126 The Electron Microscope 



according to calculations of Louis de Broglie to raise the operat- 

 ing voltage to at least 300 kv, if sufficient penetrating power is to 

 be achieved. It appears rather doubtful whether at such high 

 voltages lenses of sufficient power can be realized without auto- 

 electronic breakdown. 



Another consideration, which reduces the gain further, is that 

 it may be very difficult to keep the chromatic error at the re- 

 quired low level, as protons have to be generated by gas dis- 

 charges, which may not yield currents of as homogeneous 

 velocities as thermoelectric cathodes, in the case of electrons. 

 But it appears that this capital difficulty has been satisfactorily 

 solved, and the inventors of the proton microscope have powerful 

 sources of thermal protons at their disposal. They expect to 

 realize a gain of about 8, which would mean a resolution limit 

 below 1 A. 



One grave objection naturally presents itself. Will the pro- 

 tons with their relatively huge masses not smash up the struc- 

 tures which they are intended to explore? This is rather gen- 

 erally expected outside the circle of the College de France, 

 although the inventors themselves are optimistic. Perhaps the 

 situation will repeat itself which existed, round about 1927-28, 

 with regard to the electron microscope. IMany physicists turned 

 away from the project, as they expected the electrons to burn 

 fine organic structures to a cinder. Perhaps the optimism of 

 the College de France circle will be similarly rewarded as that 

 of Knoll and Ruska. 



