PREFACE. THE following lectures were given as a course of instruction to the senior students in electrical engiheering at Union University. They are however intended not merely as a text-book of illuminating engineering, nor as a text-book on the physics of light and radiation, but rather as an exposition, to some extent, from the engineering point of view, of that knowledge of light and radiation which every educated man should possess, the engineer as well as the physician or the user of light. For this purpose they are given in such form as to require no special knowledge of mathematics or of engineering, but mathematical formalism has been avoided and the phenomena have been d o scribed in plain language, with the exception of Lectures X and XI, which by their nature are somewhat mathematical, and are intended more particularly for the illuminating engineer, but which the general reader may safely omit or merely peruse the text. The lectures have been revised to date before publication, and the important results of the work of the National Bureau of Standards, contained in its recent bulletins, fully utilized. COMPILERS PREFACE. A SERIES of eight experimental lectures on Light and Radiation were delivered by Dr. Steinmetz in the winter of 1907-8 before the Brooklyn Polytechnic Institute. Unfortunately no stenographer was present and no manuscript prepared by the lecturer. A far more extended course of experimental lectures was however given by Dr. Steinmetz at Union University in the winter of 1908-9, on Radiation, Light, Illumination and Illuminating Engineering, and has been compiled and edited in the following. Two additional lectures have been added thereto by Dr. Steinmeta to make the treatment of the subject complete even from the theoretical side of ilIuminating engineering Lecture X on Light Flux and Distribution and Lecture XI on Light Intensity and Illumination. These two lectures give the elements of the mathematical theory of illuminating engineering. With the exception of the latter two lectures the following book contains practically no mathematics, but discusses the subjects in plain and generally understood language. The subjdct matter of Lecture XI1 on Illumination and Illuminating Engineering has been given in a paper before the Illuminating Engineering Society the other lectures are new in their form and, as I believe, to a considerable extent also in their contents. In describing the experiments, numerical and dimensional data on the apparatus have been given, and the illustrations dram to scale, as far as possible, so as to make the repetition of the experiments convenient for the reader or lecturer. Great thanks are due to the technical staff of the RlcGraw-Hill Book Company, which has spared no effort to produce the book in as perfect a manner as possible. JOSEPH L. R. HAYDEN. SCHENECTA S D e Y pt , e mber, 1909. CONTENTS. PAGE LECTURE I . NATURE A ND DIFFERENT F ORM O S F RADIATION. 1. Radiation as energy. 2. Measurement of the velocity of light. 3. Nature of light. 4. Difference of wave length with differences of color. RIeasurement of wave length and of frequency. Iridescence. The ether. 5. Polarization proving light a transversal vibration. Double refraction. 6. The visible octave of radiation. Ultra-red and ultra-violet radiation. 7. The electric waves. 8. The spectrum of radiation covering 60 octaves. 9. Electric waves of single frequency, light waves of mixed frequency. 10. Resolving mixed waves into spectrum. Refraction. 11...