The 'Opus Majus' begins with a book on the necessity of advancing knowledge, and a dissertation on the use of philosophy in theology. It is followed by books on the utility of grammar and mathematics; in the latter of which he runs through the various sciences of astronomy, chronology, geography, and music. The account of the inhabited world is long and curious, and though frequently based on that of Ptolemy, or the writings of Pliny, contains many new facts from travellers of his own and preceding times. His account of the defects in the calendar was variously cited in the discussions which took place on the subject two centuries after. The remainder of the work consists of a treatise on optics and on experimental philosophy, insisting on the peculiar advantages of the latter. The explanation of the phenomena of the rainbow, though very imperfect, was an original effort of a character altogether foreign to the philosophy of his day. He attributes it to the reflection of the sun's rays from the cloud; and the chief merit of his theory is in the clear and philosophical manner in which he proves that the phenomenon is an appearance, and not a reality. Between the two last-mentioned books is a treatise, 'De Multiplicatione Specierum,' entirely filled with discussions somewhat metaphysical upon the connexion and causes of phenomena. Our limits will not allow us to enter further into details: nor could we, in any moderate space, do justice to the varied learning of the author, or distinctly mark even the chief of the numerous singular and now exploded notions which are introduced; nor, as far as we know, does there exist any full account of the contents to which we can refer the reader. The following amusing extract will show the sort of reputation which Roger Bacon acquired:-- "How Friar Bacon made a brazen head to speak, by the which he would have walled England about with brass.