Niels Bohr (1885-1962) was a Danish physicist who played a key role in the development of atomic theory and quantum mechanics, he was awarded the Nobel Prize for Physics in 1922. First published in 1924, this second edition of a 1922 original contains three essays by Bohr dealing with the application of quantum theory to problems of atomic structure: the first essay is on the spectrum of hydrogen; the second is on the series spectra of the elements; the third is on the structure of the atom and the physical and chemical properties of the elements. The essays do not aim at a comprehensive treatment of their subjects, instead providing the reader with a more accessible, generalised viewpoint. This book will be of value to anyone with an interest in Bohr's contribution to physics.