Transmission Electron Microscope.
TEMs use electrons as “light source” and their much lower wavelength makes it possible to get a resolution a thousand times better than with a light microscope(from the Nobel Prize website)
Hypothesized in 1929 by Knoll and Ruska at the Technische Hochschule, it was first built in 1932. A North American prototype of the transmission electron microscope (TEM) was constructed starting in 1937 at the University of Toronto. For a detailed account of the history and the politics involved in its construction and its initial and later applications see Rasmussen, N, in Studies In History and Philosophy of Science Part A, 1996