INTERNATIONAL SCIENTIFIC SERIES. NOW READY. In 12mo, and bound in cloth. 1. FORMS OF WATER, in Clouds, Rain, Rivers, Ice, and Glaciers. By Prof. JOHN TYNDALL. $1.50. 2. PHYSICS AND POLITICS; or, Thoughts on the Application of the Principles of "Natural Selection" and "Inheritance" to Political Society. By WALTER BAGEHOT. $1.50. 8. FOODS. By EDWARD SMITH, M. D., LL. B., F. R. S. $1.75. 4. MIND AND BODY. BY ALEXANDER BAIN, LL. D. $1.50. 5. THE STUDY OF SOCIOLOGY. By HERBERT SPENCER. $1.50. 6. THE NEW CHEMISTRY. versity. $2.00. By Prof. JOSIAH P. COOKE, Jr., of Harvard Uni 7. THE CONSERVATION OF ENERGY. By Prof. BALFOUR STEWART, LL. D., F. R.S. $1.50. 8. ANIMAL LOCOMOTION; with a Dissertation on Aeronautics. By J. B. PETTIGREW, M. D. Illustrated. $1.75. 9. RESPONSIBILITY IN MENTAL DISEASE. By H. MAUDSLEY, M. D. $1.50. 10. THE SCIENCE OF LAW. By Prof. SHELDON AMOS. $1.75. 11. ANIMAL MECHANISM. A Treatise on Terrestrial and Aërial Locomotion. By E. J. MAREY. 117 Illustrations. $1.75. 12. THE HISTORY OF THE CONFLICT BETWEEN RELIGION AND SCIENCE. By J. W. DRAPER, M. D., LL. D. $1.75. 18. THE DOCTRINE OF DESCENT, AND DARWINISM. By Prof. OSCAR SCHMIDT, of Strasburg University. $1.50. 14. THE CHEMISTRY OF LIGHT AND PHOTOGRAPHY. By Dr. HERMANN VOGEL. 100 Illustrations. $2.00. 15. FUNGI; their Nature, Influence, and Uses. By M. C. COOKE, LL. D. Edited by M. J. BERKELEY. 109 Illustrations. $1.50. 16. THE LIFE AND GROWTH OF LANGUAGE. By Prof. W. D. WHITNEY, of Yale College. $1.50. 17. MONEY AND THE MECHANISM OF EXCHANGE. JEVONS. M. A., F. R. S. $1.75. By W. STANLEY 18. THE NATURE OF LIGHT. By Dr. E. LOMMEL. ES Illustrations and a Plate of Spectra in Chromo-lithography. $2.00. 19. ANIMAL PARASITES AND MESSMATES. By M. VAN BENEDEN, Professor of the University of Louvain. 83 Illustrations. $1.50. 20. ON FERMENTATIONS. By P. ScHÜTZENBERGER, Director at the Chemical Laboratory at the Sorbonne. 28 Illustrations. $1.50. 21. THE FIVE SENSES OF MAN. By J. BERNSTEIN, O. O. Professor in the University of Halle. 19 Illustrations. $1.75. 22. THE THEORY OF SOUND IN ITS RELATION TO MUSIC. By Prof. PIETRO BLASERNA. Numerous Woodcuts. $1.50. 23. STUDIES IN SPECTRUM ANALYSIS. By J. NORMAN LOCKYER. Illustrations. $2.50. 24. A HISTORY OF THE GROWTH OF THE STEAM-ENGINE. BY ROBERT H. THURSTON, A. M., C. E. 163 Illustrations. $2.50. 25. EDUCATION AS A SCIENCE. By ALEX. BAIN, LL. D. $1.75. 26. MODERN CHROMATICS. By OGDEN N. ROOD, Professor of Physics in Columbia College. 130 original Illustrations. $2.00. 27. THE HUMAN SPECIES. By A. DE QUATREFAGES. $2.00. 28. THE CRAYFISH: An Introduction to the Study of Zoology. By Prof. T. H. HUXLEY. 82 Illustrations. $1.75. 29. THE ATOMIC THEORY. By AD. WURTZ, Membre de l'Institut, etc. Translated by E. CLEMINSHAW. $1.50. 30. ANIMAL LIFE AS AFFECTED BY THE NATURAL CONDITIONS OF EXISTENCE. BY KARL SEMPER, Professor of the University of Würzburg. With 2 Maps and 106 Woodcuts. For sale by all booksellers; or any volume sent by mail, post-paid, on receipt of price. D. APPLETON & CO., Publishers, 1, 3, & 5 Bond St., New York. SIGHT: AN EXPOSITION OF THE PRINCIPLES OF MONOCULAR AND BINOCULAR VISION. BY JOSEPH LE CONTE, LL. D., AUTHOR OF ELEMENTS OF GEOLOGY, RELIGION AND SCIENCE," AND PROFESSOR OF GEOLOGY AND NATURAL HISTORY IN THE UNIVERSITY OF CALIFORNIA. WITH NUMEROUS ILLUSTRATIONS. NEW YORK: D. APPLETON AND COMPANY, 1881. PREFACE. IN writing this treatise I have tried to make a book that would be intelligible and interesting to the thoughtful general reader, and at the same time profitable to even the most advanced specialist in this department. I find justification for the attempt in the fact that there is not, to my knowledge, any work covering the same ground in the English language. Vision has been treated either as a branch of optics or else as a branch of physiology of the nervous system. Helmholtz's great work on "Physiological Optics," of which there exist both a German and a French edition, is doubtless accessible to scientists, but this work is so technical that it is practically closed to all but the specialist. I believe, therefore, that the work which I now offer meets a real want, and fills a real gap in scientific literature. The form in which the subject is here presented has been developed entirely independently, and as the result of a conscientious endeavor to make it clear to students under my instruction. As evidence of this, I would draw attention to the fact that, out of one hundred and thirty illustrations, only about twelve have |