Origin of the Electron Microscope.

In 1878, Ernst Abbe (1) proved that the resolution of the optical microscope is limited by the wavelength of light. This meant that even when immersion optics and ultraviolet light are used, the smallest detail that can ever be resolved optically is of the order of 100 millimicrons, or 10 angstroms. No means were conceived of resolving finer detail until two discoveries were made: (i) the wave properties of the moving electron, postulated by de Broglie, on theoretical grounds, in 1924 (2) and verified by Davisson and Germer (3) and by Thomson and Reid (4) in 1927; and (ii) the discovery by Busch (5) in 1926-1927 of the analogy between the effect of a magnetic coilthe focusing coil used since 1899 (6)on an electron beam and the effect of a convex lens on a light beam. Since the wavelength of the moving electron is smaller by many orders of magnitude than the shortest wavelength of light, these discoveries made it conceivable that extremely small objects might be imaged with an electron beam and electron lenses.