We have been handling Natural History Material of all kinds for the past fifty years, and so have accumulated a very complete stock. Our prices are as low as the quality of the specimens will permit. We are constantly preparing circulars and price lists covering our material in the different branches, which we will be pleased to send free of charge to the readers of Science. Some of our recent circulars: S-121 Rare Bird Skins S-119 Cretaceous Fossils CONCHOLOGY GEOLOGY Ward's Natural Science The Silverman Illuminator A new eye for the microscope Offers important advantages for practically every application of the Microscope a-It shows more detail. b-A clearer and better defined picture is presented to the eye and the camera. Several novel methods of illumination can be produced. d-It saves very much valuable time. It prevents eye strain, eye fatigue, and brain fag. f-It can be lowered in to deep, hollow places. 9-It gives excellent results for very low power work as well as higher magnifi cations, also in oil immersion work. k-It can be used with any Microscope, ordinary or binocular. A small circular tube lamp surrounds the objective and furnishes a diffused and uniform illumination directly where it is needed. The Silverman Illuminator marks A Great Advance in Microscope Illumination For sale by all the better dealers Ludwig Hommel & Co. Write for Bulletin 45-B 530-534 Fernando St. PITTSBURGH, PA. Our Laboratory Spectrometers Are sold in competition with instruments of European make WM. GAERTNER AND CO. Physical and Astronomical Instruments 5345-49 Lake Park Avenue CHICAGO Eastman Organic Chemicals DURING the past month the following chemicals have become available: Of the above list all those distinguished by an asterisk have been prepared or purified in our laboratory. "Chloride Accumulators" and "Exide Batteries" Are thoroughly satisfactory for scientific work, which accounts for their being used in so many of the educational and research laboratories throughout the United States. If you are equipped, and after long service your "Chloride" battery needs some renewals; or if you contemplate a new installation we are ready to serve you. By arrangement with the factory we act as Special College Representatives and are prepared to furnish portable or stationary outfits, with any number of cells for current capacity from ampere to any maximum desired. May we send you catalog? To aid us in rendering good service, please state what your requirements are. JAMES G. BIDDLE 1211 ARCH STREET NEW SERIES VOL. LIII, No. 1380 FRIDAY, JUNE 10, 1921 SINGLE COPIES, 15 CT8 Philosophy And The New hysics Discussion of The Relativity Theory and The The Quanta Lettres. Au 1129 Cowper Street, Sek John F. Newsom, y MORTON By LOUIS ROUGIER, Professeur Agrégé De Philosophie, Docte 159 Pages. The recent remarkable developments of physical theories, lly those concerned with relativity and quanta of energy, cannot fail to har-reaching influences on philosophical thought. Professor Rougier's very wide reading in. mathematical and experimental physics has enabled him to present and interpret the new advances in physics in a way which should prove of great interest to both philosopher and physicist. The book marks a measurable advance toward a confluence of the broad streams of philosophical and scientific enquiry. Chemical Reactions And Their Equations The General Principles of Balancing Equations By INGO W. D. HACKн, Ph.C., A.B., Professor of Biochemistry, College of Physicians and Surgeons, San Francisco. 12mo, viii+138 Pages. Cloth, $1.75 Postpaid. A detailed study of this volume will enable anyone to balance any chemical equation rapidly and correctly. Introduction To General Chemistry. An Exposition of the Principles of Modern Chemistry By H. COPAUX, Professor of Mineral Chemistry, School of Industrial Physics and Chemistry, Paris. Translated by and with Appendix by HENRY LEFFMANN, A.M., M.D., Member American Chemical Society and of the British Society of Public Analysts. 30 Illustrations, 12mo, x + 195 Pages. Cloth, $2.00, Postpaid. A new presentation of the fundamental laws of the atom and of chemical affinity calculated to clarify all difficulties. P. Blakiston's Son & Co., Publishers, Philadelphia TOLMAN Theory of the Relativity of Motion With Appendix A supplement to this work, written by the author, has just been issued. Under the title, Relativity Theories in Physics this Appendix gives a general description of the nature of relativity theories, together with brief discussions of Tolman's theory of similitude or relativity of size, of Einstein's theory of the relativity of uniform motion, and of Einstein's theory of gravity and general relativity. ix +242 pages (including Appendix). Appendix only (paper), 25 cents. BERKELEY, CALIFORNIA In the New Spirit of Science Founded on the belief that the pupil should be thoroughly grounded in the fundamentals of physics, it makes the pupil alive to the practical, everyday value of the subject. It is based on the authors' "First Course in Physics," the most widely used textbook in the subject. Here is a book that is vitally interesting to the high school boy or girl, a book that arouses and maintains enthusiasm. 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