A book of this character is peculiarly open to criticism, but I hope the critics will give me credit for knowing more than I have set down. In making a list of books of reference, I have had to make a selection, and works have been before me that I have decided to omit, although some would think them as important as many of those I have included. I need not extend this preface with any lengthy explanation of the objects of the book, as these are stated in the Introduction, but before concluding I may perhaps be allowed to allude to one personal circumstance.