 In the years that followed, as optical microscopes were improved "inclusion bodies" were seen in many virus-infected cells, but these aggregates of virus particles were still too small to reveal any detailed structure.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.