FEBRUARY 18, 1944 SCIENCE-ADVERTISEMENTS 9

HEALTHFUL LIVING FOR NURSESBy HAROLD S. DIEHL, M.D., Professor of Preventive Medicine and Public Health and Deanof the Medical Sciences, and RuTH E. BOYNTON, M.D., Professor of Preventive Medicineand Public Health and Director of the Students'-Health Service, University of Minnesota.-531 pages, 54x 84. $2.50

The purpose of this new book is to provide a textbook covering the essential information in personaland community health that every nurse should have. Although many of the health problems of thenurse are the same as those of other students, the profession of nursing exposes the young womenwho enter it to specific health hazards not found outside the medical and nursing field. For thisreason, the authors approach the subject of healthful living from the point of view of the nurseand her special problems, such as the care of the feet, care of the skin and hair, and tropical andother communicable diseases.

TEXTBOOK OF HEALTHFUL LIVINGBY HAROLD S. DIFML, M.D. Second edition. 634 pages, 6 x 9. $2.50

This remarkably successful textbook has established an impressive record of distribution and use andhas won the enthusiastic endorsement of teachers and reviewers everywhere. Especially adaptedfor classroom use, the text contains many valuable pedagogical aids in the form of tables, charts,and carefully selected questions. Some of the topics covered are: major health problems; foods;weight and its control; dietary dangers; digestive disturbances; stimulants; exercise-fatigue-rest;specific disease prevention; normal sex life; mental health; etc.

ELEMENTS OF HEALTHFUL LIVINGBy HAROLD S. DIEHL, M.D. 315 pages, 6 x 9. $1.75

In this condensation of Textbook of Healthful Living the author answers the urgent need for a brief,vital text suited to the shorter courses in hygiene necessitated by wartime conditions. All materialand data have been revised in the light of the latest information, and much of the subject matterhas been rewritten in more compact form. In view of the special health requirements created bythe war, the sections on physical fitness have been revised and expanded. A new section on heredityhas also been included.

Send for copies on approval

330 West 42nd Street, New York 18, N. Y.

McGRAW-HILL BOOK COMPANY, INC.

FEBwARY 18 .1944y SCIENCE-ADVERTISEMENTS 9

Aldwych House, London, W.C.2.

10

THE WORLD'S LOW TEMPERATURERECORD

IN an address before the Pennsylvania State Collegechapter of the Society of the Sigma Xi, the first of aseries of such addresses throughout the nation, ProfessorPeter Debye, professor and chairman of the departmentof chemistry at Cornell University, stated that althoughthe world's low temperature record is now within a thou-sandth of a degree of the unattainable absolute cold,there is a good hope that it will be pushed still fartherdownward.

This will be done by "attacking the disorder hiddenin the nucleus of the atom " by use of the magneticproperties of the inner core of the atom instead of thecloud of electrons about it. The influence of a magneticfield upon the spinning electrons made possible the dropin temperature from about a degree to a mere fractionof a degree.

Cooling is explained as an approach to "a state ofhighest possible order" and at a degree above absolutezero the disorder connected with the motions of the atomsand molecules has been largely removed. The next step isto bring order within the nucleus of the atom in orderto get to an even lower temperature.The phenomenon of paramagnetism will be used in

these experiments, not yet performed, just as it was usedin pushing the temperature to its present low level. Whena paramagnetic substance, like a piece of soft iron, isdemagnetized by taking the magnetic field away from it,it absorbs heat.

This method of getting temperatures lower than areattainable by liquefaction of helium gas was proposedsixteen years ago by Professor Debye, then in Berlin,and independently by Dr. William F. Giauque, of theUniversity of California, and was applied a decade agoin several laboratories here and abroad.

In measuring temperatures just above absolute zero thelow temperature gas pressure thermometers used atslightly higher ranges can not be used, but a satisfactorytemperature scale can be based on magnetic measure-ments alone.

Absolute zero is minus 273.1 degrees Centigrade.Helium, the gas that is hardest to liquefy because itsmolecules have the smallest mutual attraction, boils at4.2 degrees above absolute zero and by dropping the pres-sure to 1/200,000th of an atmosphere, a temperature ofseven-tenths of a degree above absolute zero can beobtained. For lower temperatures, the magnetic methodmust be used.

