-you are delivering an informal talk, say, on your travels; you Bausch Lomb BALOPTICON THE PERFECT STEREOPTICON This instrument projects opaque objects, including photographs, magazine il- Bausch & Lomb Optical @. Leading American Makers of Photographic Lenses, Microscopes, Projection Apparatus Highest Grade of Laboratory Apparatus and Supplies for all Laboratories Agriculture, Biology, Physics, Chemistry, Physiography, Physical Geography CATALOGS SENT UPON REQUEST D-Diplomas for Grade Schools, High 4-Loose Leaf Notebooks, Papers, Herbariums, Manuals, etc. 5C-General School Supplies. L-Lantern Slides, Microscopic slides and Balopticons. DC-Electric Meters, direct or alternating current. W-Wireless Apparatus and Supplies. Lists of apparatus for state requirements and for text books. Ask for list for your state or for your text. Manuals, on physics, physical geography and agriculture. Samples sent for examination upon request. A Sign of Quality We are pleased to announce quantity production and a consequent price reduction in Welch alternating current meters. Some of the special features and characteristics of the new Welch Alternating Current Meters are mentioned below. Complete bulletin describing in detail the superior qualities of these instruments, will be sent upon request. The Case is dust tight and handsomely finished. The Pointer is of light aluminum tubing. The Zero Corrector is provided for setting the pointer. Dead Beat action is obtained by carefully constructed damping device. The Accuracy is guaranteed to one half of one percent of full scale. The Scale is almost uniform, about 90 percent of the scale is large enough so that it may be guaranteed in full. Write for full description, list of ranges in stock and price. OUR POLICY To attain the highest possible standard of service is the purpose which dominates all other considerations in the designing and building of "WELCH" Laboratory Apparatus and Supplies. WELCH A Mark of Service SERVICE W. M. WELCH SCIENTIFIC COMPANY Manufacturers, Importers and Exporters of 1516 Orleans Street CHICAGO, ILL. A Portable Potentiometer for H-ion Determinations The Portable H-Ion Potentiometer is not only useful in laboratory work, but it will generally find a place in plant and field surveys, and wherever portability in such a set is a desirable feature. Standard cell, galvanometer, adjusting rheostat-all are enclosed in a substantial oak case 6" x 6" x 91⁄2", provided with a carrying strap. All parts are ruggedly constructed to withstand hard usage. The usual L. & N. guarantee applies to this potentiometer, as it does to all of our instruments. There are two models. Both are exactly alike in size and external appearance, and both have the same range. No. 7655 has a main dial and slide wire, and measures accurately to±0.5 millivolt (1% of a pH unit). No. 7656, with slide wire only, is accurate within ±5 millivolts (10% of a pH unit). PRICES ..... 7655 L. & N. Portable Hydrogen Ion Potentiometer, range o to 1200 millivolts, accuracy ±0.5 millivolt, sensitivity ample in nearly all solutions....... .$150.00 7656 L. & N. Portable Hydrogen Ion Potentiometer, as illustrated, range o to 1200 millivolts (or any other desired range), INSTRUCTIONS FOR OPERATING ALEX. D. MacGILLIVRAY Ph.D. Prof. of Systematic Entomology in the University of Illinois Chapters de devoted to near relatives of Coccids, Prep aration, External Anatomy, Classification, and to each The Microscope of the Subfamilies and Tribes, Twenty-seven in all. 8vo. Cloth. IX+502. $6.00 net. Sent prepaid upon receipt of price. Order direct from publisher SCARAB COMPANY 603 Michigan Ave. By SIMON H. GAGE of Cornell University 13th Edition, Published December, 1920 in this edition, special emphasis is put upon the Dark-Field Microscope Postpaid $3.00 Urbana, Illinois COMSTOCK PUBLISHING CO., Ithaca, N. Y. Made in wide range of frequency up to 5000 vibrations. Twenty-five years of experience have placed us in position to guarantee absolutely accurate rating. In rating our Forks we use a standard supplied by Dr. D. C. Miller of the Case School of Applied Science, Cleveland, Ohio. Our electrically maintained Forks are well known for their convenient mountings permitting of great range of adaptability. WM. GAERTNER & CO. 5345-49 Lake Park Ave., Chicago, Ill. 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. HIGH VACUUM PUMPS Whenever in lecture-room or laboratory practice, really high vacuum is required, we recommend the Condensation Pump developed by Dr. Irving Langmuir of the General Electric Companybelieving it to be superior to any other as yet obtainable. The Langmuir Pump operates with surprising rapidity, and there seems to be no practical limit to the degree of exhaustion that can be produced. Some form of auxiliary pump must be used; and for that purpose we offer a special G. E. two-stage oil-sealed mechanical pump, which is capable of producing a vacuum of 0.001 mm. when used alone. The picture shows a complete outfit comprising Langmuir Condensation Pump, two-stage auxiliary pump and 1⁄2 H.P. motor-all mounted together on one base. Write for descriptive bulletin S-965 JAMES G. BIDDLE 1211-13 Arch St. Philadelphia |