High Frequency Spectra. 197' 



but a small amount of energy, and have very little photographic: 

 effect. It is thus essential to have long exposures. 



The photography of the lines is complicated still further by^ 

 the photographic fog produced by the rays scattered by various- 

 parts of the apparatus (slits, crystal, etc.). Since the lines occur 

 on the film very close to where the direct rays fall, the scattered 

 rays are most intense just at the region where the lines are 

 to be observed. 



The one heavy element, the high frequency spectrum of which 

 has been carefully investigated, is tungsten. Since this element 

 has been used as the material of the target in the Coolidge type 

 of tube, it is a comparatively easy matter to excite its K series- 

 spectrum. As there was no Coolidge type of tube with a plati- 

 num target available for this research, a gas-filled tube, made- 

 by Gundelach, was used. 



The K series of platinum has been measured once previously 

 — by Lilienfeld and Seeman,^ who employed a Lilienfeld tube 

 with a target of platinum-iridium. The values obtained by these- 

 authors are given in Table II. 



The Spectrometer. 



The whole of X-ray spectroscopy is based on the fact that a- 

 crystal acts as a space grating to X-rays. W. L. Bragg* showed 

 that if a parallel beam of X-rays of wave-length A- was directed 

 on a crystal face at an angle so that \ = 2d sin 0, there- 

 would be an interference maximum at an angle 0, and other 

 maxima at values of given by ;/ A. = 2^ sin ; n is here an 

 integer and a is the lattice constant of the crystal, i.e., the dis- 

 tance between successive planes of atoms, parallel to the reflect- 

 ing face. If, then, it is desired to resolve a beam of X-rays into 

 its component parts, it is necessary that the crystal should be- 

 placed at different angles with respect to the incident beam. This 

 is done most efficiently by rotating the crystal slowly and uni- 

 formly. 



Further, W. H. Bragg^ established that if a diverging beam of 

 X-rays issued from a narrow slit S^ (fig. 1(b)), and fell on a 

 crystal face which was rotating about a point O (axis of rota- 

 tion), and if a photographic film was placed round the circle^ 



3. l>hy. Zeit, XIX., 1918, p. 269. 



4. X-Raya and Crystal Structure, p. 16. 



5. X-Rays and Crystal Structure, ^. 31. 



