124 The Electron Microscope 



reduce b ^^ Lg to about 2 mm or less. This appears by no 

 means impossible especially if use is made of the suggestion in 

 the previous chapter, to operate the lens system with a beam 

 voltage of only 5-10,000 volts, and bring up the electrons by 

 after-acceleration to optimum photographic efficiency. 



It may be noted that though by condition (54) the suggested 

 lens system is made insentitive to fluctuations of the driving 

 voltage, it is not achromatized, in the sense that it will not cor- 

 rectly focus electrons which have suffered inelastic collisions 

 in the object. This is neither necessary nor desirable in view 

 of the beneficial effect of the chromatic aberration on the contrast. 



There is one more question which is naturally provoked by 

 the suggestion of using an electron cloud as a lens : Will the 

 cloud not act on the beam like frosted glass? This is a very 

 difficult question to decide, as the only experimental data known 

 to the author are the classical experiments of I. Langmuir '^^ on 

 the scattering of electron beams in ionized gases. But these 

 w^ere carried out with 100-volt electrons, and in a plasma, in 

 which the ions might also play a part, difficult to estimate. It 

 can be said only, that both, from a somewhat bold extrapolation 

 of Langmuir's results and from an unpublished theoretical in- 

 vestigation of the author, it appears that the effect will not cause 

 serious interference. It appears highly desirable to settle this 

 point by the simple experiment of shooting a fast electron beam 

 parallel to the axis through a static magnetron. It may be 

 mentioned that such an experiment would also at once settle 

 the question how nearly Hull's simple theory agrees with actual 

 conditions in a magnetron, as on Hull's theory the beam would 

 suffer no radial deflection. 



The Proton Microscope 



A most interesting attempt toward higher resolutions is being 

 made in France, at the College de France, Paris, under direction 

 of M. Magnan and the general and theoretical guidance of Louis 

 de Broglie. A microscope with protons, that is to say atomic 

 hydrogen ions instead of electrons, is in the course of construe- 



