1212 ° 
plane of symmetry i.e. is parallel to {001}. The plane of the optical 
axes of the erystal being parallel to {100}, while the c-axis coincides 
with the first bisectrix of negative character, it is evident, that the 
homologous spots are missing in the photograph which lie in the 
directions parallel to the a-axis, i.e. parallel to the direction of the 
smaller optical elasticity of the crystal plate. 
The figure corresponding to the image of a plate parallel to {100} 
(vid. fig. 6), also possesses only one single plane of symmetry; but 
it is now just the plane {001}, which has disappeared as such, while 
{010} remains. Here we therefore miss the spots which would 
correspond to directions parallel to the c-axis; thus on the photo- 
graph the spots have vanished, completely or partially, which would 
lie in the direction of the greater optical-elasticity of the crystal-plate. 
On the other hand the image of a plate perpendicular to the c-axis 
(fig. 8) shows two perpendicular planes of symmetry, as well as a 
binary axis; the intensity of the spots is however very feeble indeed. 
It must here be remarked, that the combination of symmetry- 
properties observed in these three ROnTGEN-patterns is geometrically 
quite impossible for the crystals themselves. The case considered is 
therefore again more convincing than that of the cordierite. Hence 
the cause of the newly discovered phenomenon can „ot be sought 
in the special symmetry-character of the crystals; there must be 
still some unknown factor, which determines the phenomenon of 
the unexpected disappearance of the planes of symmetry. 
§ 6. As a second case of this kind we have reproduced here 
the RÖNrTGENograms which were obtained in the same way with 
plates of hambergite. 
The choice of this very rare mineral, got at Helyarden, Lange- 
sundford, Norway, was made with a view to its chemical composi- 
tion: Be, (OH) BO,; the compound’ being composed of the lighter 
elements in the periodical system, whose atomic weights are all 
smaller than 20. Hambergite is rhombic; its parameter-ratio is: 
a:b:¢ =0.7988:1:0.7267. The crystals were glassy and splendidly 
homogeneous; they showed the forms {110}, {100}, {O10} and {011}, 
and had a prismatic aspect. A perfect cleavage is present parallel 
to {010}, a good one parallel to {100}. The crystals are very strongly 
birefringent; the birefringency is about: 0.072. The plane of the 
optical axes is parallel to {010}; the first bisectrix, which has 
positive character, coincides with the vertical axis. The dispersion 
round the first bisectrix is only feeble, with o < v. 
In fig. 9, LO and 11 the stereographical projections of the RÖNTGEN- 
