in the ase dif- s have ing the -ording elds of hysics. tor the is for hoped 1ensive th the at cos- natural hut the Amer- ‘ology, at they to the bsolute ‘1entist. 1 coun- 1f-suffi- :nt and ‘mental ACKNOWLEDGMENTS HIS study could not have been written without the valuable and stimulating suggestions of Professor G. P. Adams and Professor F. J. Teggart. Especially to the latter I owe a debt which I have not been able to acknowledge in direct reference or in quotation marks. His books and the personal contact of years have been an inspiration for which I take this opportunity to express my gratitude. In the revision of the manuscript I have received the valuable advice and criticism of a number of friends; in particular, Miss Lola Jean Simpson and Miss Leona Fas- sett. rection. T S. EY.