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title: "Ruska: making an electron image was only the beginning"
description: "What separated a magnified grid from a microscope capable of revealing finer structure?"
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[Electron probes: reading the evidence](https://sandboxphysics.com/stories/electron-evidence/)

Diffraction · Imaging · Spectroscopy

# Ruska: making an electron image was only the beginning

Sandbox Physics · 2026-10-22

What separated a magnified grid from a microscope capable of revealing finer structure?

1. Electron illumination
2. Thin specimen
3. Objective and projection lenses
4. Recorded image and scale

An imaging signal chain, not a scale drawing of the 1931 instrument. Intermediate images must lie in the conjugate planes of successive lenses.

## First demonstrate imaging, then ask what it resolves

A magnetic field bending an electron beam is not yet a microscope. The instrument must map different specimen positions to distinct image positions with a reproducible scale. An intermediate image must also become an object for the next stage. Those requirements turn beam focusing into electron optics.

In his Nobel lecture, Ruska places the work in Max Knoll’s cathode-ray oscillograph group and describes testing Hans Busch’s magnetic-lens theory. The historical achievement was collective and experimental: construct a lens, measure an image and make a second stage work. It should not be presented as a device that appeared immediately after the wavelength equation.

Evidence and definitions: E. Ruska, The development of the electron microscope and of electron microscopy (Nobel lecture, 1986)

## 1931: two stages were a proof of principle

Ruska’s lecture dates the two-stage grid-image demonstration to April 1931. Its modest magnification established that electron images could be enlarged successively by magnetic lenses. The grid, screen and lens arrangement supplied identifiable objects and image planes; seeing a concentrated spot alone would not have established this.

Total magnification is the product of the signed stage magnifications under a consistent convention. That tells us where a feature is mapped and how large it appears. It says nothing by itself about the smallest separation the apparatus can distinguish. Enlarging an already blurred intermediate image enlarges the blur too.

$$
M_{\mathrm{total}}=\prod_j M_j
$$

Evidence and definitions: E. Ruska, The development of the electron microscope and of electron microscopy (Nobel lecture, 1986) JEOL, Objective lens — TEM glossary

## 1933: shorter focal lengths changed the practical question

Ruska describes compressing the lens field with iron polepieces to obtain short focal lengths, work associated with his thesis and Bodo von Borries. He completed a more powerful instrument in late 1933. The important transition was from demonstrating successive magnification to making finer specimen information accessible.

Read the original images with their specimen, scale and operating conditions. A large final image does not demonstrate a new resolution limit unless separable detail is actually recorded. Conversely, a short focal length is an engineering means, not a universal certificate of resolution: illumination, aperture and stability remain in the experiment.

Evidence and definitions: E. Ruska, The development of the electron microscope and of electron microscopy (Nobel lecture, 1986) JEOL, Objective lens — TEM glossary

## A short wavelength creates an opportunity

Electrons accelerated through a large voltage have a wavelength far shorter than visible light. That makes a much smaller diffraction scale possible. The calculation below uses a positive voltage, electron rest mass and positive elementary charge; it includes the relativistic correction and gives about 3.70 picometres at one hundred kilovolts.

This is a wavelength, not the resolution of a particular microscope. An ideal aperture imposes diffraction blur, while real lenses add other errors. Smaller wavelength cannot restore spatial frequencies rejected by the aperture, repair specimen motion or replace missing signal. The linked column model and a wavelength calculator therefore answer different questions.

$$
\lambda=\frac{hc}{\sqrt{eU(eU+2m_ec^2)}}
$$

Evidence and definitions: JEOL, Objective lens — TEM glossary JEOL, Spherical aberration — TEM glossary JEOL, Wavelength of electron — relativistic wavelength table

## The aperture is a compromise, not a sharpness knob

Rays travelling farther from the axis do not generally focus in the same plane as near-axis rays. This spherical aberration competes with diffraction: admitting a wider angular range may reduce diffraction blur while increasing lens blur. Chromatic aberration adds sensitivity to energy spread. A single ideal ray diagram suppresses precisely the effects that matter near a resolution limit.

In a real instrument, specify the accelerating voltage, aperture convention, aberration coefficients and criterion used to call two features resolved. Also check drift, vibration and electrical stability. Changing screen size or display zoom cannot replace these measurements. Modern aberration-corrected instruments require a separate account; their performance must not be attributed to the first two-coil microscope.

