some Carbonates of the Calcite Group. 333 



effect of the X-ray beam upon the photographic plate. 

 Some planes seem to be reflecting rays shorter than any 

 existing in the original beam values of (wA<0.25 A. U ) 

 It was observed, however, that each of these planes has 

 indices that are all odd. By taking a new set of axes, 

 which have the directions of the diagonals of the faces of 

 the rhombohedron made by the axes of the second set, 

 these "half order" reflections become first order reflec- 

 tions of twice the previous values of n\, thus removing all 







/• 

















/ 



f 

















/ 







\ 









/ 





/ 



A 





v 









V 







/ 





A 











/ 



i 













/ 











I, 



/ 

































00 



CO 



z 



Z 



CVJ 



r4 .6 .8 1.0 



WAVE LENGTH 



-..J 10, 10 ~ The sm °oth curve represents the relative intensity of X-rays of 

 different wave lengths for the "white" radiation from tungsten when the 

 voltage across the tube is 50,000 volts. The other gives the calculated 

 amount of absorption of a certain thickness of silver bromide. 



anomalies in the results from the photographs. This 

 transformation of axes is the same that had to be applied 

 to sodium nitrate. 16 The relations between the three sets 

 of axes that have been considered are shown in fig. 1. 

 The transformation from the second set of axes (those 

 used in considering the Laue photographs) to the new 

 set of axes (to be used in determining the arrangement of 

 atoms) may be effected by the following expressions : 



16 E. W. G. Wyckoff, op. cit. 



Am. Jour. Sci.— Fourth Series, Vol. L, No. 299.— November 1920 

 25 



