Juliet Hubbell Littleton, Colorado, United States Isenheim Altarpiece, 1512–1516 Matthias Grünewald Oil on panel Musée d’Unterlinden, Colmar, France One of the world’s greatest masterpieces is often and mysteriously excluded from the common pilgrimages educated tourists make in their travels. While crowds will mill about the Mona Lisa in Paris or endure hours of air travel and…