82 The Electron Microscope 



oil pumps. The manufacture of the Siemens instrument was 

 discontinued, in 1945, at the time of the occupation of Berlin. 



In the second half of 1945, the first British commercial elec- 

 tron microscope was put on the market by the Metropolitan 

 Vickers Co., Trafford Park, Manchester, England. It was 

 developed by M. E. Haine. It is a magnetic instrument which 

 operates with a carefully smoothed voltage of 50 kv, derived 

 from the 50 cycle mains but regulated by a negative feedback 

 controller. It has no objective diaphragm and is evacuated by 

 an oil pump. Several instruments have given excellent service 

 in practice, though they were mostly used for purposes where 

 50 A resolution was amply sufficient. A new model with many 

 novel features is in preparation. 



The first French commercial electron microscope appeared 

 on the market, early in 1946. It was developed by H. Bruck 

 and P. Grivet and is manufactured by Compagnie Generale de 

 Telegraphic sans Fil, Paris. This is an electrostatic instrument, 

 the only one on the market at the present time. It was already 

 mentioned that electrostatic instruments operating with uni- 

 potential lenses, that is to say with electrodes which are con- 

 nected either to the cathode or to the anode are free from the 

 necessity of very careful regulation of the operating voltage. 

 One cannot entirely dispense with regulation, as E. G. Ram- 

 berg ^^ has shown that at electron energies of 50-100 kev the 

 relativistic corrections of electron dynamics are no longer negli- 

 gible, and equation (19) on p. 28 ceases to apply rigorously, but 

 smoothing to 2-3 per cent is sufficient, and this can be achieved 

 by very simple means. The C.S.F. microscope is operated at 

 100 kv and contains two unipotential single lenses. These con- 

 sist of two outer apertured electrodes, dished inwards, connected 

 with the anode, and a central electrode, with a somewhat larger 

 aperture, connected with the cathode. As unipotential lenses 

 have a fixed focus, the magnification is fixed once for all at 

 the convenient figure of 6500. The focusing of the object is done 

 mechanically, by longitudinal displacement. The best resolution 

 achieved to date is 80 A. The C.S.F. microscope is very simple 

 compared with any instrument of the magnetic type, as may be 



