542 



point D\\, therefore, corresponds with the point 1, the points /^ and 

 Z)i.2.2 with the points 3 and 4 of fig. 5J. Wlien we lake a diagonal 

 succession of the pliases, tiien we have, starting fioni point G: G, 

 D\i2, Lb, KCl and Dm. In the P, T'-diagram tiie succession of the 

 curves must be, liierefore: 



{0)i, iDi,>.,)i, (L)6, {KClk, (A,l)6 

 As is apparent from fig. 8, this succession has been found also 

 experimentally. We find the metastable parts of these curves (not 

 drawn in fig. 8) by a similar discussion, as has led ns to fig. 6. 



{To be continut'd). 



Crystallography. — "On the Syinmetrij of tlwRb^THK^-patterns of 

 Trigonal and Hexagonal Crgntals, and on Normal and Ab- 

 normal Diffraction- Images of birefringent Crystals i7i general." 

 By Prof. H. Haga and Prof. F. M. Jaegek. ' 



§ 1. In connection with the peculiar phenomena observed some 

 time ago with respect to a number of RoNTOEN-patterns of birefringent, 

 and more especially of rhombic crystals'), we thought it necessary 

 to investigate in a rigorously systematical way, what kind of sym- 

 metry would be found in the diifraction-patterns of uniaxial crystals, 

 if radiated through in directions perpendicular to the optical axis. 

 For if the supposition were right, that the suppression of the sym- 

 metrj'-planes expected by theory in the RöNTGEN-patterns of rhombic 

 crystals were connected in any way with the double refraction, 

 — as was thought at that time by one of us, — then we 

 might expect something of the kind also in the case of the patterns 

 obtained by means of planeparallel sections of uniaxial crystals, if 

 cut parallel to the optical axis, and radiated through in a direction 

 perpendicular to that axis. 



To obtain the closest analogy in the oi'ientation with that present 

 in the case of the rhombic crystals, which were always cut parallel 

 to the three pinacoidal faces SlUOj, {OlOj and {001}, we in\estigated 

 in the case of tetragonal crystals those sections, which were parallel 

 to the first and the second prisms |100j and {ilOj; in the case of 

 trigonal and hexagonal crystals we used in the same way the sections 

 parallel to the prism-faces {1()I0| and jlSlOj. In the last mentioned 

 crystals thus the sections parallel to |l()lOj will be analogous to 

 those parallel to jlOOj in the case of rhombic crystals, the sections 

 parallel to {i"2J0} corres|)onding in the same way to those parallel 

 to jOlOj in the mentioned biaxial crystals. 



') These Proceedings, 17, 1204, (1915), 



