PREFACE the year 1912, the University of Calcutta appointed me I Reader in Physics and invited me to deliver a course of lectures to its advanced students on Optical Theories, one of the conditions of the appointment being that the lectures should be published after their delivery. The lectures were actually delivered during the months of February and March, 1912, but pressure of other work has prevented me, till now, from seeing them through the press. It has been my object in these lectures to trace the development of Optical Theories from the earliest times to the present day. I have tried to understand and help others so far as I can to understand the relation between the different theories, so that one may be clear as to how much is certainly known and how much is mere speculation. In the midst of the bewildering mass of investigations that a student of luminiferous medium is confronted with at the present day, a sketch, such as the one attempted here, describing, with such details as will make the general argument intelligible, how we have been led up to the present position and what that position really is should, as it seems to me, be of considerable use. How far I have succeeded in my attempt, it is for others to judge. To the latest developments of the optical theory including the theory of relativity, no reference has been made here. I hope to deal with them in a later volume, if the present attempt proves successful. D. N. nr. November, 1916. CONTENTS CRAP. 1. EARLY SPECULATIONS, CORPUSCU1, dkt 1. HPORY, UXDULATORY THEORY . . . . II. ELECTRO-MAGN TH E E T O I R C Y . . . . IV. ELECTRO T N HE ORY . . CHAPTER I INTRODUCTION. CORPUSCULAR THEORY. UNDULATORY THEORY Page 12, line 3, for must have read may take a-A A -. . - - - - l -- V y V l l l U U LU after all extremely limited. 2. This, indeed, is what is to be expected-illustrating, we mily remark in passing, the limit and scope of scientific inquiry in general. For, although the phenomena with which, to confine ourselves to optical investigations alone, we have to deal are simple and well-known, and although we can formulate the laws governing them-like any other body of scientific lawswhich state as Karl Pearson has put it, in concep ual shorthand, that routine of our perceptions, which forms for us the totality of the phenomena, to which they refer, when we come to inquire into the intimate nature of the processes, itssociated with these laws, we are confronted with insuperable difficulties. These are so subtle and deep-seated, that they will probably always elude our grasp. We are, therefore, reduced to preparing models, that shall approximate to the actual, as far as possible, and our task consists really in improving these models, more and more, so that they may more and more nearly approximate to the actual. Thus, of the mode of propagation of light and the nature of the medium which takes part in its propagation, me, with our . limitations, can never have any direct knowledge. 0. l T . . 1 CHAPTER I INTRODUCTION. CORPUSCULAR THEORY. UNDULATORY THEORY 1. A complete theory of optics has to furnish an adequate account, uot merely of-the nature of light but also - of- he-mode and mechanism of its propagation, as well as the nature of the L .. medium in , hicc the propagation-takes-place... --This text refers to the Paperback edition.