonfil or (Uhexagoiial s^nimoliy was present; the Röntgen- 

 pattern however, obtained by radiation through a{'rystalplate,cut perpen- 

 dicularly to an optical axis, showed immediately, that no binary 

 axes of symmetry, nor vertical symmetry-planes were present. «There- 

 fore the true symmetry of the crystals could only be that of the 

 hexagonal-hlpyramidal or of the /ledm/onal-pj/rainidal (hemimorphic) 

 class. The external aspect of the crystals could bring no decision in 

 the choice between these two possibilities ; the fact, that the faces at 

 both ends of the vertical axis showed always about the same degree 

 of development, could perhaps be considered as a very weak argu- 

 ment for the view, that no polarity of the vertical axis were really 

 present, and thus a single horizontal symmetry-plane must necessarily 

 exist. However the figures of corrosion which were obtained on the 

 faces of the prism |1010| by heating for a short time the crystals 



Fig. 3. 



Stereographical projection of the difTraclion-pallein of the Erbium-aetiiylsulpliule 



(± opt. axis) obtained by RöNTGEN-rays. 



