1561 
the patterns corresponding with a section perpendicular to the optical 
axis have been discussed already several times '). For the purpose 
of comparison the stereographical projection of the image of calcite 
parallel to 1210) is here reproduced once more. (Fig. 12 on Plate 
‚ 
rhodochrosite, as can be seen from fig. 10 on Plate IV, this being 
IV); the image parallel to (OOOL) is very analogous to that of 
a reproduction of BraGe’s photogramme. 
The degree ofanalogy between the two minerals may be judged from 
the following data: 
Caleite : CaCO,, with a:c =1:0,8543; spec. gravity : 2,75; y=3,397. 
Rhodochrosite: MnCO,, with a:ec=1:0,8259; spec. gravity : 3.66; 
y= ora Dds 
Also in this case the general analogy, at least of the images 
obtained parallel to (0001), cannot be denied; but the differences 
in the above stated respects are no less evident. It can be clearly 
seen, that in the two patterns as well the number as the intensities of 
corresponding spots are different in the case of each mineral; the 
patterns parallel to (1210), although of similar character, are even 
strongly different with respect to details. In cases as these, where 
only a single binary axis is present as an element of symmetry, 
the immediate comparison of the two patterns is often found much 
more different, than in cases, were one or more planes of symmetry 
are present too. 
$ 5. Nickelsulphate and Nickelselenate. (+ 6H,O). 
In our previous paper *) Röntgen-patterns of these tetragonal salts 
have been published; thus only the stereographical projections of 
the images parallel to (O01) are here reproduced once more for the 
purpose of comparison (fig. 14 and 15 on Plate IV). The degree of 
analogy of the two salts can be seen from the following data: 
Nickelsulphate: NiSO,,6H,O. a:c=1:1,9119; spee. gravity: 
2,065 ; 4: w = 4,053 : 7,749. 
Nickelselenate: NiSeO,,6H,O. a:¢ =1:1,8364; spec. gravity: 
Ziene en arr bs ITA VENO 
The analogy between the two images is here a very close one; 
however in no less degree the difference in intensity and the number 
of the spots in the same zones. Also in this case these differences must 
1) Vid. i.a.: W. H. Braas, Proc. Roy. Soc. A. 89. 248. (1913) Zeits. f. anorg’. 
Chem. (1914). 206. 
2) H. Haca and F. M. Jarcer, these Proceed. 18. (1915). p- 1350. 
