In every laboratory where electricity is used, there is need for variable resistances-and to meet such requirements we offer a well tried series of "JAGABI" SLIDING CONTACT TUBE RHEOSTATS Hundreds of them have been sold during the past three years, to various Government Departments, Universities, Colleges, Technical Schools, Industrial Companies, etc.-and have "made good" under every-day service conditions. WRITE FOR ILLUSTRATED DESCRIPTIVE BULLETIN 987 JAMES G. BIDDLE 1211-13 ARCH STREET, PHILADELPHIA Freas Constant Temperature Freas Tube Furnace, Supplied by all dealers in Laboratory apparatus Manufactured by The Thermo Electric Instrument Co. 8 Johnson Street DISABLED? Our repair department can assist you thru this period of cutting expenses and stretching reduced appropriations by putting any disabled instrument into good, accurate working order[at a very reasonable cost. The personnel of this organization has had wide experience with both foreign and domestic precision instruments, and our equipment is capable of the finest accuracy. And do not hesitate to consult us on that special instrument-physical or electrical-you may need for unusual investigation. Rubicon Company is particularly qualified, in men and machines for experimental and development work. RUBICON COMPANY 923 Walnut St. Newark, N. J. PHILADELPHIA WALLACE Photomicrographic and Spectrographic COLOR FILTERS The color filters illustrated here were devised by Dr. R. James Wallace, formerly PhotoPhysicist of the Yerkes Observatory and later Director of the Research Laboratory of the Cramer Dry Plate Company, primarily to enable workers in photomicrography to obtain an accurate photographic reproduction of objects, the details of which are rendered conspicuous to the eye by inherent or artificial color differentiation. By their use, the field of application of photography to bacteriology, histology, metallography and spectrography is greatly extended and the accuracy of conclusions drawn from a careful study of visual impressions, rendered permanent on the photographic plate, is increased beyond measure. The Wallace Color Filters consist of a set of ten gelatine coated glass plates two inches square, each bound with a protecting cover glass. The transmission values of these filters are the result of years of research work on the part of one who combines in a unique manner the viewpoints of both physicist and photographer, and the ranges are therefore selected to cover practically every possible combination of photographic plate and desirable or undesirable color value in the specimen to be photographed. WALLACE COLOR FILTERS, set of ten, as described above, in neat wooden case covered with imitation leather. Each plate is numbered for convenience in referring to the scale of transmission ranges on the inside of the cover..... WALLACE COLOR FILTERS, same as above but with gelatine film mounted on optical glass of first quality. These will be made up to order only.... .$12.00 .$24.00 Bulletin No. 95S, now in preparation, gives a complete explanation of the application of color filters to photomicrography and contains much information of value to those who are engaged in such work. It will be sent free upon request. CENTRAL SCIENTIFIC COMPANY 460 EAST OHIO STREET CHICAGO, U. S. A. NEW SPENCER ABBE REFRACTOMETER $200 Range of indices 1.30 to 1.71 The index scale is graduated in both directions to be read from either side. Readings are direct in Indices of the D line. The mirror slides on a horizontal bar which greatly facilitates obtaining light from various positions. Among other substances it is suitable for examination of waxes, sugar solutions, lard, oils, gums, balsams, varnish, etc. Description sent on request PALO COMPANY LABORATORY APPARATUS 153-157 WEST 23D STREET NEW YORK, N. Y. Hydrogen Ion Indicators SORENSEN'S LIST Sorensen in an extensive investigation of the correspondence between colorimetric and electrometric determinations of hydrogen ion concentrations revealed discrepancies which are attributed mainly to the influence of protein and salts. He chose those indicators which were relatively free from the socalled protein and salt errors. Sorensen's final selection, together with Pμ range of each indicator, is given below. By specifying National Stains and Indicators, you are assured of receiving dyestuffs manufactured under expert supervision and subjected, prior to sale, to the most rigid chemical and biological tests known. Catalogue upon request. National Stain and Reagent Co. THIRD EDITION-NOW READY AMERICAN MEN OF SCIENCE A BIOGRAPHICAL DIRECTORY EDITED BY J. McKEEN CATTELL AND DEAN R. BRIMHALL The third edition of the Directory contains about 9,600 sketches as compared with 4,000 in the first edition and 5,500 in the second edition. The work should be in the hands of all those who are directly or indirectly interested in scientific work. (1) Men of science will find it indispensable. It gives not only the names, addresses, scientific records and the like of their fellow workers, but also an invaluable summary of the research work of the country, completed and in progress. (2) Those interested in science, even though they may not be professionally engaged in research work, will find much of interest and value to them in the book. (3) Executives in institutions of learning and others brought into relations with scientific men will use the book constantly. (4) Editors of Newspapers and periodicals will find it to be one of the works of reference that they will need most frequently. (5) Libraries will find the book to be a necessary addition to their reference shelves. Price, Ten Dollars, Net, Postage Paid THE SCIENCE PRESS GARRISON, N. Y. LANCASTER, PA. READY JULY 10th Metallurgy of the Common Metals Fifth Edition, Revised and Enlarged By LEONARD S. AUSTIN Formerly Professor of Metallurgy and Ore-Dressing, HIS Fifth Edition has been largely rewritten and is in accord with presentday methods in the metallurgy of gold, silver, iron (to the point where pig-iron is obtained), copper, lead and zinc. Underlying principles are clearly set forth, and the details of methods and of metallurgical equipment are given. A chapter devoted to questions of the economic situation of the business of metallurgy is also included, dealing with the rapid advance in price and the consequent serious modification of the costs of operation, adds materially to the practical value of the book. AUSTIN'S "METALLURGY OF THE COMMON METALS" has 615 pages, 6 by 9 inches, and 178 figures. It is cloth-bound-$7.00 postpaid. Enter your order NOW, for a Free Examination copy when issued. JOHN WILEY & SONS, Inc. 432 Fourth Avenue NEW YORK |