Mr Kaye, The Selective Absorption of Rontgen Rays. 245 
Conclusions. 
The results indicate that a metal screen is specially transparent 
to Rontgen radiation from an anticathode of that metal, and that 
this abnormal transparency is shared in less degree by metals with 
atomic weights differing little from that of the anticathode. 
The effect seems to point to the view that the Rontgen rays 
emerging from the interior of the anticathode to the surface 
undergo selective absorption, leaving the remainder specially 
penetrating to further layers of the same substance and to a less 
extent to substances of neighbouring atomic weights. 
Using a plate of aluminium as the absorber, the amount 
of transmitted radiation was found to be approximately propor- 
tional to the atomic weight of the metal used as anticathode over 
a wide range of atomic weights. 
It may be that this selective absorption is a maximum for one 
particular degree of hardness of X-rays depending on the metal, 
and that a metal of high atomic weight shows selective absorption 
for a harder type of Rontgen ray than that which gives the effect 
for a metal of smaller atomic weight. 
The experiments are being continued. 
I should like to thank Prof. Thomson for some helpful sug- 
gestions, and for his interest in the investigation, which was carried 
out in the Cavendish Laboratory. 
