subjected to Radium Rays. 
349 
only to the a, /3, and 7 rays, but also to the emanation. The 
standard plate p 0 was also removed from the box, and placed in a 
similar small vessel, but without the radium. 
After a definite time, the two plates were replaced in the 
boxes, and after a few hours had elapsed, to give time for the 
induced activity on the plate p 1 to die away, their rates of leak 
were again measured. It was found that the exposure to radium 
had greatly decreased the photo-electric effect of the test 
plate p 1 . 
The figures are as follows : 
Copper. 
Time of exposure ... ... ... ... ... ... 4,days 
Ratio Pifp 0 after polishing, but before exposure to radium IT 3 
Ratio pilpo after exposure to radium for 4 days ... ... '082 
After repolishing p x ... ... ... ... ... T20 
After repolishing both p 0 and p x ... ... ... ... IT 2 
It will be seen from these figures, that the exposure to 
radium reduced the leak from the exposed plate p x to about 
7°/ o of its original value, that is by more than 90°/ o . The de- 
crease in the leak from the standard plate p 0 in the same period, 
only amounted to a few per cent. 
It will be seen also that the fatigue is entirely removed by 
repolishing the surface of the exposed plate. 
On account of the importance of the result the experiment 
was repeated with fresh copper plates ; but the results were 
exactly the same. 
It was found further that it was not necessary to expose the 
plate to the radium emanation in order to produce these effects ; 
but that photo-electric fatigue could be produced, to a smaller 
degree, by exposure merely to the /3 and 7 rays from radium. 
Thus using copper plates the ratio of the leak from the test 
plate p, to that from the standard plate p 0 was found to be IT 2, 
immediately after polishing. p 1 was then exposed to the /3 and 
7 rays from radium, without being removed from the box. It was 
found that after two hours’ exposure of p l to the radium rays the 
ratio of the photo-electric effects was reduced to T08, while after 
an exposure of six hours it was further reduced to T02. It 
appears, therefore, that photo-electric fatigue can be produced by 
the ft and 7 rays alone. 
It is hoped to make further investigations on this point. 
