Descrizione

The difficulties in arrangement which this fact, suggests have been enhanced by the nature of the subject itself and by the tentative character of some of its results. The writing of the book has necessarily involved attempts to extend the frontier of existing knowledge, and to fill in gaps which broke the connexion between isolated parts of the subject. The' references in the text which do not give the name of the author always refer to my own papers; in this connexion, however, I should like to out that the citations in the text are not to be regarded any means complete, and are supplemented by the list of literature at the end of the volume.