THE idea of writing this book was a result of frequent enquiries about the possibility of publishing in the English language a book on optics written by one of us more than twenty-five years ago. A preliminary survey of the literature showed that numerous researches on almost every aspect of optics have been carried out in the intervening years, so that the book no longer gives a comprehensive and balanced picture of the field. In consequence it was felt that a translation was hardly appropriate; instead a substantially new book was prepared, which we are now placing before the reader. In planning this book it soon became apparent that even if only the most important developments which took place since the publication of Optik were incorporated, the book would become impracticably large. It was, therefore, deemed necessary to restrict its scope to a narrower field. Optik itself did not treat the whole of optics. The optics of moving media, optics of X-rays and y-rays, the theory of spectra and the full connection between optics and atomic physics were not discussed; nor did the old book consider the effects of light on our visual sense organ – the eye. These subjects can be treated more appropriately in connection with other fields such as relativity, quantum mechanics, atomic and nuclear physics, and physiology. In this book not only are these subjects excluded, but also the classical molecular optics which was the subject matter of almost half of the German book.