How does Alfred de Hamal disguise himself to sneak into the school and visit Ginevra in the novel, Villette?

Over the course of the novel, we learn that the ghost of a nun who was buried alive supposedly haunts the Rue Fossette. On several occasions, Lucy and M. Paul see a nun in the garret and in the forbidden alley. This nun turns out to be Alfred de Hamal, who uses this disguise to sneak into the school and visit Ginevra.