After visiting the 1902 Bruges exhibition of Netherlandish art ("Exposition de tableaux flamands des XIVe, XVe et XVIe siècles") he published an independent critical catalogue with Alfons Siffer, highlighting the large number of mistakes in the official catalogue which had used attributions and descriptions from the owners. He went on to become a leading scholar in the field. He wrote a separate introduction to the catalogue in which he speculated about the painters' identities ("De l’identité de certains maîtres anonymes"), followed by publications about Pieter Bruegel the Elder and Jan Provoost. He was the first art historian to suggest that Robert Campin was in the fact the painter known as the Master of Flemalle and to identify Rogier van der Weyden as an apprentice to Campin.