Léon Gimpel (1873-1948) Illustration of the technical principles of the autochrome process, 1907. Microphotographs of the preparation and sorting of potato starch. Microphotograph of potato starch grains on the autochrome plate. The same plate after lamination. Microphotographs of each stripe of the flag in complementary and actual colors. Negative of the French flag in complementary colors. Positive of the flag in actual colors. 11 autochromes and two gelatin silver bromide positive glass plates. Size of glass plates: 9 x 12 cm The supplement published by the newspaper L'Illustration on June 15, 1907, which explains the technical principles of autochrome photography, based on some of the plates shown here, is enclosed. On June 10, 1907, Auguste Lumière presented the newly-developed autochrome technique at the offices of the newspaper L'Illustration. The event, organized by Léon Gimpel, was featured in a supplement to the weekly and in a short report published in the Bulletin de la Société française de photographie. In an ingenious mise en abyme, and to illustrate the conference, Léon Gimpel produced a series of micrographic autochromes detailing the principles and steps involved in making Lumière plates: sorting and mixing colored potato starch, laminating, demonstrating the notions of complementary and true colors. Autochromes testifying to this original educational exercise are particularly rare. To the best of our knowledge, only one set of similar importance is preserved in the collections of the Société française de photographie.