44:4 Dr. J. P. V. Madsen on 



100 : 31. With a 9 mm. ping, however, inserted in the 

 conical hole just over the Ra the reading with no dome was 

 to the reading with a 6 mm. dome as 100 : 39. A similar 

 effect was also obtained with a Zn radiator. 



We see, then, that hardening the primary radiation has 

 the effect of hardening the secondary, and this is in agree- 

 ment with the suggestion put forward, viz., that in the case 

 of Zn, Al, and such substances the secondary emergence 7 

 radiation consists of two bundles corresponding to, in fact 

 derived immediately from, the two corresponding bundles 

 constituting the original 7 radiation. 



In fig. 9 the dotted curve has been plotted to a larger 

 scale, and the curve D obtained previously in fig. 3, and 

 shown in that fig. curve B, has been plotted with the same 

 maximum value as C/ for comparison with it. It will be 

 noticed that the curve D reaches its maximum for a smaller 

 thickness of radiator than does (7. This we might expect 

 as the secondary rays will be able to emerge from a greater 

 depth of the radiator, when some of them come out more 

 normally, as happens in the case represented by curve C. 



In the experiment from which the curve D is derived 

 the rays emerge very obliquely, and their self -absorption 

 by the radiator is much greater for a given thickness of 

 radiator. 



Otherwise the curves are in very good agreement. It is 

 to be noticed that in curve C certain effects may show them- 

 selves which are probably not present to such a large extent 

 in the case represented by D. Soft radiation, for instance, 

 has a much better opportunity of producing its effect in the 

 former case. 



Returning now to a brief consideration of the secondary 

 incidence 7 radiation. Although no attempt has as yet 

 been made to determine whether the radiation from different 

 substances is strictly homogeneous, it was thought advisable 

 to find out how its amount varied for any substance as the 

 Ra was covered with different screen thicknesses of Pb. 



In these experiments C, Zn, and Al showed much the 

 same effect. With no plug, a 5 mm. plug, and a 10 mm. 

 plug over the Ra, the corresponding readings were as 

 100 : 60 : 35. Now, since the value of X for the soft 7 radia- 

 tion from Ra is 1*35 for Pb, it should have required about 

 8 mm. of Pb to reduce it to 35 per cent, of its original value, 

 and had the incidence secondary radiation been due entirely 

 to the original soft 7 rays it should apparently have been 

 reduced to much the same amount. It is shown above that 



