The present book is a revised version of a lecture course given by the author in the Spring Term 1968, in three weekly hours (a total of about 40 lectures), in the Department of Physical Sciences and Applied Mathematics, North Carolina State University, Raleigh, N.C. The bulk of the audience came from the faculty members of mathematics, physics, chemistry, and various branches of engineering; the students, who took the course for credit, were in the minority. It was thus indicated to adopt a more flexible and more imaginative approach than that followed in a customary university course, in which students are prepared for examinations. Here were scientific workers from all branches of science and one may well ask, whether the diversity of the audience and the largeness of the subject matter would not cancel out in advance anything that goes beyond generalities and platitudes. To the author the task appeared in a much more optimistic light.