SIX LECTURES ON LIGHT DELIVERED IN THE UNITED STATES IN 1872-1873 BY JOHN TYNDALL, D.C.L., LL,D., F.R.S. LATE PROFESSOR OF NATURAL PHILOSOPHY IN THE ROYAL INSTITUTION OF GREAT BRITAIN [Illustration: Sir Thomas Laurence PRA Pinx Henry Adlarc. Sc. Signature: Thomas Young] London: Longmans & Co. _SIXTH IMPRESSION_ LONGMANS, GREEN, AND CO. 39 PATERNOSTER ROW, LONDON NEW YORK AND BOMBAY 1906 PREFACE TO THE FOURTH EDITION. In these Lectures I have sought to render clear a difficult but profoundly interesting subject. My aim has been not only to describe and illustrate in a familiar manner the principal laws and phenomena of light, but to point out the origin, and show the application, of the theoretic conceptions which underlie and unite the whole, and without which no real interpretation is possible. The Lectures, as stated on the title-page, were delivered in the United States in 1872-3. I still retain a vivid and grateful remembrance of the cordiality with which they were received. My scope and object are briefly indicated in the 'Summary and Conclusion,' which, as recommended in a former edition, might be, not unfitly, read as an introduction to the volume. J.T. ALP LUSGEN: _October_ 1885. CONTENTS. LECTURE I. Introductory Uses of Experiment Early Scientific Notions Sciences of Observation Knowledge of the Ancients regarding Light Defects of the Eye Our Instruments Rectilineal Propagation of Light Law of Incidence and Reflection Sterility of the Middle Ages Refraction Discovery of Snell Partial and Total Reflection Velocity of Light Roemer, Bradley, Foucault, and Fizeau Principle of Least Action Descartes and the Rainbow Newton's Experiments on the Composition of Solar Light His Mistake regarding Achromatism Synthesis of White Light Yellow and Blue Lights produce White by their Mixture Colours of Natural Bodies Absorption Mixture of Pigments contrasted with Mixture of Lights LECTURE II. Origin of Physical Theories Scope of the Imagination Newton and the Emission Theory Verification of Physical Theories The Luminiferous Ether Wave-theory of Light Thomas Young Fresnel and Arago Conception of Wave-motion Interference of Waves Constitution of Sound-waves Analogies of Sound and Light Illustrations of Wave-motion Interference of Sound Waves Optical Illustrations Pitch and Colour Lengths of the Waves of Light and Rates of Vibration of the Ether-particles Interference of Light Phenomena which first suggested the Undulatory Theory Boyle and Hooke The Colours of thin Plates The Soap-bubble Newton's Rings Theory of 'Fits' Its Explanation of the Rings Overthrow of the Theory Diffraction of Light Colours produced by Diffraction Colours of Mother-of-Pearl. LECTURE III.