68 The Electron Microscope 



projector lens, and the second throw, followed by the photo- 

 graphic chamber. Every lens, the throw, the viewing chamber, 

 the object chamber, and the photographic chamber is a unit by 

 itself, accurately machined, and fitted together by means of large 

 surface flanges, lagged with gaskets of duprene, a synthetic rub- 

 ber. This sectionalized method of construction enables new 

 developments to be embodied in existing instruments. It entails 

 a very great number of vacuum joints; in addition to those 

 mentioned, there are three rubber-lagged glass windows in the 

 first throw, six in the viewing chamber, and one in the object 

 chamber. Yet a vacuum of the order of 10"*^ mm mercury can 

 be realized without difficulty, and leaks are a very rare occur- 

 rence. The exhaust system consists of a three-stage oil diffusion 

 pump, backed by a mechanical pump. No liquid air is necessary. 

 Very quick evacuation is insured by two airlocks, one for the 

 specimen, and one for the photographic plate, which can be 

 changed without breaking the vacuum in other parts of the 

 microscope. This is particularly important in the case of the 

 discharge tube, as tungsten filaments are rather sensitive to 

 atmospheric moisture. The air locks are preevacuated by a 

 separate mechanical pump. 



The discharge tube contains an electron gun, consisting of a 

 V-shaped tungsten filament and a cathode shield, the operation 

 of which has been explained in chapter 5. It is fitted with a 

 corona shield, and these are the only parts of the microscope at 

 — 60 kv against earth, all accessible parts are earthed. The two 

 sets of screws which can be seen in figure 18 below the discharge 

 tube allow two independent adjustments, so that the tube axis 

 can be exactly aligned with the condenser lens. The adjustment 

 can be watched on an oblique fluorescent screen with a small 

 central bore, visible in the figure just above the condenser lens. 

 This can be observed through a special port. A set of screws 

 just below the condenser lens (not visible in the figure) enable 

 the condenser and the discharge tube to be aligned with the 

 objective aperture as one unit. This adjustment can be watched 

 on a fluorescent screen with a small hole on top of the projector 

 lens. Finally, the objective lens, together with all the preceding 



