This book contains the extant tradition of Benvenutus Grassus' Treatise on the eye and six philological related studies. The tradition in Latin (Metz, Bibliothèques- Médiatèques, MS 176) is displayed with the four known versions in Middle English (Glasgow, Glasgow University Library, Hunter MSS 503 and 513); London, British Library, Sloane MS 661, and Oxford, Bodleian Library, Ashmole MS 1468) along with one in Provençal (Basel, Öffentliche Bibliothek der Universität, MS D.II.11). The diplomatic transcriptions of the manuscripts are synoptically arranged to ease the researchers' consultation and comparison. The philological studies deal with the versions of the Latin tradition and with the common and diverging features of the English vernacular tradition, mainly in the Hunter MSS. Both the synoptic edition and the philological studies are the result of a collaborative edition and joint research on Hunter MSS providing a state-of-the-art approach to the treatises.