424 Dr. J. P. V. Madsen on 



It will be shown in the present paper that this prediction 

 is fulfilled very exactly ; that the want of symmetry in the 

 amount of radiation from the two sides of the plate is very 

 marked, that a softening of the original rays is effected, and 

 that in addition there is in some cases a lack of symmetry in 

 the quality of the emergence and incidence y radiation. 



Kleeman, from a study of the incidence radiation, has 

 advanced a theory of selective absorption, not only for the 

 secondary but also for the primary 7 rays. In the light of 

 the more comprehensive information which can be obtained 

 from a study of both the emergence and the incidence radia- 

 tion, an attempt will be made in the present paper to show 

 that most of the secondary ray effects can be explained, 

 if we suppose that from the radium there are originally 

 emitted two sets of homogeneous 7 rays, which each 

 subsequently suffer modification by the process of scat- 

 tering, becoming softened and in some cases broken up, 

 giving rise to f3 rays. 



The present paper is intended to give a preliminary outline 

 of the experimental work and general theory. 



The measurements are in many cases small and difficult 

 to make with any very great accuracy, but the effects to be 

 described seem well marked. It will be necessary to extend 

 with greater care many of the details of the work before it 

 -can be considered as at all complete. 



The arrangement of apparatus employed in the first experi- 

 ments is shown in fig. 1 (PI. VIII.). The radium is placed near 

 the apex of a conical hole made in a block of lead. By means 

 of a powerful magnet, the poles of which are shown, most 

 of the (3 rays could be prevented from passing out of the 

 conical hole. The magnet and lead block containing the Ka 

 were surrounded by an iron case, to prevent the magnetic 

 field from producing any effect in the ionization-chamber. 

 Resting upon the iron case and suitably insulated from it is 

 placed a lead plate one inch thick, with a circular portion 

 removed from the centre. The ionization-chamber standing 

 upon this lead plate is made of lead 1*25 mm. thick, and 

 contains an insulated electrode in the form of a circular 

 wire ring, suitably protected by sulphur, and connecting to 

 the electrometer or to earth by suitable keys. 



In the path of the 7 radiation which proceeds from the 

 conical hole, plates of material which are used as radiators 



