1885
V O L U M E 2 3 N O . 12, D E C E M B E R 1 9 5 1
N E W BOOKS Union List of Microfilms. Iievisc~tl, enlarged, and cumulated ~ ~ ( l i ~ i t ~ tl i ~ , l ~ i l ~ ~ ~ 13il)liogmphical l ~ ~ l ~ ~ h i a Center and lTriiori 1.ilii.ai.y ( ' i i t a 1 0 ~ . $107 pagw. J. W,Edv-ards, 300 .John St., . \ n n .\rt)or. .\Iic.ti. SIi.50. ASTM Standards on Textile Materials (with Related InforStatistical Methods for Chemists. IV. J . l 7 o i d e , i . x 13; mation 1. $'pcic.ific,at ions, Tolerances, Methodr of Testing, 1-oi.k 16, pages. John TVilr!- k Sons. 440 Fourth .\vta., SP\\I>diriitions, anti Terms. AiSTXCommittee D-13 on Trxtilr h[atcrial*. 59; pages. .%merican Socic,t!, for, Tosting MaT.Y.; 1951. Price. $3.00. terials, 1016 Race st.,Philadelphia 3, P a . , 1931. Youden has g a t h e r 4 togethei, i i i this book +onit' of t l i t . niow Symposium on Methods of Measuring Viscosity a t High Rates of Shear. Presented to API Conimitt,w on Petroleum Produs(~Fultricks in his eapac.iouF ling ivhich he has g:trrierc~l over thtx uct,s, Chicago, Ill., Sovember 7, 1949, and to Research j ~ r i20 years working ivith statistical method5 iir vhttniist ry. Division T'II on Flow Properties, A4STlf Comniit'tee D-2 on Thv reader is led by emy and simplifitd argument? r t l ail apprePetroleum Products and Lubricants, Washington, D. C., cixtion of the power of the statistical approach ttr expi~riiiiviital Februarv 24, 1950. Special Technical Publicat'ion 111. .imerican Societ,y for Twting Materials, 1916 R a w St.., W I I I , ~ , and shown hou. 10 perloriii the kinds of cal(wlatioi~s (very 19.51. 47 pagtv. $1.3;i. Philatle1ptii:i 3. h,, rht~niistshould be ah)r t o ma kc^ in analyzing his o ~ vr ~ xi p ( ~ r i i r i t ~ n t i t 1 clara. Thc unique importaiic.1. of iiitwartions bet\\--tw vaiklil(.s in clieinical phenomena is recognized lor the first tinic, i n aiiy put)l i s t i d book and a lucid dincussion is given of its c ~ o r i s r q u r ~ i r ~ e ~ - In the revirn of "Stantlard Nethods for Testing Tar and I t s both in the analysix of csperiniental result? arid i r i the, r,cquirtAProducts" [ANAL.( ' H m r . , 23, 1343 (1951)) the price of t h e hook nients for planned experiments. should have been given as Sl lo?., postage 10d. extra. The section on deaign of experiments is brief arid tends t o follow the formalized methods developed in Britain I-JY agricultural workers rather than t,he methods found most usrful i i i recent T h t s followiiig corrections should be made in the book .'Reyt'ars by chemical experimenters. agent Chrmirala, .%nierican Chemical Society Specification*, This book should serve cthcmists as a most useful introduction 1%?0" [.ks.ii,. CHEX., 23, 1841 (1951)l: to the philosophy anti techniques of statistical met hods. The Pxge 16. Stanclarti Solutiori~for Control. Chloride. \\-eigh serious experimenter ~ i l need l to go farther in order t'o attain t h e 0.16.i grani of sodium vhlorid~,NaCl. f u l l benefits which modrrn stat.iaticd t w l i n i q u ~ scan produce. Page %% ('d(*iuni ('arbonitte, Low in Alkalies. (:tiloride \v. I.. (;ORE: t w . t h e s t w i l d a c ~ n t c ~ n rPhoultl r read: "To 10 ml. of the fi1tr;it)e."
