Siemens electron microscope, Germany, 1943.
© Science Museum / Science & Society Picture Library
The electron microscope is constructed from a tube in which electrons emitted from the cathode are focused, by magnetic and electrostatic fields, to form an enlarged image of the cathode on a fluorescent screen. By pasing the electrons through an object, such as a virus, a greatly enlarged image can be obtained on a photographic plate. The instrument has a very high resolving power compared with the optical microscope, due to the shorter wave-length asociated with electron waves.