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David W. McCamant is a physical chemist studying light–matter interactions and fast molecular processes. His Rochester research-group biography describes his Berkeley doctoral work with Richard A. Mathies on femtosecond stimulated Raman spectroscopy. The2003 method paper independently verifies his full byline as David W. McCamant.^[ref][ref]

The coauthored apparatus uses a filtered pump near793nm, a continuum probe and spectrally resolved probe/reference detection to measure vibrational gain.^[ref] This makes a molecular vibration accessible through the difference between optical frequencies.

For an original conversion example, a793-nm pump and an850-nm Stokes probe correspond to Raman shiftThe factor $10^7$ converts reciprocal nanometres to reciprocal centimetres. Subtracting wavelengths directly would not give the vibrational wavenumber. The850-nm value is stipulated for this example, not attributed as a measured band in the paper.

The method connects pulse design, spectral resolution and quantitative signal processing with questions about molecular dynamics. A rapidly changing signal still needs a molecular assignment and appropriate instrumental controls before it establishes a particular reaction pathway.

Choose a vibrational Raman shift magnitude ${\tilde{\nu }}_v=1000\mathrm{c}{\mathrm{m}}^{-1}$, excitation vacuum wavelength ${\lambda }_0=532\mathrm{nm}$ and temperature $T=300\mathrm{K}$. These are original illustrative inputs, not a measured 1928 optical spectrum or a claim about a particular liquid. They giveTake reciprocal lengths in consistent units before converting to nanometres. At $300\mathrm{K}$, $r\simeq 0.00826337$ and $\overline{v}\simeq 0.00833222$. The predicted line-integrated radiant intensity ratio is $0.0126522$, whereas the ideal photon count-rate ratio is $0.0113740$. Thus the radiometric ratio is about $1.27$ percent, rather than the $0.826$ percent obtained by using the Boltzmann factor alone.

For a transition starting at $J$ and ending at $J+2$, the molecule gainsEnergy conservation therefore gives the scattered-light wavenumbersThe first shifts below the laser are $6\tilde{B},10\tilde{B},14\tilde{B},18\tilde{B}$. Adjacent Stokes lines are separated by $4\tilde{B}$, while the first line is $6\tilde{B}$ from the Rayleigh line. The figure uses positive Raman shift ${\tilde{\nu }}_L-{\tilde{\nu }}_{\mathrm{S}}$; increasing shift means decreasing scattered-light wavenumber.

Chandrasekhara Venkata Raman and Kariamanikkam Srinivasa Krishnan jointly reported the effect in 1928. Their letter describes light from the Sun concentrated by an $18\mathrm{cm}$-aperture, $230\mathrm{cm}$-focal length telescope objective and a second optical lens onto a liquid purified by repeated vacuum distillation. With blue-violet and yellow-green optical filters together in the incident beam, the visible track disappeared; moving the yellow-green optical filter between liquid and observer restored it. The complementary arrangement rejected unchanged-colour scattering of light while admitting newly generated light. Persistence after purification and strong wave polarization in many liquids argued against ordinary fluorescent contamination.^[ref]

In his 1930 lecture, Chandrasekhara Venkata Raman describes a cobalt-glass optical filter selecting indigo-violet light from the Sun, with a separated blue-green band in the scattered optical spectrum. Substituting a quartz mercury arc and condenser gave sharp new spectral lines. Small Hilger quartz spectrographs recorded them photographically; Kariamanikkam Srinivasa Krishnan obtained more precise records with a larger instrument and established violet-displaced lines. The optical prism disperses wavelengths for spatial readout; it does not create the Raman shift. The diagram gives functional optical paths rather than a reconstruction of a specific Hilger instrument.^[ref]

Their May 1928 report strengthens the comparison: filtered mercury illumination of toluene produced extra spectral lines. Removing the $4358\text{A˚}$ incident line removed its associated longer-wavelength group; a quinine filter removing the $4047$, $4078$ and $4109\text{A˚}$ group removed a different shifted group while preserving the one associated with $4358\text{A˚}$. These are actual selective-excitation controls, rather than inferred colours.^[ref]

The inference has limits. A complementary-filter test alone establishes newly admitted wavelengths, not their precise spacing or a unique microscopic mechanism. Fluorescence can also survive purification when intrinsic to the liquid, and fluorescence can be polarized. Useful distinctions therefore combine resolved spectral lines, excitation dependence, wave polarization and sample preparation. A proposed modern extension is to change a single incident frequency: a molecular Raman shift should follow the same internal energy gap, whereas ordinary relaxed molecular fluorescence usually retains approximately its emission-band positions. This is a diagnostic under stated conditions, not a universal theorem about every fluorescent material or resonant scattering process. Raman scattering need not involve a populated, relaxed electronic excited state as ordinary molecular fluorescence does; neither a redder photon nor the word Stokes identifies the mechanism by itself.

