190 



plane of symmetry, as is quite normal for monoclinic crystals cut 

 parallel to (100) or (001), or to any other face of the orthodiagonal- 

 zone 1 ). As an instance of this in fig. 155 the R on t gen-pattern, as 

 it was obtained with racemic tri-ethylenediamine-cobalti-bromide is 



reproduced in ste- 

 reographical projec- 

 tion. 



Here the natural 

 crystals parallel to 

 (00 1 ) were used in the 

 experiment 2 ), in the 

 shape of very thin, 

 splendidly develop- 

 ed pseudo-hexago- 

 nal plates which 

 showed an exactly 

 centrical and almost 

 undisturbed axial 

 image in convergent 

 polarised light ; a 

 slight, deformity of 



the axial interferen- 

 -, 



f^f^ Tmdorfi r^f^r*oncA /"IT 



ce-iiucitie, uccausc ui 

 an a PP arent biaxia- 

 lity of the substance, 

 was the 'only abnormality observed. Analogous phenomena were 

 met with in the case of the perfectly transparent benzile-pla.tes 

 cut parallel to (001), and of cleavage-lamellae of apophyllite 3 ). 

 Potassium-ferrocyanide once gave an almost tetragonal-symmetrical 

 pattern, but in most cases abnormal images showing only a single plane 

 of symmetry. Obviously this is a fact which supports the lamellar 

 theory; for it proves that local disturbances of the structure occur, 

 which cannot but consist of a slight rotation of a part of the basal 

 plate round an axis having the direction of the perpendicular 

 to the plane of symmetry visible in the obtained pattern. If such 

 a crystal be composed of lamellae, it is easily conceivable that such 

 dislocations may occur, if a pile of lamellae is rotated round its longer 



1) F. M. Jaeger, Proceed. Kon. Akad. van Wet. Amsterdam, 18.51. (1915). 



2) H. Haga and F. M. Jaeger, ibid. 18. 1201. (1915). 



3) H. Haga and F. M. Jaeger, ibid. 17. 438. (1914). 



Fi - T 5S- 

 Stereographical Projection of an abnormal Ron t gen- 



pattern of the pseudo-tetragonal 

 rac.-Tri-ethylenediamine-Cobalti-bromide. 



