ARETINO, if he had not been convinced by the experiment he had himself made of it. The pope would have detained him at Rome; but labouring under a bodily disorder, and fearing an injury to his health from the air of the place, and the heats of the summer, which was then approaching, Guido left that city upon a promise to revisit it, and explain to his holiness the principles of his new system.
On his return home he visited the abbot of Pomposa, a town in the duchy of Ferrara, at whose request he took up his abode in the monastery. Here he composed a tract on music, entitled Micrologus, i.e., a short discourse, which he dedicated to Theobald, bishop of Arezzo; and finished, as he himself at the end of it tells us, under the pontificate of John XX., and in the 34th year of his age. Vossius speaks also of another musical treatise written by him, and dedicated to the same person. Most of the authors who have taken occasion to mention Guido, speak of the Micrologus as containing the sum of his doctrine; but it is in a small tract, entitled Argumentum novi Cantus invenendi, that his declaration of his use of the syllables, with their several mutations, and in short his whole doctrine of solmization, is to be found. This tract forms part of an epistle to a very dear and intimate friend of Guido, whom he addresses thus, "Beatissimo atque dulcissimo fratri Michaele," at whose request the tract itself seems to have been composed.
Whether Guido was the author of any other tracts is not easy to determine. It nowhere appears that any of his works were ever printed, except that Baronius, in his Annales Ecclesiastici, tom. xi. p. 73, has given at length the epistle from him to his friend Michael of Pomposa, and that to Theobald, bishop of Arezzo, prefixed to the Micrologus; and yet the writers on music speak of the Micrologus as of a book in the hands of every one. Martini cites several manuscripts of Guido; namely, two in the Ambrosian library at Milan, the one written about the twelfth century, the other less ancient; another among the archives of the chapter of Pistoja, a city in Tuscany; and a third in the Medico-Laurenziano library at Florence, of the fifteenth century: these are clearly the Micrologus. Of the epistle to Michael of Pomposa, together with the Argumentum novi Cantus invenendi, he mentions only one, which, he says, is somewhere at Ratisbon. Of the several tracts above mentioned, the last excepted, a manuscript is extant in the library of Bafil College, Oxford. (See Kiecsawetter Hist. of Music; translated by Robert Müller: London, 1848, chap. ii.)