40 Dr. M. Siegbahn and Mr. E. Friman : High-Frequency 



hour. The plate-holder is fastened on the marble plate with 

 two screws^ and may, by loosening the screws, be made to 

 approach to or to recede from the crystal. By this means it 

 is possible always to focus the rays on the middle part of the 

 range during examination. The adjustment of the plate- 

 holder perpendicularly to the line : slit-centre-the crystal 

 rotation axis, is made by an optical method. 



In order to screen off the photographic plate in the casket 

 as much as possible from the scattered radiation a lead shield 

 extending from the crystal to the casket was placed on the 

 support seen in PI. I. 



The X-ray tube had the form seen in fig. 1. The anti- 

 cathode, consisting of a copper cylinder fastened on with 



Fig. 1. 



M-Window. 



putty to the outside of the glass-grinding, was cooled with 

 water. As window, a foil of 0*05 mm. of aluminium was 

 used. The tube was exhausted with a Gaede molecular 

 air-pump. To obtain a constant vacuum it proved advis- 

 able to use a valve (fig. 2), placed between the two pumps. 



