1351 
Finally we were obliged to limit our study to the following 
erystal-species: Futile; Cassiterite; i-Erythrite; Scheelite ; Nickelsul- 
phate (6 H,O); Nickelselenate (6 H,O); Potasstumdihydrophosphate ; 
Penta-erythrite ; and Wulfenite. 
§ 2. The symmetry of the RörrceNpatterns for definite sections 
of tetragonal crystals of different classes has been already deduced 
by us ®), and the results reviewed. We can therefore refer here to 
them, and begin without delay with discussing the different crystal- 
species studied by us. 
[. Ditetragonal-bipyramidal Class. 
a. Rutile: TiO,, with axial ratio: a:¢c 1: 0.6442; our crystals 
originated from Raade in Norway. They were only imperfectly 
translucid as well as the sections of them, and full of very fine cracks. 
The obtained RoyrGENpatterns were not very beautiful, and unsuit- 
able for reproduction. Therefore only a stereographical projection 
is reproduced here (fig. 1 on Plate IV) of the image obtained in 
radiating through a erystal-plate parallel to (OOI): its thickness was 
1.50 mm. The pattern evidently shows a quaternary axis, and four 
vertical planes of symmetry passing through if. 
bh. Better results were obtained with Cassiterite: SnO,. Our 
crystals, whose parameters were: @:c = 1: 0.6724, origmated from 
Schlaggenwvald in Bohemia. Of the resulting images of crystal-plates 
parallel to {O01}, {100; and }110} only that parallel to {OOI}, (thick- 
ness of the plate: 0,20 mm.) appeared to be reproduceable (fig. 1 
on Plate I); it possesses, as can be seen from its stereographical 
projection on Plate [V (fig. 2), also a quaternary axis and four 
vertical planes of symmetry. Of the images obtained with plates 
parallel to {100} and {110} (d — 0,22 and 0,23 mm.) stereographical 
projections are reproduced in fig. 8 and 4 on Plate IV: they possess 
two perpendicular planes of symmetry and a binary axis perpen- 
dicular to the photographic plate, quite in concordance with the 
theoretical deductions. 
LT. Ditetragonal-pyramidal Class. Notwithstanding numerous 
tentatives we did not succeed in obtaining any other object of this 
class than Penta-erythrite in a suitable form. The crystals of Silver- 
jluoride were too hygroscopic, those of Lodo-succintcimide too small. 
However we were able to get very beautiful, transparent crystals 
of Penta-erythrite: C,H,,O, (= C(CH,OH),), mpt: 253° C. This 
compound, which according to Martin*) must belong to this 
1) Haca and Jagger, these Proceed. 18. 543. (1915). 
2) Martin, N. Jahrb. f. Miner. Beil. Bd. 7. 18. (1891). 
