Joseph was assigned to the Bollandists in 1888, to research Greek and Eastern saints, and taught a course on Sanskrit at the Institut Catholique de Paris. In 1896 he was appointed conservator of manuscripts in the Royal Library of Belgium. He continued to contribute to the work of the Bollandists until 1905. As conservator he systematically catalogued and considerably expanded the Royal Library's manuscript holdings. He was involved in organising the Exposition des primitifs flamands à Bruges, and published editions of several medieval illuminated manuscripts, including "Le Bréviaire de Philippe le Bon" (1909), Jean le Tavernier's "Cronicques et conquestes de Charlemaine" (1909), Loyset Liédet's "Histoire de Charles Martel" (1910), and two books of hours attributed to Jacques Coene (1911). In 1909 he was appointed chief conservator.