Commercial Electron Microscopes 79 



time it has been left open. The object is removed together with 

 its stage, which allows scanning a circular area of the specimen 

 2.5 mm in diameter. 



A striking new feature of tlie small microscope is the fact 

 that the objective lens and the projection lens form one unit, 

 energized by the same coil. The iron pole piece unit is machined 

 in one, which enables superior accuracy to be obtained and 

 obviates special alignment mechanisms. The figure shows the 

 pole piece adjusted for a magnification of 5,000. If the lower 

 magnification of 500 is desired, a separate cartridge has to be 

 removed from the projection end of the pole piece. This can be 

 done through the window of the fluorescent screen which can be 

 easily removed. No steps are provided in the supply of the 

 energizing coil, only a fine adjustment for the focusing, which 

 changes the magnification only slightly. 



The viewing chamber and the method of photography have 

 also undergone radical changes. The fluorescent screen is now 

 viewed in transmission. It wall be noted with interest that the 

 R.C.A. has developed new fluorescent materials which give a 

 three- to four-fold increase of image brightness as compared with 

 the willemite commonly used in American television, which was 

 hardly, if at all, inferior to the best sulfide screens, preferred in 

 Britain. At the same time the grain size of the fluorescent 

 powder is fine enough to resolve about one hundred lines per mm. 



Only a single photographic plate is used. An ingenious 

 mechanism enables one to tilt this in front of the fluorescent 

 screen, exposing it by opening the lid, to close the lid, and to 

 tilt it again into a horizontal position, all by the movements of 

 a single lever. The plate can be removed through a door, visible 

 just below the fluorescent screen. 



The exhaust mechanism has been also greatly simplified, and 

 its operation is made nearly automatic. Three valves are worked 

 by a lever in such a way that a complete pumping cycle cor- 

 responds to one to-and-fro motion of the lever. Only the timing 

 is left to the operator. Evacuation is effected by a single-stage 

 high-speed oil diffusion pump which is an integral part of the 

 microscope unit and a mechanical pump which stands under- 



