 It was not until the invention of the electron microscope in 1931 by the German engineers Ernst Ruska (1906–1988) and Max Knoll (1887–1969), that virus particles, especially bacteriophages, were shown to have complex structures.The sizes of viruses determined using this new microscope fitted in well with those estimated by filtration experiments.Viruses were expected to be small, but the range of sizes came as a surprise.