My work as editor has been simple. The lectures were written out in full before they were delivered, and they are printed here substantially as they stand in the manuscript. It proved unnecessary to print the recapitulation with which the second course began, but a few amplifications have been introduced from it into the text of the original lectures. I am responsible for all the divisions and subdivisions introduced into the text, for the titles of the separate 'lectures', and for the marginal analysis. I have verified the references, and have for the convenience of the reader printed at full length in the Appendix any that were not likely to be readily accessible.