In 1869 Louis Ducos du Hauron demonstrated an approach to color photography using color separations combined in a subtractive process (A similar process was published almost simultaneously by Charles Cros) (Pénichon 2013, 14). Du Hauron printed the three negatives on glass plates coated with a layer of bichromated gelatin. The gelatin on each plate was dyed in a color complimentary to the filter through which the original negative was exposed. The gelatin layer hardened to a depth proportional to the amount of light it was exposed to during printing. The three layers were then washed to remove the unhardened gelatin and the three colored plates superimposed against a white background (Ducos du Hauron 1869, 36-37).

In 1874 Ducos du Hauron patented a camera he called the Melanochromoscope, which exposed three negatives on a single glass plate at the same time through a single lens. Half-silvered mirrors were used to split the incoming light and direct it through three color filters (Les Amis de Louis Ducos du Hauron 2025). The glass plate shown here was exposed in such a camera. Like all color separation negatives, it could be used to create a final image using either subtractive or additive methods.

As du Hauron acknowledged, existing materials would need to be improved to achieve high-quality results. Black and white emulsions did not respond uniformly across the spectrum. In particular, emulsions were far less sensitive to red than blue and green. Panchromatic emulsions, which responded relatively equally across the spectrum, were developed by Adolf Miethe around the turn of the 20^th-century (Pénichon 2013, 17).