Photochromie, or the photochrome process, was an innovative method invented in the early 1870s by Léon Vidal, an active member of the photographic community. After patenting it in 1872 and 1873, the inventor continued to refine it in the studios of the Société Anonyme des Publications Périodiques, whose director he became in 1875. There he also produced monochromatic photomechanical prints, such as collotypes or Woodburytypes, which were used to reproduce some of the plates of the Trésor. Nevertheless, this work was the Société’s first publication illustrated with a substantial number of photochromes: thirty of the thirty-nine photographic plates of the Trésor’s first series were printed using the Vidal process.