Smithsonian Report, 1933. — DuBridge 



Plate i 



1. The difference in biological action between X-rays and neutrons is illustrated above. The upper figure 

 made by C. T. R. Wilson shows the tracts of electrons ejected from atoms in a cloud chamber by X-rays, 

 while the lower one made by E. O. Lawrence shows the recoil protons produced by neutrons traversing 

 a cloud chamber. 



2. Cloud chamber photograph showing the emission of positrons by radioactive nitrogen which was pro- 

 duced when the carbon plate shown at the right was bombarded by 900,000 electron-volt protons. (Ander- 

 son and Neddermeyer.) 



