426 Mr. A. Miiller on an X-Ray Bulb 



dark bands there was a clear space A, and it is easy to see 

 that the distance d could be calculated from the width of 

 the clear space. Owing to the fact that no appreciable 

 diffraction occurs while working with X-rays, the edges of 

 these dark bands were well defined. It was therefore quite 

 easy to measure the width of the clear space with sufficient 

 accuracy. In the case when the axis is outside the crystal 

 surface, the dark bands overlap each other. The measure- 

 ment of d then becomes impossible. This could be avoided 

 by shifting the crystal-holder until the axis of rotation fell 

 inside the crystal surface. 



Another point which had to be considered was the focussing 

 of the lines. Theoretically the exact focussing only occurs 

 if the distance between slit Si and the axis and the distance 

 between the axis and the line are equal. In the present 

 investigation that focussing happened only for one single 

 wave-length. In spite of that no correction had to be made 

 because the primary beam was sufficiently narrow to give 

 a sharp definition of the lines for the whole range of angles 

 over which the crystal was oscillated. It can be shown that 

 in measuring the distances / from centre to centre of the 

 lines, the correction would even be of the second order, and 

 thus immaterial under the present conditions. The distance 

 of the plate from the axis was chosen to give exact focussing 

 for the Hga line. 



The wave-lengths of the fainter lines have been measured 

 in a somewhat different way from the above. The stronger 

 lines of the mercury spectrum were taken as reference lines, 

 and the wave-lengths of the fainter lines have been calculated 

 by interpolation (in the case of longest and the two 

 shortest wave-lengths by extrapolation). A small correction 

 had to be taken into account owing to the fact that the 

 photographic plate was not exactly perpendicular to the 

 plane slit-axis. 



6. Results and General Remarks. 



The final results are given in the following table. Sieg- 

 bahn's notation for the lines has been used. The lines have 

 been identified by comparing them with the corresponding- 

 lines of gold, thallium, and lead. These elements have the 

 respective atomic numbers 79, 81, and 82. Mercury has the 

 atomic number 80. The wave-lengths of the L-spectra of 

 these elements have been measured recently by Coster* in 

 the laboratory of Siegbahn. 



* Coster, Zeitschr.f. PJnjsik, iv. p. 178 (1921). 



