* 1931: Ernst Ruska and Max Knoll, working at the University of Berlin, built the first electron microscope.

* 1933: Ruska built the first electron microscope capable of resolving objects smaller than the wavelength of visible light.

* 1939: The first commercial electron microscope was built.

The development of the electron microscope was a major breakthrough in microscopy, allowing scientists to see objects at a much higher resolution than with light microscopes.