HELICOPTERSHELICOPTERS are ideal craft for many of the Coast

Guard's regular peacetime tasks, is stated by LieutenantCommander F. A. Erickson, in an article in the U. S.Coast Guard Magazine. The uncanny ability of thesewhirligig flying-machines to slow down and " stand still"in the air, and to climb and descend at very steep anglesto blanket-sized landing spots, enables them to carry out

VoL. 99, No. 2564

such typical Coast Guard missions as close-up inspectionof suspicious-looking boats off shore, putting pilots aboardincoming vessels and removing them from out-bound ones,removing crews from stranded or foundering ships andfrom ice floes and other situations of peril, carrying linesaboard them from surface rescue vessels, and transferringsupplies and personnel to and from lighthouses, light-shipsand other isolated navigational aids.As evidence of the helicopter 's extreme maneuver-

ability, Commander Erickson lists the series of "impos-sible" stunts through which a heliocopter pilot studentis required to put his machine at twenty-five hours offlight training. A few of them are: Take off sidewiseand climb at a 45-degree angle. Come down the sameway, but stop and hover awhile before setting the shipdown; take off backward and fly tail-first. At 25 or 50feet altitude, while continuing flight in same direction,swing tail around and go on nose-first; from 200 feetaltitude, make 60-degree glide and land on marked areaapproximately 20 feet on a side; fly around the boundaryof a square, stopping in the air at each corner, withoutdeviating more than one foot, either vertically or hori-zontally, from true flight line.

FARM MACHINERYRUST-RESISTANT tillage tools, power-operated orchard-

pruning shears and saws, tractors with chassis for mount-ing various harvesting machines, are a few of the newform implements that should be developed for post-waruse, according to Professor E. G. McKibben of theMichigan State College in a report made to the AmericanSociety of Agricultural Engineers.Farm tillage tools and machines are constantly sur-

rounded by ideal rusting conditions and farmers arenotably careless in protecting them. Professor Me-Kibben suggested that corrosion-resistant metals devel-oped in recent years by metallurgists should replacethose now used. "If we really wish to keep tillage ele-ments from rusting it may be easier to change the char-acter of the tillage tool than that of the farmer.""In round figures there are well over 3,000,000 acres

of orchards, groves and vineyards in the United States,"he pointed out. "Three million acres of pruning repre-sents a lot of strong-arm work." Power-operated prun-ing shears and saws appear to Professor MeKibben to bea reasonable suggestion to eliminate some of this handpruning. "Field machines which would operate in theirown paths and turn in their own lengths would solve mostof the machine problems resulting from small, irregularfields and such soil conservation practices as contour farm-ing and strip cropping. " Machines that will operateboth in their own paths and to the side are needed forgrass-cutting in orchards and use in other places.The present draft tractor perhaps should be replaced

by a tractor which would be primarily a chassis formounting such machines as balers, forage harvesters,combines for harvesting small grains, and corn harvesters.

SCIENCE-SUPPLEMENT

SCIENCE NEWSScience Service, Washington, D. C.

FZBRUA3Y 18, 1944 SCIENCE-ADVERTISEMENTS 11

WILEY BOOKSIN MATHEMATICS

for your V-12 and AST courses

If you prefer a unified textbook covering algebra,analytic geometry, and trigonometry, we recommend-

A FIRST YEAR OF COLLEGE MATHEMATICSBy HENRY J. MILES, University of Illinois. For V-12 courses M1-2-3-4, and AST Basic Phase Curriculum courses 406-407. 607 pages;

5'A by 8¼2; $3.00

If you prefer individual textbooks, we recommend-For V-12 courses M1-2-3-4; for AST Basic Phase Curriculum courses 406-407:COLLEGE ALGEBRA

By H. P. PETTIT and P. - LUTEYN; both atMarquette University. Second Edition: 247 pages;

51 by 81; $1.90

COLLEGE ALGEBRABy LOUIS J. ROUSE, University of Michigan.Second Edition: 462 pages; 5j by 8; $2.25

ANALYTIC GEOMETRYBy EDWARD S. SMITH, MEYER SALKOVER,and HOWARD K. JUSTICE; all at the Universityof Ci'ncinnati. 298 pages; 6 by 9; $2.50

TRIGONOMETRYBy HOWARD K. HUGHES and GLEN T.MILLER; both at Purdue University. Withtables: 268 pages; 51,by 8X; $2.00

PLANE TRIGONOMETRYWITH TABLES

By HARVEY A. SIMMONS, Northwestern Uni-

versity, and GREENVILLE D. GORE, CentralY.M.C.A. College. 282 pages; 5j by 81; $2.00

