496 On an X-Ray Vacuum Spectrograph. 



This moves along a scale S, that indicates the turning angle 

 of the crystal directly in degrees. The arm M can be moved 

 to and fro by a slowly rotating eccentric-pulley. Through 

 the side of the box, placed opposite to one another, two 

 tubes R and U pass. The former is in communication 

 with the pump b}^ means of an indiarubber tube. In the 

 latter there is another tube V, the end of which is covered 

 with a plane-ground plate p, having a slit o 0*1 mm. wide 

 and furnished with edges of gold. Between this slit and 

 the crystal a screen I of lead is placed, with a slit s of 

 2 mm. width. In the tube V is a cylindrical shield of lead 

 1*5 cm. thick, with an opening of nbout 1 mm. The X-ray 

 tube (of the construction previously * described) is attached 

 to the spectrograph in such a way, that the extended part of 

 it is put into the tube U, and then an air-tight elastic 

 connexion obtained by means of picein. 



The photographic plate is placed in the casket G. This 

 is attached to the sledge L by screws passing through the 

 two feet, which are furnished with longilateral openings by 

 means of which the casket can be turned or removed. 

 Through this arrangement the plate (in position 0) can 

 be adjusted perpendicularly to the line first slit — crystal 

 rotation axis, and at the same distance as the slit from the 

 crystal. The sledge is supported by three steel bullets (comp. 

 Moseley), two of which rest in a circular groove, the third 

 in one of the radial cavities. In this way the sledge can 

 take up perfectly fixed positions, the number of which is 

 thirteen, situated at about the same angular distance from 

 each other. These angles have been very carefully deter- 

 mined. In the investigations described in the next paper, only 

 five positions have been used, which are denoted by 0, I-IV. 

 The aperture angle of the casket is about 37°, and thus the 

 plates in two adjacent positions have an angle of 4°-5° 

 common. 



The X-ray tube is evacuated with a molecular air-pump. 

 The fore-pump is employed simultaneously for the molecular 

 air-pump and the spectrograph. If the latter is separated 

 from the X-ray tube through a foil placed on the greased 

 plate p, a suitable vacuum can be obtained in a few minutes. 



Physical Laboratory, University of Lund, 

 June 1916. 



* Phil. Mag. vol. xxxii. p. 39 (1916). 



