547 



single plane of symmetry, perpendicular (o the prism-face. The 

 spots in the image of the thick crystal-plate are very heavy and 

 not oval-shaped, but rectnngulnr. We have already drawn attention 

 to this phenomenon on a previous occasion, in the cases of sod'mm- 

 chlorate, of sylvine '), etc. 



' It now becomes clear that it is principally connected with the 

 thickness of the crystal-plate: tlie formerly described patterns of 

 sodiumchlorate and sylvine are indeed also obtained by means of 

 verj/ thick plates. 



This peculiarity was also stated by us in many other cases, if thicker 

 plates of not very strongly absorbing substances were used in the 

 experiments; often the spots appear to be double ones in such 

 cases, which by joining tinally give the impression of a more 

 or less rounded rectangular shape. We think that an e.xplaiuilion 

 can be given in this way : that in the case of not powerfully 

 absorbing substances so great a number of successive molecular layers 

 contribute to the intensity of the spot on the photographic plate, 

 that the images of the outer layers of the whole pile will ai)pear 

 ill a discernible distance from each other on the tilm, because of 

 the different distance of these outer layers from the sensitive plate. 

 If the spots thus properly produced will coalesce with each other, 

 the rounded rectangular shape of the resulting image is easily 

 explained. 



The fourth turmaline-plate was cut parallel to {121Ü| ; the Rontgkn- 

 pattern shows as a single symmetry-element, a binary axis coinciding 

 with the plate-norinal. (Plate I, lig. 3). The results of the experiments 

 are therefore in this case in complete accordance with those of the 

 theoretical deductions. 



b. Phenakite. We had at our disposition very beautiful, colourless 

 and lustrous ])henakite-(tvy siixls from San Miyuël, Minus Geraës, in Brazil. 



The crystal plate cut perpendicularly to the c-axis, showed in 

 convergent polarized light, a uniaxial interference-image of positive 

 character ; it manifested however a small abnormality in the form 

 of a feebly biaxial image with extremely small axial angle. However 

 this abnormality did not appear to have any influence on the diffraction- 

 pattern. The plate had a thickness of J,l mm.; the photograjihic 

 image was not very beautiful, and the most important spots appeared 

 to be covered by the strong irradiation of the central spot. Later-on 

 we obtained by means of our newer apparatus described previously, 

 a feeble but completely symmetrical image, which was used in the 



1) Ibid. 1207, note 1. 



