116 



Tlie Electron Microscope 



stant is determined by the ratio beween electron injection and 

 removal. If the removal is negligible, electrons will accumulate 

 in the accessible space until they have depressed the potential 

 at the inner radius to the level of the cathode potential. The 

 inner radius itself moves outward during this accumulation, until 

 it reaches the magnetic field line passing through the cathode. 

 From this moment on, no further electrons can leave the cathode ; 

 the cloud is saturated. The initial velocities of the electrons will 



produce near the boundary departures from the — law, which 

 results in blurring of the edges. 



£k tended cathode, or inttracbi on. 



Fig. 42 



