858 Prof. Bragg and Mr. Grlasson on a Want of 



In this way it was easy to show that the expected want 

 of symmetry actually existed, particularly with aluminium, 

 celluloid, or paper as the radiators, substances of small 

 atomic weight. But the experiments were open to some 

 extent to the objection that a was too large compared with 

 b-c, and that possibly the excess of emergence over incidence 

 was an apparent effect due to actual variations of a under 

 different circumstances. The current a was, in fact, due to 

 several causes. There was a small natural ionization leak 

 even when the X-rays were not acting ; there was an effect 

 due to primary X-rays which had penetrated the walls of 

 the chamber, though they were made of zinc one-eighth 

 of an inch thick. But the greatest part of a was due to a 

 diffusion of soft rays about the primary beam, much of which 

 came through the hole at B at such an angle as to penetrate 

 the screen SS ; it could be largely cut out by thilckening the 

 screen. Again, part of a was due to radiation returned from 

 the open air abave the ionization-chamber. Some of these 

 radiations might be appreciably interfered with by placing 

 the radiating sheet at B or at the top of the chamber. We 

 were, however, able to satisfy ourselves by special experi- 

 ments that the want of symmetry was quite real, and that as 

 a matter of fact no valid objection could be made. But we 

 abandoned the first arrangement for a second which, as we 

 expected, would show the want of symmetry more clearly, 

 and which proved better than the first in every way. The 

 first method was exactly the same as that used by Madsen 

 in examining the secondary y rays ; but it was clear that the 



enormous difference which these rays showed was not going 



to be repeated in the case of the X-rays. 



Our new arrangement was, as shown in fig. 1, or, inverted, 



in fig. 2. Two cylinders of brass, each 2 in. long, but of 



different diameters — 4 in. and 2 in. — were 



fixed to a connecting piece DD, shown in Fig. 4. 



plan in fig. 4. The latter resembled a 



light brass wheel with four spokes, and 



various thin screens cut in the form of flat 



rings could be attached to it, filling up all 



the spaces between the spokes. In fig. 1 



the double cylinder is shown as arranged 



for the measurement of incidence secondary 



radiations ; the radiating sheet was placed at C, supported 



by a sheet of celluloid lying flat on the top of the cylinder. 



A hole was cut in the centre of the celluloid sheet big enough 



to allow the primary beam to pass through without touching 



the edges ; and a fluorescent screen was used to make sure 



