The Hartridge reversion spectroscope is a specialised optical instrument designed to determine the precise wavelengths of absorption lines in a specimen's spectrum by employing a double-pass, or "reversion," optical arrangement. Unlike the standard hand spectroscope or direct-vision spectroscope used in routine gem identification, the Hartridge design superimposes two images of the same spectrum — one reversed relative to the other — so that a given absorption line appears as a matched pair that can be brought into coincidence. The wavelength is then read directly from a calibrated scale, eliminating much of the subjective estimation inherent in simpler instruments. The instrument is named after H. Hartridge, the British physiologist who developed the reversion principle in the early twentieth century for precise colorimetric and spectral work.

Optical Principle

In a conventional direct-vision spectroscope, the observer estimates the position of an absorption line against a wavelength scale that has been calibrated by reference to known spectral lines — typically the sodium doublet or the Fraunhofer lines of the solar spectrum. Positional accuracy depends heavily on the quality of the prism train, the resolving power of the instrument, and the experience of the observer. The reversion spectroscope addresses this limitation by splitting the dispersed beam and inverting one half, then recombining the two. When the instrument is adjusted so that a particular absorption feature coincides exactly with its mirror image, the reading on the micrometer drum or scale corresponds directly to the wavelength of that feature. This bisection method substantially reduces parallax and interpolation errors, yielding wavelength determinations accurate to within approximately one nanometre or better under favourable conditions.

Construction

The instrument typically comprises a collimating slit and lens, a dispersing prism or prism train, a beam-splitting and reversion arrangement (often achieved with a Dove prism or equivalent reversing element), an eyepiece with a fine cross-wire or bisection marker, and a calibrated micrometer drum. The mechanical construction is considerably more substantial than that of a pocket spectroscope, and the instrument is designed for bench use with a stable light source — usually a tungsten lamp or, in more modern configurations, a fibre-optic illuminator. The slit width is adjustable to balance resolution against light throughput, a consideration of particular importance when examining small or deeply coloured stones that transmit little light.

Gemmological Applications

In gemmological practice, the Hartridge reversion spectroscope finds its principal application in research and forensic identification contexts where the exact wavelength of an absorption feature carries diagnostic significance. Several situations particularly benefit from this level of precision:

• Distinguishing natural from synthetic corundum: Certain chromium-related absorption features in ruby and pink sapphire, and iron-related features in blue sapphire, occur at wavelengths that may differ subtly between natural and flux-grown or hydrothermal synthetic material, though the differences are often at the limit of resolution of standard instruments.
• Colour-cause determination: Identifying whether a green stone's absorption is attributable to chromium, vanadium, or iron requires confident wavelength assignments; the reversion instrument reduces ambiguity in borderline cases.
• Alexandrite and colour-change stones: The precise position of the chromium doublet near 680 nm in alexandrite and related minerals benefits from accurate measurement when distinguishing natural alexandrite from synthetic or from other colour-change species.
• Research on treated stones: Beryllium-diffused sapphires and heat-treated rubies may exhibit modified spectral profiles; documenting these modifications for laboratory reference databases requires the repeatability that a reversion spectroscope provides.

Limitations and Availability

The Hartridge reversion spectroscope has never been a common instrument in commercial gemmological laboratories. Its cost, bulk, and the time required for each measurement make it impractical for the throughput demands of a trade laboratory. Modern research facilities have largely superseded it with fibre-optic spectrometers coupled to charge-coupled device (CCD) arrays, which offer comparable or superior wavelength accuracy, digital data capture, and the ability to record a full spectrum simultaneously rather than scanning point by point. Nevertheless, the reversion spectroscope retains historical and pedagogical importance as an example of precision optical design applied to gemmology, and functional examples are still encountered in university mineralogy departments and older institutional collections. Its operating principle also provides a clear conceptual illustration of how bisection methods reduce measurement uncertainty — a principle that underpins several modern spectroscopic calibration techniques.

Further Reading

• Gems & Gemology — GIA research journal archive
• International Gem Society: Spectroscope Use in Gem Identification