G88 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



had passed through a screen of another metal than when they came 

 direct from the tube ; in the language of the subject, the rays were 

 " hardened " by passage through the first screen. The following ratios 

 of transmission show this : 



Copper alone, 0.587 

 Aluminium alone, 0.875 

 Aluminium alone, 0.85 

 Copper alone, 0. 53 



Copper behind silver, 0.608 



Aluminium behind tin, 0.928 

 Aluminium behind copper, 0.91 

 Copper behind tin, 0.65 



TABLE VIII. 



Effect of Transmission through Silver on Penetrating Power for 



Aluminicm. 



In these experiments the thicknesses of the various screens were : 

 copper, 0.0044 cm. ; silver, 0.0019 cm. ; aluminium, 0.027 cm. ; tin. 



