Cyanometer

The cyanometer asked a human eye to do a spectrophotometer’s job: hold a numbered blue swatch up against the open sky and say which one matched. Horace-Bénédict de Saussure built the first version in 1787, gluing 16 strips of paper dyed in graduated shades of Prussian blue onto a disc, and carried it up Mont Blanc that August, reading the zenith at the 39th degree on what he’d eventually extend to a 53-stop scale running from white to black. He presented the refined instrument to the Académie Royale des Sciences on 9 May 1790, publishing an illustration of a 40-stop version in that year’s Mémoires. His working theory was that the sky darkened with altitude because there was less water vapor to scatter light back toward the eye — a reasonable guess from a man standing on top of Europe with no way to measure the physics directly, only to compare a swatch to a wheel. Alexander von Humboldt carried a cyanometer across the Atlantic and through the Andes, and on 23 June 1802, climbing Chimborazo in Ecuador, recorded the 46th degree of blue, the darkest sky anyone had matched to the disc — a record that outlasted Saussure’s own by fifteen years and a much taller mountain.

The measurements accumulated for decades and explained nothing. Saussure’s water-vapor theory turned out to be wrong, and no amount of additional readings from additional peaks converted a stack of numbered swatch-matches into a theory of why the sky is blue in the first place. That answer came from physics, not from more paper discs: John Tyndall’s experiments on light scattering by suspended particles, through the 1860s, gave a quantitative account of atmospheric color that made a hand-held color comparison look like exactly what it was — a proxy, not a measurement. The cyanometer got one more life as the Linke–Ostwald blue-scale, a cardboard version numbered 2 to 16 that meteorologists used well into the 20th century to survey atmospheric turbidity, before photoelectric photometers and spectrophotometers took that work over too, recording the wavelength and intensity of scattered skylight directly instead of asking an observer which of sixteen blues looked closest. The physics the cyanometer was built to chase — Rayleigh scattering, now taught in any introductory optics course — is thoroughly understood; the instrument that tried to read it off a paper wheel by eye is a museum curiosity, and Saussure’s 39 and Humboldt’s 46 are numbers on a scale nobody calibrates against anymore.