Twenty-five years have passed since the writing of this volume in its first edition. Such a
long interval has inevitably made necessary a fairly thorough revision and expansion of the
book for its second edition.
The original choice of material was such that, with some very slight exceptions, it has not
become obsolete. In this part, only some relatively minor additions and improvements
have been made.
It has, however, been necessary to incorporate a considerable amount of new material.
This relates in particular to the theory of the magnetic properties of matter and the theory
of optical phenomena, with new chapters on spatial dispersion and non-linear optics.
The chapter on electromagnetic fluctuations has been deleted, since this topic is now
dealt with, in a different way, in Volume 9 of the Course.
As with the other volumes, invaluable help in the revision has been derived from the
comments of scientific colleagues, who are too numerous to be named here in their entirety,
but to whom we offer our sincere thanks. Particularly many comments came from V. L.
Ginzburg, B. Ya. Zel'dovich and V. P. Krainov. It was most useful to be able to hold regular
discussions of questions arising, with A. F. Andreev, I. E. Dzyaloshinskii and I. M. Lifshitz.
We are particularly grateful to S. I. Vainshtein and R. V. Polovin for much assistance in
revising the chapter on magnetohydrodynamics. Lastly, our thanks are due to A. S.
Borovik-Romanov, V. I. Grigor'ev and M. I. Kaganov for reading the manuscript and
for a number of useful remarks.