The circumstances that have led to the production of this little book are, I think, sufficiently explained in the introductory chapter; there is, therefore, no need to dwell upon them here. It is only necessary perhaps to warn the reader that he will find in what follows but little that is original. With the exception of small contributions and suggestions upon special points, in the last few chapters alone does there exist anything that has not already found a place in the literature dealing with the subject; and probably it will be the earlier rather than the later portions of the book that will most often be consulted. Nevertheless, a work of compilation, such as the present for the most part aims at being, may have its justification and a certain sphere of usefulness; especially so perhaps in the present case, since a certain proportion of the original papers to which reference is here made is contained in books and periodicals that have at no time been readily accessible to the English-speaking public and were for some years practically unobtainable.