The course from which this book has developed was given at Ни invitation of the National Bureau of Standards in the summer ill 11)52 at the American University, Washington, D.C. Notes of I he lectures were taken by Mr. M. Ticson and reproduced in a small Miniiher of copies as the working paper CL-52-2 for the internal use of the NBS.
As this course contained a great deal of unpublished material, It appeared advisable to publish the whole as a book. For this purpose, tlit- whole course was worked through anew and completely rewritten, 11ми It new material was incorporated in the lectures, and 11 appendices wrir added to deal with problems which could not be treated in the nmi 111 text of the book. The treatment is still far from being complete, ImwrviT.