The 1930 Nobel Prize in Physics went to Chandrasekhara Venkata Raman alone for his work on scattering of light and the discovery of the effect. That award allocation should be distinguished from the joint experimental report with Kariamanikkam Srinivasa Krishnan.^[ref]

A Raman band-area map assigns each spatial position an integrated, usually background-corrected spectral intensity over a stated Raman shift range. If $I(x,y,\tilde{\nu })$ is the spectrum and $B$ is its estimated baseline,This is one way to summarize the spectra collected in Raman mapping. It is useful for following a band's spatial distribution while preserving a different interpretation from a Raman peak-position map.^[ref]

Peak height alone does not determine area. For an isolated Gaussian function band of height $H$ and width parameter $s$, integrating the complete band givesTwo hypothetical bands of equal height10 intensity units and $s=2$ or4cm⁻¹ have full areas about50.13 and100.27 intensity-units·cm⁻¹. Thus an area map can change while a height map stays constant. A finite integration window captures only part of the complete band, so the actual window must accompany the value.

Raman integration-window bias arises if shifting or broadening moves part of the chosen band outside that fixed range. Neighboring bands and an incorrectly subtracted background can also change the result. For a sampled spectrum, a quadrature uses the actual channel spacing, for example $A\approx {\sum }_i(I_i-B_i)\Delta {\tilde{\nu }}_i$. A bare sum without the spacing factor has a different numerical convention when sampling changes.

Comparing areas across positions also requires consistent exposure, illumination and focus, or an appropriate normalization and measurement calibration. A larger area is therefore not automatically a larger concentration of one material. The manufacturer's discussion of spectral analysis explicitly cautions that broad background can contaminate simple intensity-based images.^[ref]

A Raman baseline-induced center shift is a displacement of an estimated band center caused by the spectral background used or omitted in the analysis. It is one mechanism of Raman peak-model bias. A background slope can move the local maximum of a band-plus-background signal even if the underlying band has not shifted. A fitted center can also move when an incomplete background model transfers part of that slope into the fitted band position.

For an original local model, write $x$ for Raman shift relative to the true center, and consider a Gaussian function band plus a small residual slope:Its stationary-point equation near the band center is $-(Hx/s^2)e^{-x^2/(2s^2)}+b=0$. When the displacement is small compared with $s$, the local maximum is displaced byFor $H=10$ intensity units, $s=2\mathrm{c}{\mathrm{m}}^{-1}$ and $b=0.05$ intensity units per $\mathrm{c}{\mathrm{m}}^{-1}$, this is approximately $0.020\mathrm{c}{\mathrm{m}}^{-1}$. A positive residual slope moves this local maximum toward increasing Raman shift. This local spectral-window model does not assert a physically unbounded linear background.

A least squares fitted center answers another question. If $j_i$ is the derivative of a chosen band model with respect to its center, omitted residual background $r_i$ produces the first-order fitted displacementfor fixed other parameters and positive weights $w_i$. An omitted background component matters when it projects onto the center-shift response. For symmetric samples and weights, a constant residual is orthogonal to the odd derivative of a symmetric band, whereas a residual slope is not. Asymmetric sampling or weights can remove even that constant-background cancellation.

The three-channel fit in Raman peak-model bias gives approximately $0.0330\mathrm{c}{\mathrm{m}}^{-1}$ for the same slope and band parameters, because estimating a center from those samples is not the same operation as finding the continuous local maximum. At sample positions $-s,0,s$, the center derivative is proportional to $x$; allowing both a center change and an unknown linear background therefore introduces a first-order identifiability problem in that restricted model. Additional spectral samples or independent background information can help separate their responses.

