134 



The Electron Microscope 



Atoms with an atomic number larger than Z ^ 2 ought to be 

 above the visibiHty level. This estimate is more optimistic than 

 Schiff's, who puts the limit to Z = 7, because he takes only the 

 elastic scattering into account. 



\\ hat is the apparent size of heavier atoms ? The discussion 

 is much simplified if we consider only the elastic part of the 

 scattering, and replace the atoms by a convenient model, such 

 as the Thomas-Fermi atom.* (Mott-Massey,^^ p. 126.) This is 



!* II 'V .* \ |\ 



















(. 



o 



Fig. 48. Electron density diagram of NaCl crystal, obtained by W. L. 

 Bragg's method of Fourier analysis from a great number of diffraction 

 photographs. (H. G. Grimm, R. Brill, C. Hermann and CI. Peters, 

 Naturzviss, 26, 479, 1938) 



* This part was written and figure 47 drawn before I was aware that 

 SchifT "^2 had discussed the problem in almost exactly the same way. 



