The book written in 1941 by the late Charles Coulson served to introduce a great many students to the basic ideas that underlie linear
wave motion. Just prior to his death he was beginning to give thought
to the task of revising his book in order to take account of the changes
in the teaching of the subject that have inevitably taken place since it
was first written. It is to be regretted that he left no plan indicating the
modifications he intended to make. Consequently, when accepting his
last request to me to see that a new edition of his book should appear in
print, I had to undertake my own re-planning of this work without
benefiting from any of his ideas or the special gift of physical insight
that he brought to his teaching.