For V-12 courses M5-6-7; for AST Basic Phase Curriculum course 408:

CALCULUSBy EDWARD S. SMITH, MEYER SALKOVER,and HOWARD K. JUSTICE. 558 pages; 6 by 9;$3.25

CALCULUSBy FREDERIC H. MILLER, The Cooper UnionSchool of Engineering. 419 pages; 6 by 9; $3.00

CALCU LUSBy HENRY B. PHILLIPS, Massachusetts Insti-tute of Technology. 353 pages; 5 by 7*; $3.00

DIFFERENTIAL EQUATIONSBy HARRY W. REDDICK, The Cooper UnionSchool of Engineering. 245 pages; 51 by 81; $2.50

DIFFERENTIAL EQUATIONSBy HENRY B. PHILLIPS. Third Edition: 125pages; 5 by 71; $1.75

JOHN WILEY & SONS, INC., 440-4th Ave., New York 16, N. Y.

-wI----~~~~~~~~~~~~~~~~~~~~~~i1

F-WRUARY 187 1944 SCIENCE-ADVERTISEMENTS 11

voTi, 99, No. 2564SCIENCE-SUPPLEMENT

Many of these present self-propelled machines are in useonly a relatively small number of days in a year. Re-liable light gasoline engines, equipment to give betterweed control, precision planters for sugar-beet and othercrops, conveying and elevating equipment, potato har-vesters that will separate the potatoes from stones andclods, and "once-a-season lubrication" for importantfarm machines-all of these are among other new farmmachine developments needed.

BOLIVIAN TINBOLViA 's tin is a number one war essential to the

United States and the United Nations generally. Theoverturn of the Bolivian government should not decreasethe supply of tin ore, since Bolivia recently declared waron Germany, but directly or indirectly it may.

Bolivia is primarily a mining country. In pre-war daysit produced about one-sixth of the annual world's supplyof tin ore. It produced many other metals; but tinbecame the all-important metal after sources in BritishMalaya, the East Netherlands, Thailand, China and otherplaces in the Far East were cut 'off by the Japs. Thelarge tin smelter constructed as a war measure in Texasby the United States government uses Bolivian ore.Bolivia has no smelters of its own.

Bolivia, located at high elevation on the Andean pla-teau, has no seaport. Between it and the Pacific Oceanlie Peru and Chile. On its north and east is Brazil, onits southeast and south are Paraguay and Argentina.Some of its drainage is into the Paraguay River (calledParana south of the Paraguayan border) but most of itis by tributaries of the Amazon.

The principal railroad of Bolivia connects its capital,La Paz, which is near the Peruvian boundary, with Aricaon the coast of Chile. The road, from 250 to 300 milesin length, runs southwesterly from La Paz without touch-ing Peru. About one third of its mileage is in Chileanterritory. Nearly 100 miles of Chile separates Boliviafrom the Pacific.

In size Bolivia is nearly as large as Arkansas, Louisi-ana, Oklahoma and Texas combined. It contains 416,000square miles, less than 60 per cent. of its area in 1900.Its population in 1940 was approximately 3,500,000, threefourths of which was Indian or mixed bloods.

Bolivia and Paraguay are the only South Americancountries without seacoasts. Paraguay has river trans-portation down the Parana to the La Plata and the SouthAtlantic. Bolivia's desire for a little of northern Chile,or a little of southern Peru, has been the cause of interiortrouble in the past and is reported to be one of the fac-tors in the present situation. Wages for miners is an-other source of dissatisfaction and caused conflicts in1942 between the miners and the government resultingin bloodshed.One result of this confliet was an investigation by a

special United States commission, as interruption to mill-ing would make impossible the delivery of tin andtungsten ore for which the United States had previouslyentered into contract with Bolivia. Under this contractthe United States agreed to buy 18,000 tons of tin annu-

ally for five years, and 3,000 tons of tungsten for threeyears. In July, 1942, the American government agreedto increase the amount of both tin and tungsten to bepurchased. Also it loaned Bolivia $25,000,000 for theconstruction of four major highways, the establishmentof a national sugar industry, the stimulation of petroleumproduction, and other economic purposes.

Tin ore constitutes over 75 per cent. of Bolivia's exporttrade. Tungsten, antimony, copper, silver, lead, zinc andbismuth are other important mining products. Prior tothe war Bolivia was producing approximately 240,000barrels of petroleum annually. In 1942 the Bolivian gov-ernment agreed to pay the Standard Oil Company of NewJersey $1,750,000 for oil property expropriated in 1937.

