Commercial Electron Microscopes 81 



ment was just able to separate two particles which can be seen 

 below the line indicating a length of 1 M', near one end of it. 

 From this it appears that under favorable conditions a resolution 

 limit of 100 A can be expected, sufficient for a wide range of 

 applications. 



In 1944, the Radio Corporation of America completed the 

 development of two new commercial models. These are the 

 Type EMU Universal Model, shown in figure 25 and the Type 

 EMC Console Model, shown in figure 26. Apart from the strik- 

 ing elegance and convenience of its design the EMU model 

 represents a progress in comparison with the Type B one, by 

 embodying an almost continuous range of magnifications be- 

 tween 80X and 20,000X, an object stage for stereoscopic photo- 

 graphs, and the dififraction adapter described in chapter 14. 

 The main features of the EMC model are substantially the same 

 as described in connection with the experimental desk model. 



3. Other Commercial Electron Microscopes 



The Siemens & Halske A.G. in Berlin were the first to appear 

 on the market, in 1939, with a commercial electron microscope, 

 developed by E. Ruska and B. v. Borries. This very successful 

 instrument was in many respects simpler than the American 

 electron microscopes. It could be operated up to 100 kv, but 

 the high voltage supply was of the kind used in commercial 

 X-ray equipments. The lens currents were supplied by accumu- 

 lators. In spite of the less perfect smoothing, this instrument 

 achieved resolutions down to 18-20 A, a performance which has 

 been surpassed only quite recently by the R.C.A. microscope. 

 There appear to be two reasons for this. One is that the electron 

 gun of the Siemens instrument produced at the specimen a 

 current density perhaps a hundred times better than the R.C.A. 

 instrument prior to the redesigning of its electron gun, in 1945. 

 The other is that the Siemens instrument was always used with 

 an objective aperture which, unlike the one in the R.C.A. in- 

 strument, seldom gave trouble probably because the Siemens 

 microscope was always evacuated with mercury pumps, not with 



