Question: Who started the development of the transmission electron microscope, the first electron microscope, in 1931?
About the article "Microscope", section "Electron microscopy".

Candidate passages:
In the early 1900s a significant alternative to light microscopy was developed, using electrons rather than light to generate the image.
Ernst Ruska started development of the first electron microscope in 1931 which was the transmission electron microscope (TEM).
The transmission electron microscope works on the same principle as an optical microscope but uses electrons in the place of light and electromagnets in the place of glass lenses.
Use of electrons instead of light allows a much higher resolution.
Development of the transmission electron microscope was quickly followed in 1935 by the development of the scanning electron microscope by Max Knoll.
Electron microscopes quickly became popular following the Second World War.
Ernst Ruska, working at Siemens developed the first commercial transmission electron microscope and major scientific conferences on electron microscopy started being held in the 1950s.
In 1965 the first commercial scanning electron microscope was developed by Professor Sir Charles Oatley and his postgraduate student Gary Stewart and marketed by the Cambridge Instrument Company as the "Stereoscan".
An electron microscope uses a beam of electrons to illuminate a specimen and produce a magnified image.
Two common types of electron microscope are the scanning electron microscope (SEM) and transmission electron microscope (TEM). 
 Electron microscope *Transmission electron microscope Optical microscope uses reflectiveness or refractiveness of light to produce an image.
Scanning acoustic microscope Scanning probe microscope *Atomic force microscope (AFM) *Scanning electron microscope *Scanning tunneling microscope (STM)  Focus variation X-ray microscope Sub-microstructural properties of condensed matter, gas.
Imaging technology, Microscope.
See also the article on spectroscopy and the list of materials analysis methods. 
Electron crystallography is a method to determine the arrangement of atoms in solids using a transmission electron microscope (TEM). 
The first electromagnetic lens was developed in 1926 by Hans Busch.
According to Dennis Gabor, the physicist Leó Szilárd tried in 1928 to convince Busch to build an electron microscope, for which he had filed a patent.
The German physicist Ernst Ruska and the electrical engineer Max Knoll constructed the prototype electron microscope in 1931, capable of four-hundred-power magnification; the apparatus was the first demonstration of the principles of electron microscopy.
Two years later, in 1933, Ruska built an electron microscope that exceeded the resolution attainable with an optical (light) microscope.
Moreover, Reinhold Rudenberg, the scientific director of Siemens-Schuckertwerke, obtained the patent for the electron microscope in May 1931.
In 1932, Ernst Lubcke of Siemens & Halske built and obtained images from a prototype electron microscope, applying concepts described in the Rudenberg patent applications.
Five years later (1937), the firm financed the work of Ernst Ruska and Bodo von Borries, and employed Helmut Ruska (Ernst’s brother) to develop applications for the microscope, especially with biological specimens.
Also in 1937, Manfred von Ardenne pioneered the scanning electron microscope.
The first "practical" electron microscope was constructed in 1938, at the University of Toronto, by Eli Franklin Burton and students Cecil Hall, James Hillier, and Albert Prebus; and Siemens produced the first "commercial" transmission electron microscope (TEM) in 1939.
Although contemporary electron microscopes are capable of two million-power magnification, as scientific instruments, they remain based upon Ruska’s prototype.