In the Preface to my former volume I expressed a hope that the remaining volume might be completed for publication before the end of the year 1881. This hope has been disappointed partly owing to the labour of expanding into a separate work the Sketch of Ancient Philosophy, which formed part of the Introduction to the First Book, and partly from the unforeseen difficulties which I have encountered in the endeavour to explain fully the scientific views of the Ancients, as they are reported by Cicero in his Second Book. The consequent increase in the size of the Commentary has made it necessary to devote a whole volume to this Book, and the publication of the third and last Book must still be deferred to another year.

In the present volume I have been enabled to improve on the Apparatus Criticus of my former volume, owing to the kindness of the authorities of Merton College, Oxford, in lending me their valuable Codex (Oxf. o, here denoted simply as Oxf.) written in the 12th century. It is older than any other English ms of the De Natura Deorum with the exception of the fragmentary Harleian no. 2622 (k), and is closely allied with the oldest of all the MSS, the Vienna Codex of the 10th century (v). I have inserted a full collation of the Merton Codex amongst Mr Swainson's Collations of English mss, showing such a remarkable resemblance between it and v, that the one might easily be supposed to have been copied from the other.