Examining residuals and changing physically justified background models are useful tests before interpreting a Raman peak-position map as a map of material changes. Manufacturer guidance distinguishes spectral intensity, background contamination and fitted curve parameters.^[ref] A small conditional Raman peak-fit uncertainty alone does not establish that the assumed background is adequate.^[ref]

Raman integration-window bias is a systematic difference between the spectral-band area one intends to compare and the area captured by a selected finite Raman shift interval. It can create contrast in a Raman band-area map when the complete band area is unchanged: shifting or broadening redistributes intensity across the fixed interval's edges. The target must be stated clearly, because an area over that interval can be correctly measured even when it is a biased proxy for the complete band.

For an original calculation, take an isolated background-corrected Gaussian function bandIts complete area is $A_{\mathrm{full}}=Hs\sqrt{2\pi }$. Integrating only from $a$ to $b$ gives the Raman captured-area fractionwhere $\Phi$ is the cumulative distribution function of the standard normal distribution. The substitution $z=(\tilde{\nu }-{\tilde{\nu }}_0)/s$ gives this expression directly. The complete area depends on height and width; the captured fraction also depends on center position relative to the window.

Suppose $s=2\mathrm{c}{\mathrm{m}}^{-1}$ and the fixed window runs from996 to1004cm⁻¹. A center at1000cm⁻¹ places its edges at two standard widths on each side, giving $f=\Phi (2)-\Phi (-2)\approx 0.95450$. Shift the center to1004cm⁻¹ without changing height or width: now $f=\Phi (0)-\Phi (-4)\approx 0.49997$. The measured fixed-window area falls to about52.38% of its previous value although the complete band area is identical. This contrast is therefore insufficient evidence for a decrease in the amount of material.

A window that follows the fitted center can keep the captured fraction constant for this ideal fixed-width model. Broadening, neighboring bands, errors in a Raman peak-position map and an imperfect background still matter; a larger window can admit unwanted signal. Manufacturer documentation explicitly distinguishes simple intensity images from fitted-band analysis and warns that broad background can contaminate intensity contrast.^[ref] Reporting the actual interval, background treatment and whether the interval moves with the band makes the Raman band-area map scientifically interpretable.

Raman mapping records Raman spectra at known sample positions and constructs spatial maps from their spectral features. A map can show composition or structural variation that is absent from an ordinary reflected-light image. A point-scanning apparatus combines a focused laser, microscope objective, rejection of unshifted light, a spectrometer and controlled sample positioning. A primary university facility description identifies single-point acquisition and two-dimensional mapping as measurement modes.^[ref]

The acquired data can be represented as $I(x,y,\tilde{\nu })$, with position coordinates $x,y$ and Raman shift $\tilde{\nu }$. It is a form of hyperspectral imaging. The displayed image is a derived summary of those spectra. For example, a baseline-corrected Raman band-area map useswhile a Raman peak-position map records a fitted band's position. Different summaries can reveal different properties of the same sample. Manufacturer examples include a silicon-carbide wafer whose spectral map separates inclusions and structural variation.^[ref]

For an illustrative20µm-by20µm point map sampled every1µm with both boundaries included, there are $21\times 21=441$ positions. At0.10s exposure per position, exposure time alone is44.1s; motion and readout add overhead. Reducing step size increases sampling density, while optical resolution remains constrained by the excitation/detection point spread function and instrument configuration. A finely interpolated display cannot recover an unsampled small inclusion.

Intensity also depends on illumination, focus and the spectral background. A single-frequency brightness map can mix a desired Raman band with broad background, whereas fitted or integrated features can answer a more specific question. Quantitative composition or stress estimates require an appropriate measurement calibration; a colourful spatial contrast is not by itself a calibrated concentration or stress measurement.^[ref]

A Raman microscope combines an optical microscope with Raman spectroscopy to measure a spectrum from a localized specimen region. A microscope objective focuses a laser and collects scattered light; rejection filters suppress the intense excitation line, and a spectrometer separates the remaining wavelengths for detection. A facility instrument at the National University of Singapore supports both single-point measurements and Raman mapping.^[ref]

In a common backscattering arrangement, the same objective supplies excitation and collection. Moving the specimen or focus associates each Raman spectrum with a position, so a chosen spectral band can map chemical or structural variation. Confocal microscopy can additionally reject some out-of-focus light; a small map step by itself does not establish correspondingly fine spatial resolution.

