*This article is substantially the same as one which was composed for publication in Germany, and has been brought out as a book by the Verlag S. Hirzel. I am much indebted to the criticism and advice of my mathematical colleague Mr. L. A. MacColl, and have profited by some of the additions and rearrangements made for the German version by the translator, Dr. E. Rabinowitsch. I have also taken heed of many good suggestions made by an anonymous critic while the manuscript was in the hands of a publisher.
