will have largely served its purpose if it draws attention to the point of view. In questions of this kind the most fruitful source of progress is perhaps the process of mutual approximation of different standpoints; any single attempt at effective adaptation of fundamental conceptions must involve detail that is only provisional, and leave points of difficulty unsolved or imperfectly analyzed, while in many cases it will originate more problems than it settles. This latter feature of general theoretical physics cannot in fact be better illustrated than by the original foundation of all such theories as given by Maxwell, which in its broad outlines was the culmination of the greatest advance in modern times: yet it presented itself with so many gaps in its formal development, and raised so many new questions for discussion, that finality with regard to the mode of formulation of the subject is possibly yet far off.
Acknowledgment is due to the officials of the University Press for the exact and obliging manner in which they have executed the printing and corrections.
St John's College, Cambridge Mar. 6, 1900
