must necessarily fit, but they cannot be accepted as constituting in themselves a final theory of light. The study of Physics must be based on a knowledge of Mechanics, and the problem of light will only be solved when we have discovered the mechanical properties of the æther. While we are in ignorance on fundamental matters concerning the origin of electric and magnetic strains and stresses, it is necessary to introduce the theoretical study of light by a careful treatment of wave propagation through media the elastic properties of which are known. A study of the theory of sound and of the old elastic solid theory of light must precede therefore the introduction of the electromagnetic equations. г The present volume is divided into two parts; the first part includes those portions of the subject which may be treated without the help of the equations of dynamics, although a shortdiscussion of the kinetics of wave motion is introduced at an early stage. The mathematical treatment has been kept as simple as possible, elementary methods only being used. I hope that rigidity of method is nowhere sacrificed thereby, while the advantage is gained that students obtain an insight into what is most essential in the theory of Interference and Diffraction, without introducing purely mathematical difficulties such as are involved in the use of Fresnel's integrals. Even accurate numerical results may be obtained by a proper use of Fresnel's zones. The second part of the book is intended to serve as an introduction to the higher branches of the subject. It has not been my object as regards this more advanced portion to write a treatise which shall be complete in itself, but rather to introduce the student to the writings of the original authorities. As a teacher, I consider this to be the correct method, being convinced that students should be encouraged at an early stage to consult the literature of the subject. It is a necessary consequence of the point of view adopted that the treatment is somewhat unequal. Where the author has nothing to say which is novel, or may remove obscurities, the best thing he can do, is to content himself with a short summary, referring the reader for details to the available sources of information. A more lengthy exposition is justified where a simplification or some new matter can be introduced. It may be mentioned in this connexion that as far as I know the consideration of absorptive regions of finite range of frequency in the theory of selective dispersion is new, and has not previously been published. I have purposely abstained from entering into details of methods of observation or instrumental appliances. These belong more properly to the courses of laboratory instruction. I hope that the short biographical notices of deceased authors who have made important contributions to the science will be found to be of interest. The greater part of this book was already in type when Lord Kelvin's Baltimore Lectures appeared; I was still able to add some references to these lectures, though not to the extent I should have wished. In some of the later chapters repeated reference is made to Drude's Lehrbuch der Optik. Students who desire to pursue the subject further, should also have access to Mascart's Optique and Lord Rayleigh's Collected Works. My own indebtedness to Lord Rayleigh's writings and personal inspiration is greater than can be acknowledged by mere references to his papers, and I am therefore glad to be allowed to dedicate this volume to him. I am obliged to Prof. Wilberforce and Mr W. H. Jackson for having looked through the proofs of the greater portion of the work, and favoured me with their corrections and suggestions. I have also to thank Mr J. E. Petavel for the very valuable help he gave me in drawing out the figures, and Mr H. E. Wood for taking the photographs of interference effects which have been used in preparing the plates. August, 1904. ARTHUR SCHUSTER. CONTENTS. Normal oscillations under the action of forces varying as the 3. Energy of a particle in periodic motion 4. Composition of periodic motions. 5. Combination of any number of oscillations 6. Combinations of oscillations at right angles to each other Composition of linear vibrations of slightly different periodic |