AUTHOR'S PREFACE. THIS little book has been written from the point of view of the usefulness of the Theory of Relativity in the development of physical science. It is principally based on a short course of lectures which I delivered at Indiana University in the Fall of 1912 and on my recent papers in the Physical Review, vol. 35, pp. 153-176, and Second Series, vol. 1, pp. 161-197. No attempt is made to give any applications of the theory other than what is incidental to the derivation of the fundamental results concerning length and time, the transformation of coordinates, mass and energy, and experimental verification. It is believed, however, that the presentation is such as to keep always close to concrete experience, so that the results obtained may be directly useful in suggesting experiments for the laboratory. My indebtedness to other writers is indicated by the references in my recent papers referred to above. I wish also to speak of the useful suggestions made by my students in their frequent discussion of the Theory of Relativity during the course last Fall.