The mathematical treatises known as Taḥrīr al-mutawassiṭāt, “Revision of the Middle books”, are a collection of Greek and Arabic treatises which were re-edited by Naṣīr al-Dīn al-Ṭūsī. This paper explores the nature of the editorial work Ṭūsī did, showing that it is not a simple rendering of the original text but that it involves major revisions and in fact original additions. For the first time, a collated version of Ṭūsī’s revision on the treatise on the measurement of the circle (by Archimedes) is provided. A translation of the Arabic text is put side by side with a rendition of the treatise as it was circulated before Ṭūsī’s time. An in-depth analysis of the text elucidates the nature of Ṭūsī’s contributions to the text. This article also provides modest corrections to earlier studies by W. R. Knorr and P. Luckey.