The Speculum musicae of Magister Jacobus de Ispania from Liege is the ultimate Medieval Summa of music. Compiled probably during the 1330s, it comprises no fewer than seven books, altogether totaling more than 375,000 words. Princeton musicologist Rob C. Wegman offers a translation of the seventh and final book, which deals with contemporary polyphony. This part of the Speculum is the notorious-and uncommonly impassioned-diatribe against new musical and notational practices that had gained currency in France in the second quarter of the fourteenth century. Jacobus proves himself thoroughly...