Fig. 2. 



432 



cordierite of Bodenmais ; and a pink cordiei'ite of' Mount Vnlij on 

 Madagascar. 



a. From a magnificent, almost colourless, homogeneous and slnyle 

 crystal of cordierite, after its label from Madagascar, three plane- 

 parallel plates about 1 or 1,2 mm. thick, 

 ^rh'ym^iy were carefully cut, and the following optical 

 properties of them determined (fig. 2) ; the 

 arrows indicate the direction of the luminous 

 vibrations, for which the mentioned colours 

 were observed in the crystals; obviously 

 thus the absorption-scheme with respect to 

 the crystal-axes is: a^f/^c. The axial 

 plane was parallel to jlOOj ; the c-axis was 

 first bisectrix (a). The birefringence is about 

 0,008, and of negative character. Of each 

 of these plates we obtained a RöxNTGEN-pat- 

 tern, after they had been carefully orientated 

 in the way formerly described by us. '). The distance of the photographic 

 plate and the crystal was 45 mm., while the time of exposure varied 

 between 1'/^ and 2'/^ hours respectively. In connection with the question 

 of the orientation, attention must be drawn here once more to the fact, 

 that deviations of the theoretically riglit orientation, even so slight 

 that they cannot be controlled any more by means of optical test, 

 will however always manifest themselves by a slight dissymmetry in 

 the RöNTGEN-pattern. For instance, the image obtained by radiation 

 through |00J| in several experiments, appeared to be always im- 

 symmetrical to a more or less degree, while by the optical test in 

 any of these cases no ajij)reciable deviation of the optical image 

 and of the right orientation of the first bisectrix could be proved. 

 Thus even the greatest attainable degree of precision in this orien- 

 tation can never exclude the necessity, to acknowledge certain 

 imperfectibilities of the expected symmetry of the obtained 

 RöNTGEN-patterns as of only secondary importance in the com- 

 parison of these images, and to neglect them presently in drawing 

 conclusions from the photographs. This j)oint must always be con- 

 sidered in all following discussions of the obtained results; without 

 this restriction it simply appears absolutely impossible to draw any 

 valuable conclusion from the results obtained by experiment. At the 

 same occasion we wish further to remai-k, that the use of a phos- 

 phorescent screen (species "Eresco") behind the photographic plate 



1) H. Haga and F. M. Jaeger, these Proc, loc. cit. (1914). 



