544. Mr. C. G. Barkla on Energy of 
im positions shown in the figure, the screen OC being just 
outside the primary beam and D parallel to C ata distance 
of 15 centimetres. 

In C and D were two square apertures, R and 8, 5 em. sq., 
placed so that the lines joining corresponding points of the 
two were perpendicular to their planes. 
Behind the aperture S was placed an electroscope M * 
which was carefully shielded from radiations proceeding from 
all directions except through the two apertures R and 8. 
This radiation entered by a thin paper and aluminium window. 
Lead plates E and F protected it from secondary and tertiary 
radiation from metals. The rate of fall of the gold-leaf in 
electroscope M was then only affected by secondary radiation 
from air in the primary beam opposite aperture R. 
A second similar electroscope N was placed behind thick 
leaden screens Z in the primary beam, and received a narrow 
pencil of primary radiation through a small hole in this 
screen. Absorbing plates could be placed immediately in 
front of both electroscopes. The rays then entered the pri- 
mary .electroscope N by passing normally through the ab- 
sorbing plate. The secondary rays of greatest obliquity 
entering the electroscope M, in their passage through the 
* For description see previous paper. 
