One of a handful of problematic scenes. James holds up a chalice and explains something to two men, one of whom wears what might be read as the hat of a Jewish Magus. The legends of St James include a lengthy 'duel' with the magus Hermogenes, much of it fought through the proxy of the latter's servant Philostratus, who is quickly converted by James to Christianity. Eventually his master also converts and is instructed at length by James in the finer points of Christian theology. My 'best guess' for this scene is therefore that it shows James teaching Hermogenes and Philostratus about the Eucharist.