The _Opus Tertium_ contains the author's last revision, in the form of an abridgment and improvement, of the _Opus Majus_; and was drawn up at the command of Pope Clement IV., and so called from being the _third_ of three copies forwarded to his holiness; the third copy being not a _fac-simile_ of the others, but containing many most important additions, particularly with regard to the reformation of the calendar. It also throws much light on Bacon's own literary history and studies, and the difficulties and persecutions he had to surmount from the jealousies and suspicions of his less-enlightened contemporaries and rivals. The _Opus Tertium_, according to the sketch given of its contents by Bacon himself, is not complete either in the Douay MS. or in that in the British Museum, several subjects being left out; and, among others, that of Moral Philosophy. This deficiency may arise, either from Bacon not having completed his original design, or from no complete MS. of this portion of his writings having yet been discovered. M. Cousin says, that the _Opus Tertium_, as well as the _Opus Minus_, is still inedited; and is only known by what Jebb has said of it in his preface to the _Opus Majus_. Jebb quotes it from a copy in the Cottonian Library, now in the British Museum; and it was not known that there was a copy in France, till M. Cousin was led to the discovery of one, by observing in the Catalogue of the public library of Douay, a small MS. in 4to. with the following title, _Rog. Baconis Grammatica Græca_. Accustomed to suspect the accuracy of such titles to MSS., M. Cousin caused a strict examination of the MS. to be made, when the discovery was communicated to him that only the first part of the MS. consisted of a Greek grammar, and that the remaining portion, which the compiler of the Catalogue had not taken the trouble to examine, consisted of many fragments of other works of Bacon, and a copy of the _Opus Tertium_. This copy of the _Opus Tertium_ is imperfect, but fortunately the deficiencies are made up by the British Museum copy, which M. Cousin examined, and which also contains a valuable addition to Chapter I., and a number of good readings.