422 



Mr. A. Muller on an X-Ray Bulb 



approximately 4*5 cm. The crystal-holder turned in a conical 

 bearing. It was connected by a transmission to a lever, the 

 free end of which was in contact with a revolving cam. 

 This scheme allowed the crystal to be oscillated within 

 convenient angles. The readings of these angles were made 

 on a circle divided in degrees. The cam was slowly turned 

 by a clockwork. 



Fkr. 2. 



s, 



s* 



Crystal 



photo- 

 plates 



in 



the 



P/ate 



The plate-holder had a rectangular opening which was 

 covered with black paper. The reflected X-rays passed 

 through this opening and fell on the plate. The 

 graphic records have been obtained on llford X-ray 

 which were cut in small strips. 



The time of exposure of the plates, which varied 

 present investigation between twenty minutes and an hour, 

 depends a good deal upon the relative position of the anti- 

 cathode and the slits of the spectrometer. By placing the 

 slits near the anticathode, they can be kept very narrow 

 without losing much intensity. This means that only a small 

 portion of the crystal is concerned in the reflexion, and that 

 one gets good definition of the spectral lines from a crystal 

 which would not give good images if used with wide slits. 

 With the present design of the bulb it was possible to put 

 the slit Sx at a distance of only about 1 cm. from the centre 

 of the anticathode. The plane of the two slits very nearly 

 coincided with the plane surface of the anticathode. This 

 scheme was used first by Bragg, and has two advantages. 

 The first is that the total intensity of the rays which fall on 

 the crystal becomes independent of the wandering of the 



