565 



Fig 3. Steieogi-apliical Projeclion of Ihe RoNTGENOgrani of Anhydrite. 

 Plate parallel to (lOiti. 



photos of arnKjonitc, obtained witli ervftalplates ofdiftei-eiil Ihicknoss. 



Thus it seems nndoiibtedly necessary ; to (ake into considefafioii 

 the tliickness of tiie crystal-plates in the calcnlations of the angles v, 

 and to diminish the distance of 50 m.m., if rather thick plates are 

 nsed, with half the thickness of them '). The |>rojections reproduced 

 on Plate IV of this pajjer all relate to such stereogra|)hical projec- 

 tions, for which Ihe distance (JP is 75 m.m. ; in such cases the 

 diameter of the figure is also kept equal to 75 m.m. * 



In most of the drawings of Plate IV we have calculated (he 

 symbols \hko\, \okl\ and jAo/jof the zone-circles, (whose centres lie in 

 the direction of the axes), in the way formerly described ; the sym- 

 bols of the most important spots in the figure can then be imme- 

 diately seen from the indices of fhe zone-circles, after the method 

 mentioned above. 



1) This is connected with the specific atisorbing power for RöNïGEN-rays of the 

 crystallised material. As this absorption is stronger, the distance of 50 m.m. will 

 have to bo diminished with a smaller pari of the thirkness d. 



