546 



aeliüu on that place of llie photographic plate. We were able to 

 eliminate this obstacle (or the greater pait. by cutting from the 

 centre of the screen a small disc of about 1 cm. diameter, and to 

 cover the inner rim of the hole with a layer of black ink. On the 

 photo however a very small halo was still visible in some cases ; 

 but this could be easily removed by covering the central part of 

 the negative during the reproduction with a small disc of black 

 paper. In this way the disturbance of the image by Ihe above men- 

 tioned causes was finally completely prevented. 



§ 3. Krom the representative of each ci'ystal-class, necessary for our 

 purpose, not all could be obtained in a sufficiently excellent quality, 

 or they could not be used from some other cause in our experiments. 



So for instance the sodmm-period'ttc-cvyslals were unsuitable, because 

 of their very rapidly occurring efflorescence and loss of their water 

 of crystallisation ; the crystals of benzil on the other hand appeared 

 to show optical anomalies and peculiar phenomena to be described 

 in a later communication. Notwithstanding much trouble it was 

 impossible to obtain larger crystals of cinnabar, which were not at 

 the same time twins or appeared to be too inhomogeneous. From 

 zincite we could have only badi} disturbed and lamellar crystals ; 

 in the ease of nephelite the obtained crystals still appeared finally 

 to be polysyntlietic twins, notwithstanding the choice of very small, 

 clear-looking individuals. 



Completely reliable results we obtained finally in the case of the 

 following minerals : jthenakitc, dolomite, quartz, turinaline, calcite, 

 apatite and lieryl, while also our experience with some nephelite- 

 preparations, and with ri})nahar cut perpendicularly to the c-axis, 

 can be judged as to be in agreement with the theoretical deduction. 



§ 4. Description of tlu' e.ramined substances. 



a. I'll rma line. For our observations we used a beautiful, dark 

 green lurmaline-crystal of Brazil. The image obtained by radiiition 

 through the direction of the optical axis, was already formerly 

 reproduced'); it possesses the expected symmetry, namely: one 

 ternary piincipal axis and three vertical symmetry-planes (vid. the 

 stereographical projection in lig. J, I^late VI). 



The first crystal-plate parallel to jlOJOj had a thickness of 3,05 

 m.m. ; a second one however only of 1,15 m.m. Both images 

 (vid. Plate 1, fig. 1 and 2, and Plate VI fig. 2.) show only one 



1) Vid. these Proceedings, 17. 1204. (1915); Plate 1, fig. 4; Plate IV, fig. 4. 



