However, although there has been no thorough-going investigation of this question, much has been accomplished by students interested in sections of the _Etymologies_, such, for example, as those on music and law. Classical scholars also have investigated his sources in a more general way, but their efforts have been not so much directed to the elucidation of Isidore himself as inspired by the hope of recovering some fragments of the classical authors. The varying conclusions reached show that no great certainty has been attained, but it is possible to give a tentative list of sources which will indicate roughly the nature of the influences which contributed to form Isidore’s ideas.[54] It seems probable that his working library contained works of the following authors: Lactantius, Tertullian, Jerome, Ambrose, Augustine, Orosius, Cassiodorus, Suetonius, Pliny, Solinus, Hyginus, Sallust, Hegesippus, the abridger of Vitruvius, Servius, the scholia on Lucan, and Justinus. [54] Dressel, _De Isidori Originum Fontibus_, in _Rivista di filologia_, 1874–75, discusses Isidore’s method of using his sources, and gives a list of writers and works to which he traces passages in Isidore, giving usually a list of the latter. The writers include Sallust, Justinus, Hegesippus, Orosius, Pliny, Solinus, the abridger of Vitruvius, Lucretius, Hyginus, Cassiodorus, Servius, the scholia on Lucan. Nettleship, _Lectures and Essays_, Oxford, 1885, devotes attention chiefly to the encyclopedic tradition, treating of Verrius Flaccus, the _Glosses_ of Placidus, the _Noctes Atticae_ of Gellius, Nonius Marcellus, and Servius. He treats of Isidore only by the way, and lays stress on his debt to Suetonius, _Prata_, and Verrius Flaccus, _De Verborum Significatu_. See pp. 330–336, and for opinion of Latin encyclopedic tradition, pp. 283–285. Reifferscheid, _Suetoni Reliquiae_, recovers several passages of Suetonius from Isidore. C. Schmidt, _Quaestiones de musicis scriptoribus Romanis imprimis de Cassiodoro et Isidoro_, traces Isidore’s _De Musica_ to an unknown Christian writer. G. Becker, editor of _De Natura Rerum_, Berlin, 1857, discusses the sources of that work especially, tracing it to Suetonius, Solinus, and Hyginus on the one hand, and Ambrose, Clement, Augustine, on the other. H. Hertzberg, _Die Chroniken des Isidors, Forsch. zur deutschen Geschichte_, 15, 280 _et seq._, discusses the sources of Isidore’s _Chronica_, which he traces to Jerome’s translation of Eusebius with later continuations. The same writer also treats of the sources of _The History of the Goths_ (Gött. 1874). H. Usener, _Anecdoton Holderi_ (Bonn, 1877), p. 65, asserts that Isidore did not use Cassiodorus’ encyclopedia of the liberal arts.