ITEMSA MEDAL and a prize of $200 will be awarded each year

by the newspaper El Universal for the best doctoral thesisin a Mexican university. This Justo Sierra prize is namedafter the Mexican educator and philosopher who re-established at the turn of the century the University ofMexico after it had been decentralized for a century.

NINETY per cent. of all dates grown in the UnitedStates are produced in the Coachella Valley in Cali-fornia. The date crop this year amounts to nearly 20,-000,000 pounds, worth about $5,000,000, according to Dr.Walter T. Swingle, who had much to do with the in-troduction of date culture in this country. Date grow-ing on a major scale was begun in the Coachella Valley,which lies north of Imperial Valley, about the turn of thepresent century, and is now the principal industry of theregion. Irrigation water comes from deep wells. Culldates, unmarketable for food purposes, are fermentedfor the production of industrial alcohol.

A RECORD-BREAKING electric motor, which operates at120,000 revolutions a minute, has been developed, builtand thoroughly tested by the General Electric Company.This new three-horsepower motor weighs only sevenpounds and is so small that it will fit into the palm of aman's hand. The motor is water-cooled and is equippedwith oil-mist lubricating bearings. Tests, including acontinous run of eight hours, are said to have proved itsperfection. The ordinary three-horsepower electric motorweighs 105 pounds. The speed of rotation of this newmotor is 65 times faster than the conventional motorused in home washing machines and refrigerators. Motorsof this type will be used for the grinding and drillingof essential parts of airplanes and other war equipmentand will not be available for general use until after thewar.

Through an editorial slip on a Science Service type-writer Dr. C. A. Browne was credited with the statementthat Humboldt "at the beginning of the nineteenth cen-

tury set Mexico at the head of all Western Hemispherenations in cultural attainment. " (Science News, Jan. 21,p. 12.) The article should have stated that Humboldtplaced Mexico at the head of all Latin American countries.

12

FEBRUARY 18, 1944 SCIENCE-ADVERTISEMENTS

Chemistry__

Organic Chemistry* By G. Albert Hill, Wesleyan University, and

Louise Kelley, Goucher CollegeThis text offers material for a rigorous full year 's college course. Helpful ques-

tions and well selected problems are given at the end of each chapter. Reflectingvaluable experience in teaching organic chemistry to beginning college students,the text is highly praised by teachers for its clarity of expression and logicalarrangement. 919 Pages. $4.00 (1943)A Teacher says, "I am especially impressed by the simple and direct treatment oftheoretical aspects."

Introductory Organic ChemistryBy E. Wertheim, University of Arkansas

This is a text for the short course which presents the material in a concise manner

stressing vital concepts. It will ground the student in such fundamentals as anappreciation of the significance of the graphic formula, the importance of func-tional groups in the reactions of compounds, and relationships of simple compoundsto each other. Summaries, review questions and charts are provided. 82 Illus.,482 Pages. $3.00 (1942)

A Teacher says, "The material is logically arranged and well selected . . . it canbe covered in a six-hour course . . . the questions are outstanding in interest."

Experiments in Organic ChemistryBy E. Wertheim, University of Arkansas

Designed to accompany "Introductory Organic Chemistry," this manual offers awide selection of experiments in elementary organic chemistry, covering both "prep-arations " and "properties." 34 Illus., 221 Pages. $1.35 (1942)

A Teacher says, "We are using this manual with considerable success. Organiza-tion of the material is splendid."

Fundamentals of Physical Chemistry2nd Ed.

* By Earl C. H. Davies, West Virginia UniversityThis text provides the necessary background for applications of physical chemistryto biological problems. It is being successfully used for a one-semester, nonmathe-matical course. 86 Illus., 447 Pages. $3.50 (1940)

A Teacher says, " It presents a good selection of topics consistent with the interestsof students for whom it was written."

Philadelphia 5, Pa.

L ~THE BLAKISTON COMPANY

13

14 SCIENCE-ADVERTISEMENTS

flflT(RIfI mEDial,TOXICOLOGY &G

*DAVISON'S NEW3YdEDITIONAlthough the first edition of this favorite appeared only four years ago, alreadyit is in its third edition-just off the press. Revision has been thorough, in keep-ing with recent developments in the field.

