Paul Klee, Perseus (Der Witz hat über das Leid gesiegt) [Perseus-The Triumph of Brain over Body], 1904, etching on zinc, National Gallery of Art, Washington. https://dartabase.art/cite/nga/8109

Paul Klee, Still Life with Four Apples (Stilleben mit vier Früchten in Schale vor dunkelgrünem Grunde), 1909, Oil and gouache on board, The Museum of Modern Art, New York. https://dartabase.art/cite/moma/33358

Paul Klee, Quarry at Ostermundigen (Steinbruch Ostermundigen), 1909, Ink on paper on board, The Museum of Modern Art, New York. https://dartabase.art/cite/moma/35208

Paul Klee, The Friends (Die Freundinnen), 1909, Ink on monotype on board, The Museum of Modern Art, New York. https://dartabase.art/cite/moma/36676