72 Dr. L. Vegard on 



We observe the position of the chamber. If the position 

 angle o£ the chamber for spectra of the order n and i be 

 a n and di, then 



cot &i = . cosec — x cot — 5 — {6) 



l Z — 



For each face the intensity of the strongest maximum is 

 put equal to 1. Thus it is only the intensities of different 

 orders corresponding to one and the same face which should 

 be comparable. 



The intensities of the reflexion are measured in the 

 following way : — 



The slit, which the primary beam has to pass before striking 

 the crystal, is made quite narrow, while the slit in front of 

 the chamber is kept open, and the ionization is measured for 

 angles which are near to the glancing angle of the spectrum 

 in question. 



The maximum ionization current (when the ionization of 

 the " white radiation" has been subtracted) is taken as a 

 measure of the intensity of reflexion. 



Iu order to make certain that the intensities thus measured 

 correspond to the same strength of the primary beam, the 

 intensity measurements for each face were carried out rapidly 

 and in symmetrical order. 



§ 5. Interpretation. 



The interpretation is based on the fundamental equation of 

 Bragg, combining the spacing d, the glancing angle 0, and 

 the wave-length \ : 



n\ = 2d sin n (4) 



From the values of 6 given in Table II. we can calculate 

 the spacing for any reflexion-face. 



In Table III. are given the absolute values of ^100 = ^010 

 and d m , as calculated from (4) (\=0'607 X 10" 8 ). Column 3 



gives the ratios —■ and column 4 the ratios c'/a' of the 



"100 

 crystallographic axes, as taken from P. Groth, Chemische 

 KrystallograpJiie . 



We see that in the case of zircon and rutile, 



"ooi G _ 



dioo a r 



In the case of kassiterite the reflexion from (001) has not 

 been found ; but from the similarity between the spectra of 



