26 



The Electron Microscope 



between the two devices is that in the spherical type relatively 

 small voltages (of a few thousand volts) are sufficient to produce 



Fig. 7 



cold or auto-electric emission from the sharp point. The mag- 

 nifications obtainable with these simple devices are of the order 

 of a few hundred thousand, and their resolving power is remark- 

 ably high, of the same order as of the supermicroscope, but 

 their field of application is rather limited. They have been used 

 by M. Benjamin and R. O. Jenkins ^"^ in a very thorough study 

 of the auto-electric emission of various clean and contaminated 