The section on Toxicology has been greatly expanded. Material has been addedon drug eruptions, sources of acute poisoning, common poisons, chemical warfareagents; a scheme of chemical analysis has been incorporated.

Vitamins, in the section on Biologicals, have been given a great deal of attention.Their presentation has been rearranged on the basis of medium of solubility. Newvitamins discussed include choline, biotin, inositol, para-amino benzoic acid, citrin.Penicillin is discussed under microbiotic agents. Blood and blood substitutes arecovered in detail.

The section on Sulfonamides has been completely revised. These drugs are firstdiscussed generally as to their pharmacologic action and properties. Then theindividual compounds are taken up. A plan for assay is given and use for localsepsis is presented in detail.

SYNOPSIS OF MATERIA MEDICA, TOXICOLOGY AND PHARMACOLOGYby Forrest Ramon Davison, B.A., M.Sc., Ph.D., M.B., Medical Department,

The Upjohn Company, Kalamazoo, Michigan.

766 pages, 40 illustrations, 4 in color, 34 tables. $6.50

The C. V. Mosby Company Sci. 2/443525 Pine Blvd.

Use Coupon St. Louis, Mo.Gentlemen: Send me the new 3rd edition of Davison's

"SYNOPSIS OF MATERIA MEDICA, TOXICOLOGY ANDTo Order PHARMACOLOGY," price, $6.50

D Attached is my check. Q Charge my account.

Today! | Dr...............................................................Address ..................................................................................................................................................................................................................

VOLD. 99, No. 256414 SCIENCE-ADVERTISEMENTS

FERUR 18 194SINEAVETSMN1

- BORZOI BOOKS

Two Superb Reports

On the Amazing Conquests of Modern Science

by Williams HaynesAuthor of This Chemical Age

A clear and thrilling account of thespectacular role that chemistry is play-ing in our modern technological war.Mr. Haynes writes dramatically of themany new materials, munitions, drugs-of the men who discovered them,how they are made and used, whatthey promise.

"He has first-hand knowledge both ofchemistry and industry.... He knowshow to tell a story dramatically....He ranges over the whole field ofchemistry as it is applied at present."-WALDEMAR KAEMPFFERT, New YorkTimes. "Interesting, informative,stimulating."-HARRY M. BEARDSLEY,Chicago News.

Illustrated with charts and platesin full color. $3.oo

by Rogers D. RuskProfessor of Physics and Chairman ofthe Department, Mt. Holyoke College

A challenging and revealing book in

which the momentous discoveries and

revolutionary ideas of modern phys-ics are brilliantly surveyed and in-

terpreted.

"He has succeeded m'agnificently in

this broad and sweeping survey. He

has given us a story of the develop-ment of science that is as fascinatingas a novel."-Natural History Maga-

zine. "Physicists may safely recom-

mend it to their friends in otherprofessions."-E. U. CONDON, West-

inghouse Laboratories.

With 28 illustrations * $3.50

ALFRED * A * KNOPF * 501 Madison Ave. * N. Y. 22 G- - - AL o . --~ a

PzBp.uAp.y 18, 1944 SCIENCE-ADVERTISEMENTS 15

T'..2;..~ ~~ ^m

AM01-M m W1 01 xi a &L.m 4% 5 a' 0

qi

.. A~ ENVE~

sq ]KIVIT

,d~. 1.

11%.; 1 0.

16 SCIENCE-ADVERTISEMENTA9

School administrators In all the states from Maine toCalifornia Inclusive are requesting that we recommendteachers to them for their teaching staffs. We are in-terested In further registration of teachers both ex-perienced and inexperienced, who have in mind continuedadvancement In the teaching field.

AMERICAN COLLEGE BUREAU28 E. Jackson Blvd. Chlcago, Iiirn

SPRAGUE - DAWLEY, INC.P~iee4~ dif "w 4 4 4aoda2d I Aa

Madison, Wisconsin

Philadelphia, Pa.

A Pioneer in Small PumpsTHE MILLER WATER CIRCULATORA complete electrically driven unit

weighing 2.8 lbs.Carl D. Miller

327 Salem St. North Andover, Mass.

RESEARCH CHEMICALSBile acids, hormones, and special bio-

chemical products are available for re-search purposes. Send for our latest pricelist.

George A. Breon & CompanySynthetic Chemicals Division

Box 769, Kansas City 10, Mo.

