Originally published in 1934, this book was primarily intended as a reference guide to equip undergraduate students with the introductory and principle knowledge of the theory of quantum mechanics. The book contains ten chapters in total, ranging from 'The wave equation' to 'Electrons in crystals'. Multiple diagrams also supplement the text, with their aim being to elucidate the problems of quantum mechanics via graphical methods and to illustrate the tangible array of challenges still faced by scientists in the field, including the problems of valency and the properties of molecules. Notably, Gurney identifies and examines the principles, considering and keeping always in view the 'physical significance'. This book will serve as a salient reference work for specialists and non-specialists alike and will be of great value to anyone wishing to further their understanding and knowledge in the history of quantum physics.