The term "optics" has always been interpreted in this series in a broad sense, and the range of topics covered by the articles in this volume reflect this fact once again. Two of the articles, those on self-focussing and self-induced transparency, deal with subjects that owe their origin largely to the laser. Both concern the behavior of the polarization of a medium through which light passes and, particularly, the way in which it depends upon properties characteristic of laser light. The growing influence of laser theory on other aspects of physics is exemplified by the article on the phase transition concept and coherence in atomic emission.
Considerable advances have also occurred in experimental techniques in optics and in spectroscopy. Indicative of such advances are the articles on modulation techniques in spectrometry and on beam-foil spectroscopy. In the same spirit, another article in this volume describes a relatively new and beautiful technique, employing monomolecular dye layers, for studying a variety of phenomena involving the generation and propagation of light waves.
It is, as ever, a pleasure to welcome to the pages of this scries a new group of authors, each active and well known in his own field.