Ii.—a Symposium on Electron Staining

SYNOPSIS Differential contrast in an electron image is determined mainly by differential scattering in the object. The theory of electron scattering is outlined, leading to the conclusion that contrast arises almost entirely from differences in density and thickness of different parts of the object. Numerical data are quoted for the elastic and inelastic cross‐sections of a number of elements; the variation with aperture angle is appreciable only for elements of low atomic number at the voltages at which an electron microscope is usually operated. The possibility of substance differentiation by photometry of the electron image is very limited in unstained material. Differential staining may improve the prospects, but only if a relatively large uptake can be achieved. Since it is the weight‐for‐weight uptake of stain which counts in enhancing contrast, it is not essential that the stain should contain a heavy element. On the other hand, contrast can be greatly increased simply by reducing the operating voltage, and substance discrimination should then be possible even in an extremely thin unstained section.