1213 
patterns obtained are reproduced. When perpendicular to the c-axis, 
the photograph, remained very feeble even after 34 hours’ exposure; 
in both the other principal directions however, even after much 
shorter exposure, the photographs were very sharp. 
The plate perpendicular to the c-axis, in fact shows two normal 
planes of symmetry, as well as the binary axis: in the direction of 
the a-axis (parallel to the plane (010), the density of the spots 
seems to be greatest, but this is only slight in comparison with that 
of the very numerous and intense spots on both the other patterns. 
When the RöNreeNogram is perpendicular to the a-axis (fig. 9), 
the expected symmetry is also unmistakably present; the circles con- 
taining most spots here lie in the direction of the c-axis (parallel to 
the plane (010). 
The photograph however, obtained with a plate perpendicular to 
the J-axis manifests only one single plane of symmetry, namely that 
parallel to {001}. It is evident, that this combination of symmetry- 
properties would be also erystallographieally quite impossible, and 
the only suitable explanation in this case is, that the plane of sym- 
metry (parallel to {100)) has disappeared for some reason. Fig. 11 
proves, that the above mentioned symmetry-plane is really expressed 
in that photo; but if the RöNrGeN-rays are transmitted parallel to 
the direction of the optical normal (i.e. perpendicularly to the direct- 
ions of maximum and minimum optical elasticity) evidently the spots 
which would be expected in the direction of maximuin elasticity 
either come out in the photo not at all or only partially. 
In table IL moreover two of the original photographs in figs. 7 
and 8 are also reproduced; they show the said phenomenon very 
clearly. 
In the case of cordierite only the two vertical planes of symmetry 
were found, for plates cut parallel to {100} and {O10}. In cordierite 
{100} is the optical axial plane, and the c-axis is the first bisectrix, 
and of negative character. However cordierite is a but feebly birefringent 
mineral (about: 0.010) and the optical elasticity in the directions of 
a- and b-axis is only slightly different in it. 
Both pinacoides {100} and {010} have thus in an optical sense 
about the same relation to the direction of the c-axis, and therefore 
we observe that the plane of symmetry parallel to {001} disappears 
on the corresponding photos. lm both plates the spots which lie 
in directions parallel to the greater elasticity have thus disappeared 
completely, just as in the case of hambergite. 
§ 7. Finally we point out the peculiarities found in the study 
