194: R. W. G. Wyckoff — Symmetry and Crystal 



tinct preparations were studied. Some pertinent data 

 from the interpretation of one of these photographs are 

 given in Table I. In this table the wave-lengths of the 



Table I. 



Laue Photographic 



Data. 



Indices of plane 



Intensity 



Wave Length 



381 



m 



0.232 A. U 



581 



f 



.239 



781 



f 



.227 



161 



s 



.261 



341 



m 



.263 



521 



s 



.217 



381 



m 



.206 



1 10 1 



f 



.243 



392 



f + 



.206 



2 13 3 



f 



.241 



6 13 3 



f 



.244 



7 10 1 



f 



.253 



572 



m — 



.211 



752 



f 



.179 



732 



f . 



.233 



11 5 4 



f 



.268 



752 



ra 



.256 



3 14 3 



f 



.240 



592 



f 



.243 



Note: — In this table spots having an intensity / are faint, those marked 

 m are of medium intensity and those designated by s are amongst the 

 strongest appearing upon the photograph. 



reflected X-rays have been calculated on the basis of a 

 small unit containing three chemical molecules. The 

 voltage applied to the X-ray tube during the making of 

 these photographs was such that no reflections are ordi- 

 narily present in wave lengths shorter than A = 0.24 A.U. 

 Since, as reference to Table I shows, appreciable effects 

 with values of nX as low as 0.180 are to be found upon 

 the photographs, the correct unit must contain 24 and not 

 three molecules. 



This result was sufficiently unexpected, especially in 

 view of the fact that a simple structure containing three 



