= Lippmann colour-recording experiment
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In his 14 December 1908 Nobel lecture, <Gabriel Lippmann> described a <photographic plate> bearing a transparent, even, extremely fine sensitive layer. A holder put <Mercury> against that layer as a mirror during exposure. Incident and reflected <light> formed a <standing wave> within the <photographic emulsion>; after <photographic processing> and drying, white <light> revealed colour by selective <reflection>. The reflector served during recording; the developed structure supplied the later colour selection.<Colour Photography by Gabriel Lippmann>[Colour photography, PDF p. 1, paragraphs beginning The method is very simple and This result is due: apparatus, exposure, development and reflected readout]{ref}
The lecture's control was to project a spectrum photograph, wet it, and project it again. Its colours disappeared as the gelatin swelled, then returned during drying, red before the shorter <wavelengths>. This tested a spacing-dependent explanation. Lippmann also reported roughly one-minute sunlight exposures and identified portrait exposure time as a limitation.<Colour Photography by Gabriel Lippmann>[PDF p. 1, wetting demonstration; p. 2, final paragraphs on exposure time]{ref}
A reversible colour change supports a structural explanation, but does not independently measure every fringe spacing or the <refractive index>. For a conceptual comparison, record a narrow band and compare reflected <optical spectra> at fixed illumination and viewing <angles>; an unexposed processed plate tests background. A second comparison changes only viewing geometry. A third compares the dry state before and after a reversible spacing change. These are proposed discriminating controls, not additional experiments attributed to Lippmann. Background surface <reflection> must be separated from the selective signal. A photograph's appearance also depends on the illuminating <optical spectrum>; viewing under an arbitrary lamp is not a calibrated spectral measurement.
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