Analyzes the role of images in forming monastic prayerfulness through a study of Fra Angelico's fresco cycle at the Observant Dominican convent of S. Marco, Florence. Proposes that the source was an illustrated Dominican prayer treatise of the mid-13th c., De modo orandi (Rome, Biblioteca vaticana, MS ross. 3), and attributes iconographical inventions to Fra Angelico, himself a Dominican of the Observance.