244 Wychoff — Crystal Structure of Alabandite. 



and to locate the atoms of manganese sulphide with all 

 of the accuracy which the present knowledge permits. 

 In this instance this comparison is most readily carried 

 out by a study of the powder reflection. 



The Powder Photograph. — A small piece of the crystal 

 of alabandite was powdered, formed into a film with 

 collodion as the binding material and a powder photo- 

 graph prepared in the usual manner using molybdenum 

 radiation which had been rendered monochromatic by 

 a zirconium oxide filter.'^ The reflections were regis- 

 tered upon a hemicylindrical film so that spectrum lines 

 were obtained upon either side of the undiffracted image 

 of the slits. Measurements of the distance of these reflec- 

 tions from the zero image were made by halving the 

 distance between corresponding spectrum lines on either 

 side of the center. A check was furnished by observing 

 the coincidence of the zero lines as determined in this 

 fashion. The spectrometer was standardized through a 

 measurement of a sodium chloride spectrum. Accurate 

 spacings of the planes producing each of the lines in the 

 spectrum were determined with the aid of the usual 

 expression 



;zX = 26? sin ^^ (1) 



by remembering that the ratio of the distance of a spec- 

 trum line from the central image to the distance from 

 the powder to the film measures 20 in radians. For the 

 identification of the different lines and for their approxi- 

 mate measurement, a scale constructed for the particular 

 spectrometer used and reading the values of the spacings, 

 d/n, directly, is very convenient (figure 1). 



An approximate calculation of the relative intensities 

 of the reflections from the simple planes, which will 

 furnish the strongest spectrum lines in the powder pho- 

 tograph, can be readily made for each of the possible 

 arrangements with the aid of the intensity expressions 

 already derived for these groupings. For the reasons 

 already outlined these computations need be extended 

 only far enough to include the first order reflection from 

 the (111) face and the reflections from two or three 

 other simple planes which will serve as comparisons. 

 Before these calculations can be made, however, it is 



^ A. W. Hull, Phys. Rev. (2), 10, 661, 19] 7. 



