704: Mr. H. G-. J. Moseley on the 



been much improved by reducing the breadth of the de- 

 fining slit to about 0°2 mm. The most convenient type of 

 X-ray tube is drawn to scale in fig. 1. The aluminium 

 trolley which carries the targets can be drawn to and 

 fro by means of silk fishing-line wound on brass bobbins. 

 An iron screen S fastened to the rails is furnished with a fine 

 vertical slit which defines the X-ray beam. The slit should 

 be fixed exactly opposite the focus-spot of the cathode-stream, 

 though a slight error can be remedied by deflecting the cathode 

 rays with a magnet. The X rays escape by a side-tube 



2 J cm. diameter closed by an aluminium window O022 mm. 

 thick. The X-ray tube, which has a capacity of over 



3 litres, was exhausted with a Gaede mercury-pump, for the 

 loan of which I am indebted to Balliol College. 



Fiar. 1. 



The radiations of long wave-length cannot penetrate an 

 aluminium window or more than a centimetre or two of air. 

 The photographs had therefore in this case to be taken 

 inside an exhausted spectrometer. Fig. 2 gives a vertical 

 section to scale of the X-ray tube and spectrometer. The 

 former consists of a bulb containing the cathode, joined by a 

 very large glass T-piece to a long tube of 4 cm. diameter, 

 in which are the rails R and the carriage C. S is the 

 defining-slit and W a window of goldbeaters' skin which 

 separates the tube from the spectrometer. This material, 

 which is usually air-tight, though sometimes it may 

 require varnishing, is extremely transparent to X rays. 

 A circular window of 2 cm. diameter will easily with- 

 stand the pressure of the atmosphere if left undisturbed. 

 In these experiments, however, the pressure was relieved 



