"In a style singularly lucid, considering the abstruse nature of the subject treated, Dr. Lommel unfolds the learning of the scientists on the nature and phenomena of light."--_Philadelphia Inquirer._ "As a popular introduction to physical optics, it would be difficult to find a more satisfactory work than the one by Dr. Lommel, which has just appeared in the excellent 'International Scientific Series.'"--_The English Mechanic._ D. APPLETON & CO., PUBLISHERS, 549 & 551 Broadway, N. Y. THE INTERNATIONAL SCIENTIFIC SERIES. D. APPLETON & CO. have the pleasure of announcing that they have made arrangements for publishing, and have recently commenced the issue of, a SERIES OF POPULAR MONOGRAPHS, or small works, under the above title, which will embody the results of recent inquiry in the most interesting departments of advancing science. The character and scope of this series will be best indicated by a reference to the names and subjects included in the subjoined list, from which it will be seen that the coöperation of the most distinguished professors in England, Germany, France, and the United States, has been secured, and negotiations are pending for contributions from other eminent scientific writers. The works will be issued in New York, London, Paris, Leipsic, Milan, and St. Petersburg. The INTERNATIONAL SCIENTIFIC SERIES is entirely an American project, and was originated and organized by Dr. E. L. Youmans, who has spent much time in Europe, arranging with authors and publishers. _FORTHCOMING VOLUMES._ Prof. W. KINGDON CLIFFORD, M. A. _The First Principles of the Exact Sciences explained to the Non-mathematical._ Prof. T. H. HUXLEY, LL. D., F. R. S. _Bodily Motion and Consciousness._ Dr. W. B. CARPENTER, LL. D., F. R. S. _The Physical Geography of the Sea._ Prof. WILLIAM ODLING, F. R. S. _The Old Chemistry viewed from the New Stand-point._ W. LAUDER LINDSAY, M. D., F. R. S. E. _Mind in the Lower Animals._ Sir JOHN LUBBOCK, Bart., F. R. S. _On Ants and Bees._ Prof. W. T. THISELTON DYER, B. A., B. Sc. _Form and Habit in Flowering Plants._ Mr. J. N. LOCKYER, F. R. S. _Spectrum Analysis._ Prof. MICHAEL FOSTER, M. D. _Protoplasm and the Cell Theory._ H. CHARLTON BASTIAN, M. D., F. R. S. _The Brain as an Organ of Mind._ Prof. A. C. RAMSAY, LL. D., F. R. S. _Earth Sculpture: Hills, Valleys, Mountains, Plains, Rivers, Lakes; How they were Produced, and how they have been Destroyed._ Prof. RUDOLPH VIRCHOW (Berlin University). _Morbid Physiological Action._ Prof. CLAUDE BERNARD. _History of the Theories of Life._ Prof. SAINTE-CLAIRE DEVILLE. _An Introduction to General Chemistry._ Prof. WURTZ. _Atoms and the Atomic Theory._ Prof. De QUATREFAGES. _The Human Race._ Prof. LACAZE-DUTHIERS. _Zoölogy since Cuvier._ Prof. BERTHELOT. _Chemical Synthesis._ Prof. C. A. YOUNG, Ph. D. (of Dartmouth College). _The Sun._