Charged-particle optics is still a young subject. The first experiments on electron diffraction were made in the 1920s, shortly after Louis de Broglie associated the notion of wavelength with particles, and in the same decade Hans Busch showed that the effect of a rotationally symmetric magnetic field acting on a beam of electrons traveling close to the symmetry axis could be described in optical terms. The first approximate formula for the focal length was given by Busch in 1926–1927. The fundamental equations and formulas of the subject were derived during the 1930s, with Walter Glaser and Otto Scherzer contributing many original ideas, and by the end of the decade the German Siemens Company had put the first commercial electron microscope with magnetic lenses on the market. The latter was a direct descendant of the prototypes built by Max Knoll, Ernst Ruska, and Bodo von Borries from 1932 onwards. Comparable work on the development of an electrostatic instrument was being done by the AEG Company.