Exposure Determinationin Photomicrography

by direct light measurement in plane of image

PHOTOVOLT Electronic Photometer MOD. 511A highly~ans~tsve light meterwith phototube and amplifier

Also suitable for spectrophotometry, densitometryof spectrographs, micro-colorimetry, measurementof luminiescence and fluorescence, ultraviolet emis-sion and absorption. Obtainable with phototubessensitive In the ultraviolet visible, infra-red.

PHOTOVOILT CORP.95 Madison Ave. New York 16, N. Y.

No. 10 Analytical Balance

HENRY TROEMNERThe Standard of Excellence for 10* years.

Manufacturer of the highest grade Analytical,Assay and Pulp Balamce and Weights of Pre-cision.

Catalog 1929S

911 Arch Street

I

16 SCIENCE-ADVERTISEMENTS Voi,,. 99, No. 25,64

FEBRUARY 18, 1944 SCIENCE-ADVERTISEMENTS 17

BRINGS YOU THIS

\FREE 500Kt1...

Fifty pages are devoted to a simple as well as atechnical explanation of pH control; precautionsto be observed in making determinations; and dis-cussion of the application of pH and chlorine con-trol to 34 different fields. The remaining 33 pagescontain descriptions of Taylor Slide Comparatorsfor pH, chlorine and phosphate control and forwater analysis.It provides information which cannot be obtainedelsewhere except by reference to many text booksand original articles. You are sure to profit bykeeping it on your desk.Write for This Book Today- IT'S FREE!

_ TAYLOR SLIDE COMPARATORSA Quick, easy to operate, absolutely accurate.

Base and slides are made of durable plastic.All Taylor Liquid Color Standards carry an

unlimited guarantee against fading.See your dealer

I1

I

FOR LATEST AUTHORITATIVEREVIEWS ON LABORATORY

PROCEDURES

Subscribe toI

A monthly publication corn-piled for laboratory direc. -tors, technicians and physi-cians. Full of essential factsand reviews of current literature on labora-tory procedure, professionally edited by R.B. H. Gradwohl, M. D., Sc. D. A neces-sity in. the modern laboratory. Published by

L SCHOOL OFLABORATORYGRADWOH ~TECHNIQUE

3514 Lucas Ave St. Louis, Mo.

Zenker Fixed Tissues Reported Routinely Within 24 Hours From the Time the FreshTissues Are Received In the Laboratory.

METHOD*Fixation Zenkers Fluid 4 hourwashing Water 1 hourDehydration 70 per cent Alcohol 1/ hour

80 per cent Alcohol 1/ hour95 per cent Alcohol 1 hour95 per cent Alcohol 1 hour100 per cent Alcohol 1 hour100 per cent Alcohol 1 hour

Clearing Chloroform or Xylol 1 hourChl. or Xylol and Paraffin 1 hour

Impregnation Paraffin No. 1 2 hoursParaffin No. 2 2 hours

Pormalin can be substituted for Zenker flwation by eliminating the washing and adding two hours to the floationperi od.The above method for preparing tissues for sectioning is now used extensively. The whole process isaccomplished by means of the AUTOTECIDICON from 5 P.M. to 9 A.M daily.* A complete booklet of methods for preparing tisses for sectioning as used in pathological laboratoriei onrequest.

'e TECHNICON CO.NEW YORK - * N. Y.

1.

ISCIENCE-ADVERTISEMENTS 17FEBRUARY 18, 1944

18 SCIENCE-ADVERTISEMENTS VOL. 99, No.2564

Spencer Stereoscopic Microscope in use at a Western Naval Air Station.

0 . . and all returned safelyEnemy anti-aircraft shells and fighter

planes are not the only hazards toAmerican Airmen. Successful operationsdepend also on the mechanical perfec-tion of the planes.Fatigue and stresses caused by millions

of revolutions in service sometimes pro-duce dangerous internal weaknesses inairplane propellers. Periodically, there-fore, every propeller is subjected totests and inspection. The surface is etch-ed with acid which reveals any minutecracks or other evidences of internalstrain.

Stereoscopic Microscopes are used forexamining these telltale clues-anotherexample of how microscopes aid inmaintaining the health and safety ofour fighting men.

Spencer LENS COMPANYBUFFALO, NEW YORK

SCIENTIFIC INSTRUMENT DIVISION OFAMERICAN OPTICAL COMPANY

18 SCIENCE-ADVERTISEMENTS VOL. ^9 NO. 2564