This book is a translation of the 6th to 8th edition of the author's Kleines Lehrbuch der Physik. The circle of readers to which it hopes to appeal and the author's purpose in writing it have been set out in the Preface to the first German edition, published in 1948. The present book consistently follows the principles of the theory of quantities, the beginnings of which date back to James C. Maxwell. This means that in all equations in this book the symbols invariably stand for physical quantities and not for the numerical values of quantities. Only then are the equations gene rally valid and independent of the choice of units used in their evaluation. The units used are always the "metric" units which have been gaining ground increasingly also in the English-speaking countries. A conversion table for some of the more important Anglo-American units is given on page XIV. I would like to record my sincere gratitude to Mr. Ewald Osers for his pains­ taking work in making this translation and to Mr. P. C. Banbury, Ph. D., of the Department of Physics, University of Reading, England, both for the advice he has given hirn throughout and for devising the problems specially for this edition.