COLUMBIA UNIVERSITY LECTURES LIGHT BY RICHARD C. MACLATJRIF, LL. D., So. D, PRESIDENT OF THE MASSACHUSETTS IKSTJTXJTE OF TECHNOLOGY mm COPYRIGHT, 1909, Br THE COLUMBIA IWIVEESITY PBESS. Set up and electrotyped. Published June, 1909, J, fl. Cushingr Co, Berwick Smith Co, Norwood, Mass., U. S. A. NEWTON PREFACE THESE lectures were given at tiie American Museum of Natural History during the winter of 1908-9, when I had the honor of occupying a chair of Mathematical Physics at Columbia University in the City of New York It is not easy, in such a place, for a man of science to sit in cloistered calm, far from the distractions of the busy world of action, and to pursue research merely for self illumination or for the edification of a caste of intel lectuals. The throb of life is all around him, and it impresses him with the duty of responding to the de mands of an active-minded people for reliable informa tion on the most recent developments of science. He is expected to know but not only to know, but also to communicate. And so, on being invited to give the Jesup Lectures, I attempted to describe the salient features of the modern theory of light within the narrow compass of ten lectures, and undertook in doing so to avoid techni calities as much as possible. I have had specially in view the man of intelligence who lays no claim to scientific knowledge, but who wishes to know what all the talk of science is about, and, in particular, why the physicists make such strange postulates as ether and electrons, and why they have so much confidence in the methods that they employ and the results that they obtain. For this purpose I have had to show him how wonderfully the theory fits the facts, down to the minutest numerical vi PKEFACE detail although, of course, the full force of the argument is lost owing to the necessity of eschewing mathematics and merely stating the results of theory without giving the actual demonstrations, except in the simplest cases. I hope, too, that the book may be found useful to the large body of teachers of physics throughout the country. They will find in it much that is scarcely touched upon in the ordinary text-books, and their appreciation of the dif ficulties of presenting such a subject in non-technical lan guage will put them in that sympathetic frame of mind that helps so much towards the understanding of a writer. My thanks are due to Mr, Farwell for the care and skill with which he conducted the experiments that illus trated the lectures, and to my colleague, Professor E, P. Nichols, for reading the proof-sheets and displaying a keen interest in the progress of the course. B. C. M. A man of science does well indeed to take Ms views from, many points of sight, and to supply the defects of sense by a well-regulated imagination tout as his knowledge of Nature is founded on the observation of sensible things, he must be gin with these, and must often return to them to examine his progress by them. Here is his secure hold and, as he sets out from thence, so if he likewise trace not often his steps backwards with caution, he will be in hazard of losing his way in the labyrinths of Nature, Colin Maclaurin An account of Sir Isaac JSTewtons Philosophical Discoveries. 1748. vil CONTENTS LEOTTTKIB I. EARLY CONTRIBUTIONS TO OPTICAL THEORY . . 1 II. COLOR VISION AND COLOR PHOTOGRAPHY ... 24 III. DISPERSION AND ABSORPTION 47 IT. SPECTROSCOPY 70 V. POLARIZATION .95 VL THE LAWS OF BEFLECTION AND REFRACTION . . 118 VII. THE PRINCIPLE OF INTERFERENCE .... 154 VIII. CRYSTALS 175 IX. DIFFRACTION 202 X. LIGHT AND ELECTRICITY 229 INDEX 249 LIGHT EARLY CONTRIBUTIONS TO OPTICAL THEORY THEY tell us, said Matthew Arnold, that when a candle burns, the oxygen and nitrogen of the air combine with the carbon in the candle to form carbonic acid gas... --This text refers to the Paperback edition.