For an original wavelength calculation, illuminate at ${\lambda }_0=532\mathrm{nm}$ and observe a Stokes Raman shift of $1000\mathrm{c}{\mathrm{m}}^{-1}$. The scattered wavelength satisfiesBoth reciprocal wavelengths must use the same length unit. The spectrometer measures the shifted light, while a filter suppresses the much brighter Rayleigh scattering near $532\mathrm{nm}$. Spectral resolution determines how nearby spectral features separate; objective focusing and the detection geometry determine spatial discrimination.

A concrete coupled apparatus placed a hydrated mouse bone in a mechanical tester mounted on a Raman microprobe stage. It followed mineral-band changes during elastic loading, connecting local molecular response with specimen mechanics.^[ref] Interpreting such a measurement requires the specimen position, loading conditions and laser exposure to be controlled. Local heating, fluorescence and instrumental response can change a recorded spectrum; focused illumination is not automatically non-destructive.

Raman peak-model bias is a systematic displacement of an estimated Raman shift or another fitted band parameter caused by an inadequate spectral model. The discrepancy can persist in repeated low-noise measurements. It differs from the random scatter contributing to Raman peak-fit uncertainty. Manufacturer documentation explains that fitted band parameters depend on theoretical curves and warns that a broad spectral background can contaminate simpler intensity measurements.^[ref]

An original three-channel example isolates a Raman baseline-induced center shift. Write $x$ for displacement from the true band center and let the true sampled signal be a Gaussian function plus a small residual baseline slope:Suppose an equal-weight least squares fit omits that slope and estimates only the center $\mu$, keeping $H$ and $s$ fixed:Near $\mu =0$, its derivative with respect to the center is $j_i=(H{x}_i/s^2)e^{-x_i^2/(2s^2)}$, so linearizing gives $m(x_i;\mu )\approx m(x_i;0)+\mu j_i$. Minimizing the resulting squared residuals givesThe odd slope in the unmodeled background therefore projects onto the odd center-shift response of the symmetric band.

For $H=10$ intensity units, $s=2\mathrm{c}{\mathrm{m}}^{-1}$ and $b=0.05$ intensity units per $\mathrm{c}{\mathrm{m}}^{-1}$, the first-order center bias is about $0.0330\mathrm{c}{\mathrm{m}}^{-1}$. This is a deliberately restricted toy fit, not a universal bias formula: fitting the background, width and height too, or using other sample positions and statistical weights, changes the result. The calculation nevertheless demonstrates why reducing random noise alone cannot remove a wrong-model displacement.

In Raman mapping, a spatially varying background or unresolved neighboring band can consequently create an apparent Raman peak-position map contrast. Comparing physically defensible background/line-shape models and examining their residuals tests that interpretation. A good residual plot is useful evidence but does not prove uniqueness of a spectral decomposition. The GUM requires significant corrections and inadequately represented influences to be included in the measurement model; its discussion of fit-parameter uncertainties does not make an incomplete model adequate.^[ref]

A Raman peak-position map assigns to each sample position the fitted Raman shift of a selected spectral band. Starting from position-resolved spectra, the analysis estimates ${\tilde{\nu }}_{\mathrm{peak}}(x,y)$ and displays its spatial variation. It answers a different question from a Raman band-area map, which records an integrated spectral intensity. Manufacturer documentation uses fitted band position to reveal structural variation, including strain-related changes in a silicon-carbide wafer.^[ref]

For a simple isolated Gaussian function band above background $B$,the fitted center ${\tilde{\nu }}_0$ is the peak position. A maximum chosen from discrete channels can instead jump in steps of the channel spacing. Fitting uses the neighboring samples too, although its result still depends on the line-shape model, background and noise. Raman peak-fit uncertainty belongs with the resulting map.

For an explicitly hypothetical band, suppose a reference region is at1000.0cm⁻¹ and another region at1000.5cm⁻¹. The shift difference is0.5cm⁻¹. If an independently established local stress calibration were $\Delta \tilde{\nu }=K\sigma$ with $K=2\mathrm{c}{\mathrm{m}}^{-1}/GPa$, that model would imply0.25GPa. The coefficient is illustrative, not a material constant asserted for the manufacturer's wafer. A quantitative stress map requires the applicable measurement calibration and control of other causes of spectral change.

