His wife played the piano, and throughout the 1890s Denis had a growing interest in the connections between music and art. He painted a portrait of her at the piano in 1890. He designed a flowing lithograph, featuring Marthe, for the cover for the sheet music of "La Damoiselle élue" by Claude Debussy, as well as another lithograph for the poem "Pelléas et Mélisande" by Maurice Maeterlinck, which Debussy transformed into an opera; and in 1894 he painted "La Petit Air", based on the poem "Princesse Maleine" by Stéphane Mallarmé, the most prominent literary proponent of symbolism. In 1893 made a collaborative project with the writer André Gide, which combined art and literature; he provided a series of thirty lithographs to accompany a long essay by Gide, called "Le Voyage d'Urien". The Lithographs did not illustrate the text, but approached the same topics from an artist's point of view.