Commercial Electron Microscopes 



77 



Fig 24, Test photograph of simplified R.C.A. microscope. Thin membrane 

 spattered with heavy metal. Electronic magnification X3,100 



and the specimen, very near to the latter, in order to reduce the 

 cross section of the beam and the heat development in the 

 specimen. Such wide bundles produce large spherical aberration, 

 and cannot be used for photographs with high resolution, but 

 they are very useful for focusing the specimen. Apart from the 

 higher intensity, a wide bundle has the advantage of smaller 

 depth of focus and enables more accurate focusing. When the 

 picture is taken a potential of — 73 volts is impressed on the 

 grid, which not only cuts down the current, but brings the cross- 

 over very close to the cathode and makes the illuminating beam 

 at the specimen very nearly parallel, with a divergence near to 

 the irreducible minimum, discussed in chapter 5. 



The specimen is no longer contained in a special object 

 chamber with an air lock. The whole volume of the microscope 

 is so small that a vacuum of the order of 5.10"^ mm mercury 

 can be established in 1^-3 minutes, according to the length of 