Overlapping bands can move the maximum of their sum when only their relative intensities change. A stable center estimate must therefore track a specified band rather than interpret every maximum shift as strain. Instrument drift, temperature and laser-induced heating can also affect position. Retaining the underlying spectra and fit results makes it possible to test those interpretations rather than rely only on the map's colours.

The signed Raman shift subtracts scattered from incident spectroscopic wavenumber:Here $c$ is the speed of light, and both wavelengths are in vacuum. Positive shifts denote Stokes Raman scattering; negative shifts denote Anti-Stokes Raman scattering. Some plots reverse the horizontal direction or report separate unsigned magnitudes, so check the convention. Spectroscopic wavenumber has no $2\pi$ factor, unlike the common angular wavenumber $k=2\pi /\lambda$. A shift of $1000\mathrm{c}{\mathrm{m}}^{-1}$ means $100000{\mathrm{m}}^{-1}$, not a frequency of $1000\mathrm{Hz}$.

For a one-quantum change of molecular vibration with ordinary frequency ${\nu }_v$, the magnitude is ${\tilde{\nu }}_v={\nu }_v/c$. This makes Raman spectroscopy attractive: the same molecular energy spacing can be read using different visible excitation wavelengths. Equal and opposite Raman shifts are not equal and opposite changes measured in nanometres, because wavelength is the reciprocal of spectroscopic wavenumber.

A Raman shift as a probe of bond stiffness interprets a measured vibrational frequency through a mechanical model of the assigned mode. For one stretching coordinate modeled as a harmonic oscillator,where $k_{\mathrm{eff}}$ is an effective spring constant and ${\mu }_{\mathrm{eff}}$ an effective mass. For an isolated diatomic stretch the latter is the reduced mass. A collective normal mode can involve many atoms, so its effective stiffness is not automatically the force constant of one selected bond.

An original same-mode example shifts from $1500$ to $1530\mathrm{c}{\mathrm{m}}^{-1}$. If the effective mass is unchanged, the harmonic model givesor a $4.04\%$ increase in effective stiffness. The small-change version isA ten-percent mass increase at fixed stiffness would instead reduce the frequency by $1/\sqrt{1.10}\approx 0.9535$. This reduced-mass ambiguity in a Raman stiffness inference is why a frequency change alone does not uniquely measure a stiffness change.

Mechanical strain can shift Raman bands by changing the local vibrational dynamics. HORIBA's application documentation illustrates stress mapping in microfabricated silicon, including shallow-trench regions, and describes a correlation between applied stress and peak shift.^[ref] That calibrated material response is more specific than treating every shift as a direct scalar bond-stiffness measurement.

A motivating experiment would track the same assigned band while varying a known load, holding temperature, excitation and polarization conditions fixed and measuring the wavelength reference. Heating, isotopic composition, mode mixing and structural changes can also alter frequencies. The Raman shift gives an energy spacing; mechanical interpretation additionally needs the mode assignment, mass model and appropriate stress/strain calibration.

Raman spectroscopy measures shifted scattering of light to infer internal energy spacings and the molecular response that makes them visible. In a modern illustrative arrangement, narrowband laser illumination replaces the historical arc, an optical filter rejects the strong excitation line, and a spectrograph resolves the weak shifted spectral lines. The illumination, rejection and spectral dispersion are separate jobs. Increasing rejection can conceal small Raman shifts close to the excitation line.

A proposed measurement should record the excitation wavelength, a dark signal, a blank container and the liquid, then compare calibrated line positions and integrated signals under the same collection conditions. Measure optical filter transmission and detector response before using the Thermal Raman line ratio for temperature. Repeat at lower incident power to look for heating or altered sample response. These are suggested checks, not an assertion that the 1928 apparatus implemented modern calibration procedures.

Its appeal is the conversion of internal molecular energy differences into visible spectral lines: changing the excitation can move the whole pattern while preserving characteristic Raman shifts. Combined with vibrational infrared activity, Raman vibrational activity supplies complementary information about molecular vibration. A line pattern can constrain a material's identity or environment, but overlapping bands, fluorescence backgrounds and the finite spectral resolution element prevent an unqualified identification from one peak alone.

