In 1921, the U.S. Bureau of Standards published a quiet little Scientific Paper (No. 422, price 15 cents) describing a technique that would astonish almost any modern photographer: you could take an ordinary, garden-variety blue-sensitive photographic plate, soak it in a beaker of pink or green dye, dry it in the dark, and pull out a plate that suddenly "saw" colors it had been blind to moments before.

The book itself is a working chemist's manual. Walters and Davis — an associate physicist and a photographic technologist at the Bureau — were trying to give working photographers and scientists a way to extend the spectral range of cheap commercial plates without buying expensive panchromatic stock. Their key figure shows it plainly:

"Spectrograms in the upper left-hand corner show the regions of color sensitiveness of the three types of commercial dry plates. The other spectrograms show the sensitiveness conferred to ordinary blue sensitive plates (Seed, 26X in this case) by bathing in solutions of sensitizing dyes. The name of the dye used is given under each spectrogram."

The roll call of dyes reads like a forgotten apothecary: Pinaverdol, Cyanin, Homocol, Erythrosin, Pinacyanol, Rose Bengal, Dicyanin. Each one extends the plate's sensitivity to a different band — green, yellow, orange, deep red. Erythrosin (a food-dye cousin of the red dye still used today in maraschino cherries) gave you the green. Pinacyanol pushed you into the red. Dicyanin, a notoriously unstable cyanine dye, was the one that finally let astronomers and spectroscopists photograph the infrared.

Is the technique real? Entirely. This is dye sensitization, discovered by Hermann Vogel in 1873, and it is, almost unbelievably, the same mechanism that makes every color photograph, every digital camera sensor's color filter array, and every dye-sensitized solar cell work. Silver halide crystals only absorb blue and ultraviolet light. The dye molecule sits on the crystal surface, absorbs a photon of the "wrong" color, and injects an excited electron into the silver halide — exactly the trick Michael Grätzel rediscovered in 1991 for solar cells and won fame for.

What's been forgotten isn't the physics. It's the kitchen-sink accessibility. In 1921, a high-school chemistry teacher with a darkroom, a tray, and a 50-cent vial of erythrosin could custom-tune the color response of any plate they bought. The book gives the exact concentrations, bathing times, and drying procedures. Today the same capability is locked inside a fab-grown CMOS sensor with a Bayer filter you cannot modify.

• Modern echo: dye-sensitized solar cells (Grätzel cells), CMOS color filter arrays, fluorescent labels in biology — all are direct intellectual descendants of these dye baths.
• Lost capability: the ability of an amateur to tune the spectral sensitivity of their own imaging medium with a tray and a bottle.