The Mundaneum – one of Wallonia’s major archives centres and exhibition spaces, has been collecting glass plates since 1905, when it was run by its founder, Belgian cosmopolitan Paul Otlet. Otlet, one of the founders of modern information science, recognized the value of iconographic materials as an information source. With the collaboration of the editor of the Revue Belge de Photographie Ernst de Potter, Otlet strived to include an iconographic repertory into the Mundaneum’s plans of becoming a modern, central archive advancing knowledge and documentation in all domains. This initial project, which was particularly active in 1905-1921, was never fully realized. However, the Mundaneum today holds a collection of approximately 30.000 glass slides. These include hand-painted, photographic, and commercial projection plates, as well as photographic negatives intended for reproduction on paper. The slides, dating from 1900 to 1950, feature various topics, from art history to landscapes and the history of polar expeditions, to excavations in Egypt, and the catechism. The slide sets include the legacy of prominent Belgian amateur photographers, such as Norbert Ghisoland and Léon Losseau, as well as parts of Ernst de Potter’s own collection of photographic slides.