A Raman spectrum records the strength of Raman scattering as a function of scattered optical frequency or, usually, Raman shift relative to the excitation. For vacuum wavelengths in nanometres, a common wavenumber convention isHere the numerical wavelengths are in nanometres. Stokes Raman scattering has positive shift under this convention; Anti-Stokes Raman scattering has negative shift. The strong unshifted Rayleigh scattering line is not a vibrational Raman band.

For an original chosen excitation ${\lambda }_0=500\mathrm{nm}$ and a $1500\mathrm{c}{\mathrm{m}}^{-1}$ vibration, the Stokes wavenumber is $20000-1500=18500\mathrm{c}{\mathrm{m}}^{-1}$, giving ${\lambda }_s\approx 540.54\mathrm{nm}$. The corresponding anti-Stokes wavelength is $10^7/21500\approx 465.12\mathrm{nm}$. Changing the excitation changes these optical wavelengths while the same assumed transition retains its Raman shift. The One-mode Raman spectrum example gives an existing level diagram and a separate numerical spectrum.

The vertical quantity must be stated too: detector counts per spectral bin, calibrated scattered power density and a stimulated gain ratio are different observables. Femtosecond stimulated Raman spectroscopy can plot the Raman pump-on pump-off gain ratio rather than spontaneous photon counts. Its primary apparatus disperses probe and reference beams onto an array and calibrates shifts with known solvent lines.^[ref]

Spectral-density conversion between wavelength and Raman shift also changes the density's units: rebadging the horizontal axis alone need not preserve a spectral integral. Background, excitation rejection, instrument response and spectral resolution affect observed band heights and widths. An integrated band area is not determined by peak height alone. A chemical inference should compare calibrated positions and responses under specified conditions, rather than identify a material from one unqualified peak.

Spectral-density conversion between wavelength and Raman shift preserves the integrated physical quantity while changing its horizontal coordinate. Fix the excitation wavelength ${\lambda }_0$, and let $s$ be the Raman shift. Using numerical vacuum wavelengths in nanometres and shifts in $\mathrm{c}{\mathrm{m}}^{-1}$,Let $D_{\lambda }$ denote detected counts per nanometre and $D_s$ counts per unit Raman shift. Conservation of counts under a change of variables givesThe conversion factor carries the corresponding density-unit change. The same argument applies to calibrated power densities, provided both sides describe the same quantity.

For an original narrow bin centred at $600\mathrm{nm}$, a count density of $1200$ per nanometre becomes $43.2$ per $\mathrm{c}{\mathrm{m}}^{-1}$. A one-nanometre bin there spans approximately $27.78\mathrm{c}{\mathrm{m}}^{-1}$, so its integrated count remains about $1200$. At $800\mathrm{nm}$, the same wavelength density becomes $76.8$ per $\mathrm{c}{\mathrm{m}}^{-1}$. Relabeling bin positions without adjusting the density would conceal this coordinate dependence.

For finite bins, use their exact transformed edges and preserve each bin's integral. Raman spectral rebinning with conserved integrated counts distinguishes this operation from interpolating peak heights onto a new grid. Instrument response correction is another step; the Jacobian alone does not calibrate detector sensitivity or remove background.

The primary FSRS apparatus calibrated Raman shifts using known solvent lines and dispersed the probe spectrum onto an array.^[ref] Its dimensionless Raman pump-on pump-off gain ratio is a different observable: a ratio of matched spectral densities can cancel their common coordinate factor. It should not be treated as a count density merely because it is plotted against Raman shift.

W. F. Finney, published in full as William F. Finney, collaborated with M. D. Morris on using Raman spectroscopy to study bone under mechanical loading. Their experiment connected a change in a Raman spectrum to deformation of a living material's mineral component, rather than merely identifying its chemical ingredients.^[ref]

The apparatus held hydrated mouse femora in a mechanical tester on a Raman spectroscopy microprobe stage. A load cell monitored the applied loading, which stayed below yielding, while the experiment followed shifts of a phosphate vibrational band. This joined a mechanical measurement to a molecular-scale probe of the loaded bone.^[ref]

For an original illustration of the mechanical scale, a specimen segment of initial length $10\mathrm{mm}$ that extends by $0.02\mathrm{mm}$ has engineering strain $\epsilon =0.02/10=0.002$, or $0.2\%$. A simultaneous Raman shift measurement could track how a molecular vibration changes during that extension. The numerical extension is illustrative; it is not a reported value from Finney's experiment.