1359 
diffraction-images of the /ead-salt will show a somewhat different 
character, if compared with the very closely analogous patterns of 
the three first mentioned compounds, as well with respect to the number 
as to the particular way, in which the relative intensity of the spots 
is manifested. However, just in the case of cerussite, the pseudo- 
hexagonal character of the erystal-structure remains manifested very 
clearly, as is shown on comparing the stereographical projection of 
the Röntgen-pattern, obtained by means of a crystal-plate parallel 
to (001). (See fig. 9 on Plate IV). 
The Röntgenpatterns of arragonite parallel to (100), (010), and 
(001) have already been published by us 1). In improved form their 
stereographical projections are here reproduced once more for the 
purpose of comparison, in the fig. 1,3, and 6 on Plate IV. Of the Réntgen- 
patterns of strontianite and whiterite, only the not absolutely centra- 
ted images of plates parallel to (OOL) could be reproduced (Plate I, 
fig. 2 and 3), and of strontiimite also the one parallel to (100). (See 
Plate I, fig. 1). The cerussite, however, gave the magnificent patterns, 
provided with numerous spots, reproduced in fig. 4 on Plate I, and 
in tig. 5 and 6 on Plate II; the last image is somewhat disturbed. 
However of all these minerals the suitable images are drawn on 
Plate IV in stereographical projection, thus a direct comparison of 
those for corresponding crystal-sections being immediately made 
possible (vid. fig. 1 to 9, on Plate IV). 
The analogy in the general structure of homologous patterns is 
undeniable, even where this structure appears to differ not inconsi- 
derably in particulars. This analogy doubtless can be considered an 
important argumentation in favour of the analogy in the total special 
arrangement of the space-lattices of such isomorphous -crystal-species 
suspected already long by erystallographers. Doubtless this agreement 
would in many cases be again more evident, if a greater certainty 
about the indices to be attributed to the different zone-circles were 
present, — especially to the circles with the greatest and with the 
smallest radius among them. Because of the imperfections of the 
photographie images, of the exact orientation of the erystal-plates, 
and because of an inevitable uncertainty in the choice of the right 
value of the distance A between plate and erystal, it is, however, 
hardly possible to attain this, and thus it is beyond any doubt, that in 
attributing the indices to some circles, the choice of them is some- 
what arbitrary. However, it is hardly doubtful, that if ideal cireum- 
stances were realised, the indices of corresponding circles in the 
1) H. Haca and F. M. Jaeger, These Proceed., 18, 559, /1915). 