Evidence and definitions: JEOL, Spherical aberration — TEM glossary JEOL, Objective lens — TEM glossary

## An electron image is not automatically an atom map

The specimen changes the transmitted electron amplitude and phase. Apertures and lenses transform those changes before detection. Depending on thickness, orientation and imaging conditions, dark and bright regions can encode scattering, phase transfer or mass-thickness contrast. There is no universal rule that the darkest point marks an atom.

For weak-phase imaging, changing defocus can reverse or suppress contrast at selected spatial frequencies. An inferred structure therefore needs an imaging model and independent checks across conditions. The existing phase-transfer Lab demonstrates this later concept with a supplied synthetic specimen; it does not reproduce Ruska’s early observations or solve a unique structure from one image.

Evidence and definitions: JEOL, Phase-contrast transfer function — TEM glossary

## The specimen is part of the achievement

Thin specimens must survive the illumination long enough to provide useful information. Heating, radiation damage, contamination and specimen preparation can erase or alter the detail sought. Ruska’s account makes these obstacles visible alongside lens development; they belong in the history of microscopy, not in a footnote after the instrument is declared successful.

Use the two linked models in sequence. First locate object and image planes in the column; then hold the reference specimen fixed while changing phase transfer. The first asks whether the optics forms an image. The second asks which specimen information survives into it. That distinction is the lasting lesson behind the move from magnification to microscopy.

Evidence and definitions: E. Ruska, The development of the electron microscope and of electron microscopy (Nobel lecture, 1986) JEOL, Phase-contrast transfer function — TEM glossary

## Continue with the models

These existing teaching Labs produce simulated records, separate from the historical evidence.

### [Follow the conjugate planes](https://sandboxphysics.com/labs/electron-column/)

Change a lens setting and follow the intermediate image and final image together.

The column is a paraxial thin-lens teaching model in one transverse plane. It does not solve the historical magnetic field or predict an instrument resolution.

### [Compare image contrast with its transfer function](https://sandboxphysics.com/labs/hrtem-ctf/)

Change defocus while keeping the synthetic specimen fixed; compare the image with the separate known reference.

This later weak-phase model omits multiple scattering. The reference atom overlay is supplied information, not a structure reconstructed from the image.

## Sources and reading notes

We read Ruska’s 1986 Nobel lecture, especially the short-coil experiments, two-stage images and polepiece lens development (pp. 357–365), using the Terasaki-hosted copy after the official PDF returned 403. JEOL’s instrument glossary supplies the modern lens, aberration and phase-transfer explanations. No historical micrograph is reproduced or claimed to have been reconstructed.

1. [JEOL, Wavelength of electron — relativistic wavelength table](https://www.jeol.com/words/emterms/20121023.071258.php)

   Manufacturer table read: the independently published value at 100 kV is 3.7014 pm.

2. [E. Ruska, The development of the electron microscope and of electron microscopy (Nobel lecture, 1986)](https://www.nobelprize.org/uploads/2018/06/ruska-lecture.pdf)

   Primary lecture read in a hosted copy; apparatus and early imaging, pp. 357–365. Official endpoint returned 403. [Read the hosted copy](https://mterasaki.us/cellbio/em/ruska.pdf)

3. [JEOL, Objective lens — TEM glossary](https://www.jeol.com/words/emterms/20121023.060558.php)

   Manufacturer explanation of lens field, focal length and aberration requirements read.

4. [JEOL, Spherical aberration — TEM glossary](https://www.jeol.com/words/emterms/20121023.020959.php)

   Manufacturer definition read; used for modern limitations, not a historical performance claim.

5. [JEOL, Phase-contrast transfer function — TEM glossary](https://www.jeol.com/words/emterms/20121023.002700.php)

   Weak-phase and no-multiple-scattering conditions, defocus and contrast transfer read.

[Continue: probes, light and measurement](https://sandboxphysics.com/stories/probe-evidence/) [Electron diffraction: two experiments, one wavelength test](https://sandboxphysics.com/stories/electron-evidence/electron-diffraction-crystal-records/) [Kai Siegbahn: when electron energies began to reveal chemistry](https://sandboxphysics.com/stories/electron-evidence/siegbahn-electron-spectroscopy-chemical-shifts/) [Before the apparatus: de Broglie’s matter waves](https://sandboxphysics.com/stories/quantum-evidence/de-broglie-matter-waves-phase-packets/)