André Baranne designed precision astronomical spectrographs, including the ELODIE spectrograph, which was used for Michel Mayor and Didier Queloz's discovery of the planetary companion to 51 Pegasi. The instrument combined broad spectral coverage with precise radial velocity measurements.

The scientific goal was to distinguish tiny stellar Doppler shifts from instrumental changes. Its design and calibration supported long-term velocity stability sufficient for detecting giant planetary companions. The original instrument paper reports the construction, data treatment and measured performance, making the observing hardware part of the explanation of the discovery.^[ref]

Jean-Pierre Rivet coauthored measurements of stellar spatial coherence using intensity interferometry. Two separated optical telescopes recorded photon arrivals, and fast electronics measured the correlation of their intensity fluctuations.

Unlike combining optical field amplitudes directly, the experiment estimated a statistical correlation from many detected events. Observations of three bright targets tested how the contrast changed with angular source structure. The work demonstrated that modern photon-counting detectors could recover useful stellar visibilities with metre-class telescopes.^[ref]

Farrokh Vakili coauthored a modern stellar intensity interferometry experiment using two one-metre optical telescopes separated by fifteen metres. The team correlated photon-counting signals from bright stars, reviving a technique historically associated with Robert Hanbury Brown.

The measured quantity was the zero-delay correlation between detected light intensities. Its contrast contains information about angular source structure. The team measured reliable visibilities for two marginally resolved stars and found reduced contrast for a binary star, consistent with its more extended apparent structure.^[ref]

Frank Watson Dyson coauthored the report of the 1919 eclipse observations with Arthur Eddington and Charles Rundle Davidson. The experiment tested the predicted bending of stellar light passing near the Sun.

Comparing stellar positions on eclipse and reference plates tested a small radial displacement. Changes in plate scale and focus could alter the signal, so the report assessed instrumental errors alongside its gravitational interpretation.^[ref]

Arthur Eddington helped carry out the 1919 eclipse test of general relativity, observing the apparent directions of stars near the eclipsed Sun. He coauthored the resulting report with Frank Watson Dyson and Charles Rundle Davidson.

Comparing stellar positions on eclipse and reference plates tested a small radial displacement. Changes in plate scale and focus could alter the signal, so the report assessed instrumental errors alongside its gravitational interpretation.^[ref]

A chosen boundary in a frequency response, commonly where the voltage amplitude gain falls to $1/\sqrt{2}$ of its pass-region value. The convention must specify the reference gain.

For a linear time-invariant system, a sinusoidal wanted signal at ${\omega }_s$ and an unwanted sinusoidal component at ${\omega }_n$ acquire their separate frequency response factors. The ratio of their output amplitudes, divided by their input amplitude ratio, is $\eta =|H({\omega }_n)|/|H({\omega }_s)|$. An amplitude attenuation factor of $F=1/\eta$ corresponds to a factor $F^2$ in squared-amplitude power ratios under a common impedance convention. The unwanted component's own absolute attenuation is $|H({\omega }_n)|$, which need not equal its attenuation relative to the wanted signal.

Cited experimental account: Figures 70–77.

Chapter IV; inspected §§70–75.

Chapter III, pp. 119–155; inspected §§61–64.

The metre, symbol $\mathrm{m}$, is the SI base unit of measurement for length. It is defined by fixing the speed of light in vacuum to $299792458\mathrm{m}{\mathrm{s}}^{-1}$, with the second defined independently. Thus light in vacuum travels one metre in $1/299792458$ of a second.

The exact defining constant does not make every measured length exact. Realizing a length standard can involve optical interferometry, frequency references, alignment and corrections for propagation through air; their imperfections contribute to measurement uncertainty. The definition specifies the reference, whereas an experiment supplies an imperfect realization.^[ref]

In 1947, Willis Eugene Lamb Jr. and Robert Curtis Retherford dissociated hydrogen in a tungsten oven, crossed its atomic jet with electrons for collisional excitation, and detected surviving $2S$ atoms by electron ejection from metal. An electrometer and galvanometer read the current. They interrupted fixed-frequency microwaves while sweeping the static magnetic field; a flip coil calibrated the field. Their Zeeman splitting comparison placed $2S_{1/2}$ about $1000$ $\mathrm{MHz}$ above $2P_{1/2}$; the plotted $1000$ $\mathrm{MHz}$ displacement was explicitly not a best fit.^[ref]

The 1955 lecture's apparatus cross-section identifies movable slits, an electron bombarder, a radiofrequency transmission line with beam slots, a d.c. quenching electrode, metal target, electron collector, and magnet poles. A $2S\to 2P$ transition followed by rapid spontaneous emission leaves a ground-state atom unable to eject detector electrons. The scan holds radiofrequency and amplitude fixed. Natural linewidth and unresolved hyperfine structure broaden resonances, whose analysis yields zero-field separations.^[ref]

Static electric field quenching supplied a control. The 1947 report identifies natural linewidth, hyperfine structure, excessive microwave intensity, and field inhomogeneity as width contributions, and flags imperfect magnetic calibration. It does not specify an absolute microwave-frequency calibration procedure.^[ref]

The surname is Robert Curtis Retherford's: “Retherford” and “Rutherford” name different people. The historical Lamb-Retherford experiment should retain the former spelling.^[ref]

The zero-field energy levels here are averaged over hyperfine structure; $h$ is the Planck constant. Electron self-energy and vacuum polarization produce radiative corrections. Self-energy describes the bound electron's radiative interaction with its own electromagnetic field. Precision predictions also include nuclear recoil and proton charge radius effects; assigning recoil terms to the reference or Lamb shift is convention-dependent. A finite proton charge radius changes binding independently of radiative effects.^[ref]

A vacuum-electric field “jiggling” picture motivates one contribution, not every hydrogen atom energy correction.