The theoretical and practical limits to the use of the optical microscope were determined by the wavelength of light. But when the oscilloscope was developed, scientists realized that cathode-ray beams could be used to resolve much finer detail because their wavelength was so much shorter than that of light. The electron microscope revolutionized biological research, and for the first time scientists could see the molecules of cell structures, proteins, and viruses.

The electron microscope we know today was developed in numerous steps, with many companies and scientists inventing better and more precise instruments. In 1928, German physicist Ernst August Friedrich Ruska (1906–1988) and German electrical engineer Max Knoll (1897–1969), using magnetic fields to “focus” electrons in a cathode-ray beam, produced a crude instrument that gave a magnification of 17; by 1932, they had developed an electron microscope having a magnification of 400. By 1938, Canadian inventor and physicist James Hillier (1915–2007) and others had advanced this magnification to 7,000—the first practical electron microscope; he also helped push to make the instrument standard equipment in hospitals, universities, and laboratories around the world. More improvements came later; for example, Russian-American inventor and scientist Vladimir Zworykin (1889–1982; he was also a pioneer in television technology) helped, in 1939, to develop an electron microscope with a magnification of 100,000 times, whereas today’s electron microscope can display a power magnification of up to 2,000,000.