Question of the Homogeneity of 7- Rays. 737 



use plates so large that no radiation gets through the edges.. 

 I£ instead of plates of infinite area we use truncated hemi- 

 spheres, the part of the radiation escaping from the sides of 

 the truncated hemisphere can be allowed for. 



The total initial radiation I over the whole 180° is divided 

 into two parts, S and P, to represent the portions escaping: 

 through the sides and top of the truncated hemisphere. 

 Then S = I (T/R) where T is the thickness of a truncated' 

 hemisphere of radius R. Then 



S*=I„(|)e- XK ; 



t/r 



*=f e- XT / co90 d(-cos0) 



cos 9 = 1 



Xr 



e 



y 



Xt 



h = e -x« +xT[Ei(-XT)] -\T[Ei( -XR)] . . (6) 



Equation (6) has been tested experimentally with hemi- 

 spheres of zinc and lead built up of circular plates of 

 diminishing radius from base to top, the radioactive prepara- 

 tion being placed at the centre of the base. Fig. 3 shows 

 the apparatus employed. The ionization chamber consisted 

 of two copper concentric hemispherical bowls, A and B, of 

 diameter 40 cm. and 20 cm. respectively with the absorbing 

 hemisphere C concentrically placed within the inner bowl. 

 The wall thickness of B was about 0*5 mm. The radium was 

 placed at the centre of C at the point marked D. Thus the 

 paths of all rays over the whole 180° within the ionization 

 chamber were equal. The ionization chamber was fixed to 

 the under side of a table E, carrying the electroscope F and 

 reading microscope. The base of the electroscope was 

 protected by a circular block of lead, Gr, 5 cm. thick, of the 

 same diameter as the electroscope, through central holes in 

 which was placed an insulating plug H bearing the electrode 

 K. The latter consisted of a construction of wires similar 

 to an open umbrella frame at its lower end, which carried on 

 its upper end a gold leaf. J was an additional lead plug. 

 The absorbing hemisphere rested on a platform which could 

 be raised or lowered at will. The dotted lines show the 

 hemisphere in its lowered position. The platform consisted 



Phil. Mag. B. 6. Vol. 19. No. 113. May 1910. 3 B 



