His magnum opus, Iconographie de l'Art Chrétien, in six volumes, encompasses all of Europe, including the Eastern Orthodox tradition.From 1911 to 1913, he was Director of the Institut français de Saint-Pétersbourg [fr]; working with his counterparts in the Mir Iskusstva movement.In 1928, he was placed in charge of a mission to Russia, to establish a catalogue of French art works being held at museums in Moscow and Saint Petersburg.From 1930 to 1938, he was Director of the Institut Français de Vienne, then served as Professor of Medieval art at the Sorbonne; a position he held until 1951.He was elected to the Académie des Beaux-Arts in 1947, becoming the first person to occupy the newly created Seat #11 in the "Unattached" section.
