334 R. W. G. Wychoff— Crystal Structures of 



ft = K + L; ft = H + L; Z = K + H, where H, K, L, 

 are the indices according to the second set and h k I those 

 according to the new axes. 17 Calculations show that 

 there are two molecules associated with the new unit 

 rhombohedron. 



The Effect of the Voltage applied to the Tube upon the 

 Resulting Photograph. — It is a matter of considerable im- 

 portance to know the general shape of the curve obtained 

 after plotting n\ against intensity for the various spots. 

 The following considerations were made in order to deter- 

 mine the effect upon the curve of impressing different 

 voltages upon the X-ray tube and to find out if filtering 

 screens would prove useful. Two factors are known to 

 influence the photographic effect of a beam of X-rays : the 

 distribution of energy in the beam from the tube, and the 

 selective absorption by the silver "emulsion" of the plate. 



The relative intensity of the X-rays of different wave 

 lengths in the region of the "white radiation" from a 

 tungsten target for a certain voltage is shown in fig. 10. 18 

 The higher the voltages, the greater becomes the intensity 

 of the X-rays of all wave lengths and the shorter is the 

 wave length of maximum intensity. The X-ray spectrum 

 of tungsten has been mapped for various voltages 19 and 

 careful measurements have been made in the region of the 

 white radiation for voltages up to 50 kilo volts. 20 



It has also been shown that the effect of X-rays upon the 

 photographic plate, at any rate in this region of the X-ray 

 spectrum, is quite closely proportional to the absorption 

 by the silver bromide of the plate. 21 If this proportional- 

 ity is assumed to be true, the relative effect of X-rays of 

 different wave lengths can be simply calculated from the 

 mass absorption coefficients of silver and bromine. The 

 absorption can be represented over a considerable range 

 of wave lengths with some degree of accuracy by the 

 equations : 22 



17 This transformation of course applies equally well to the passing from 

 the crystallographic to the second set of axes. A change in the opposite 

 direction can be made by remembering that H = 1c -\-l — h; K = h -\- I — lc; 

 ~L = h±1c — l. 



18 C. T. Ulrey, Phys, Eev., (2), 11, 401, 1918. 



19 A. W. Hull, Proc. Nat. Acad. Sei., 2, 265, 1916. 



20 C. T. Ulrey, op. cit. 



21 C. G. Barkla and G. H. Martyn, Phil. Mag., (6) 25, 296, 1913. 



22 W. Kossel, Verhandl. deutsch. phys. Ges., 16, 898, 1914. 



