= Early two-stage electron microscope experiment
<Ernst Ruska> and <Max Knoll> developed <electron optics> while improving the writing spot of a <cathode-ray oscilloscope>.<The Development of the Electron Microscope and of Electron Microscopy by Ernst Ruska>[Ruska, Nobel lecture, §C; RMP reprint pp. 628–629]{ref} A structured object tests <image formation> more strongly than a single bright spot: each feature must retain its own image position.
On 7 April 1931, a <cold cathode> source and two short magnetic coils imaged aperture grids at $50\,\mathrm{kV}$. A <platinum> grid preceded the first coil; a bronze grid preceded the second. The <fluorescent screen> showed one-stage grid images and the first grid reimaged through both stages. Comparing these patterns tested successive <image formation>, not merely beam concentration. Heating could destroy even metal grids. This prototype did not establish resolution beyond an <optical microscope>.<The Development of the Electron Microscope and of Electron Microscopy by Ernst Ruska>[Ruska, §E, Figs. 2–3; RMP reprint p. 630: dated one-stage and two-stage comparison]{ref}
In 1933, Ruska measured $3\,\mathrm{mm}$ <focal lengths> at $75\,\mathrm{kV}$ with a pole-piece lens, then built an instrument giving $12000$-fold <magnification>.<The Development of the Electron Microscope and of Electron Microscopy by Ernst Ruska>[Ruska, §E, Figs. 4–5; RMP reprint p. 631]{ref} TU Berlin dates images surpassing conventional <optical microscope> resolution to 1933.<Ernst Ruska by Technische Universität Berlin>[TU Berlin, indexed historical biography: 1933 prototype surpassing light-microscope resolution]{ref}
A <fluorescent screen> converts the spatial <Electron> distribution into visible <light>.<Transmission Electron Microscope (TEM) by JEOL>[JEOL, The difference between Electron Beam and Light: fluorescent image readout]{ref} A collector reporting only total <electric current> would discard that spatial pattern. In a two-stage image relay, the first lens makes an intermediate <real image>; the second treats that image as its object. The diagram below illustrates this geometry with invented distances and an ideal beam.
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