Wyclcoff — Crystal Structure of Magnesium Oxide. 145 



in the usual fasliion.^ A typical curve obtained by 

 plotting the estimated intensities of reflection of the 

 planes appearing in a single photograph against the wave 

 lengths they are reflecting, is shown in figure 2. ^ The 

 important characteristics of these photographs are illus- 

 trated by the graph : no planes are found which give a^iy 

 first order reflection except those, all of whose indices are 

 odd; and the second order reflections of planes giving 

 both first and second order reflections are by far the 

 stronger. 



The interference effects to be expected from each of 

 the possible arrangements of atoms can be calculated. 

 The spacing between like planes in a simple cubic 

 arrangement of points is 



a = the length of the side of the unit cube which can be 

 told from the spectrum measurements and d = t'he 

 spacing between like planes whose Miller indices are ]i,]c,l. 

 The intensity of reflection from any plane (hkl) in the n^^ 

 order can be written in the usual manner. 



Intensity oc/ (-) [A^-f B^] , 



97/ 



Where A and B are cosine and sine terms to be evaluated 



in the particular cases ; the form of the function of the 



d 

 spacing, / ( - ), need not be assumed for the present 



71/ 



purpose. 



Possible arrcmgement (a). 



It can be shown that this does not represent the struc- 

 ture of magnesium oxide because, while in the Laue 

 photographs the only planes giving reflections in the first 

 order have indices that are all of them odd, this grouping 

 should give good reflections with planes whose indices are 

 also two odd and one even. 



Possible arrangement (b). 

 This can be eliminated -for the same reason. 



«Ealph W. G. Wyckoff, This Journal (IV) 50, 317 (1920). 



Am. Jour. Sci.— Fifth Series, Vol. I, No. 2.— February, 1921. 

 10 • 