+
Corrections
Symposium on Electron Physics t:. A . 11.1>131, General .Aniline & Film Corp., Easton, I'u. 7
~1118eymposiunl, probably the first of it,s kind, was held at
1 the National Bureau of Standards on Sovember 5 , 6, and 7 ,
IDR1. It was spon*ored by the National Bureau of Standarde a3 one of twelve sympoaia designed to celebrate its fiftieth anniversary. The physics of the free electron, involving electron scatteriiig, electron optics, and electron microscopy, constituted the innin theme. The generous cooperation of the Office of Kava1 Research made possible the participation of many dist,inguished foreign scientists. Of t,he approximately 100 attendants, some 20 represented countriw abroad. The symposium was particularly stimulating because Iiiuch of the discussion was contributed by j)hysicists who are largely responsible for the fundamental aspwts of research on the electron. Most of the papers were highly throretiral, but several new practical applications proved t o be c st reniely interesting. It is not the purpose of thi* review to describe each paper per se'hut instead t o highlight the meeting as a whole, keeping iri m i r i d the interests of the readers of this journal. The classical methods of geoinetncal electron optice are giving \\-:iy to the newer phyeicd or wave-mechanical approaches. The h s i c problems of paraxial image formation derived from the Svhroedinger equation, the definition and significance of focal length, and the derivation of a formula describing the limit. of resolut,ion were outlined by W. Glaser, Technische Hochschule. Vienna, Austria, using wave-mechanical theory. .4 new limit of resolution, 4 A., was derked from this theory for a 60-kv. electron microscope. A new e~periment~al technique, microanalysis by means of an electron probe, KH,E described by R. Castaing, Office r a t i o n a l
d'kltudes et de Heeherches Aeronautique\. C'liatilloii, France. This novel and potentially useful procedure involves H Geiger counter spectrometer analysis of the soft x-rays emitted from the sample irradiated by means of a concentrated electron beam. Elenients identifiable must have an atomic number greater than 12, and the qwantitative analysis is limited to about 1yo. Thc resolving power of the method, or the minimum particle dianieter, is about 2 microns. Metallic diffusions and inorganic. precipitates have been studied to date. A review of some 30 years of experimentation on the scattering of electrons by metallic. nurlei, electron-electron scattering, and positron-electron scattering was given by W. Hothe, University of Heidelberg arid llas-Planck Institute for SIedical Research. The intensity of the Lack-scattering of electrone as a function of atomic number was demonstrated quantitatively. Cloud chamber photographs of electron-electron and positron-electron collisions were illustrated. The range of electrons in metals as a function of energy and atomic number, respectively, was discussed by Jerome Fleeman, National Bureau of Standards. A cloud chamber ionization meter behind windows of various metals of varying thickness was used t o determine the necessary data. The extrapolated value of zero ionization current was used to derive an empirical equation relating electron range, electron energy, and atomic number. Use of t h e electron microscope in determining atomic crosssection scattering was outlined by C. E. Hall, Xlassachusetts Institvte of Technology. An empirical set of formulas was demonstrated in which t h e electron transmissivity of various
1886 thicknesses of thin films was used. blicrophotometer tracings of the electron microscope film edge image were used to calculate the transmissivities. By a somewhat similar token, S. G. Ellis, RCA baboratories, Princeton, 1;.J., discussed the limits imposed by plural scattering in electron diffraction. The contrast (ring contrast-background contrast) with a voltage and specimen thickness dependence was described for experiments on aluminum and thallium chloride. Experimental verification of an empirical theory showed that this contrast suffers with increasing specimen thickness when using voltages up to 150 kv. The use of electron scattering from gaseous systems in collaboration with spectroscopic data for determining vibration frequencies and interatomic force models was illustrated by J. Karle, U. S. Naval Research Laboratory, Washington, D. C. XeIv typw of microscopes and present trends in electron microscopy were thoroughly discussed by several speakers from allroad. The driving iorres in this type of research are many, but two appear to be the most powerful: the improvement in resolution down to atomic dimensions and the ability to study living subjects. V. E. Cosslett, Cavendish Laboratory, University of Cambridge, outlined present trends in regards to both these aspects. The elimination of spherical aberration, a serious limiting fact,or in resolution, has been considered by Scherzer, Gabor, XIarton, d al. In general, aberrations are deliberately introduced eo as to cancel the inadvertent aberrations normally encountered. Scherzer’s split field lens utilizing eight radial electrodes, Eonie of which are positive, introduces a counterbalancing astigmatic~ focu9. Gabor’s helical lens is primarily designed for eliminating chromatic aberration. To date these attempts have not proved to be practical. However, ii combination of optics has had some success. For esample, Gabor’s reconstructed wave-front diffraction microscopy has been verified espeiimentally and improved, notably by Dyson and IIaine. The spherical aberration present in the electron diffraction pattern is eliminated during the reconstruction with light optics. D. Gabor, Imperial College, London, has recently directed his attention to the elimination of the long exposure times required for preparing the “hologram,” or electron diffraction p t t k r n . This is important because unlike the case for classical electron microscopy, resolution in diffraction microscopy is seriously impaired by the exposure time. Resolutions of 1 to 2 A. are theoretically possible. However, the phase contrast due to diffraction effects will undoubtedly be the limiting factor. It develop paradosically that once the instrumental difficulties for resolving structure on the order of several Angstroms have been overcome, the proper specimens may not be unavailable. .%sa sidelight, Gabor also described the use of a polished graphitc surface as an electron mirror. It was surprising to learn that this surface reflects almost, quantitatively electrons in the 2- to 6kv. range. Apparently these electrons penetrate only up to about 100 atoms’ depth. The second potent driving force in presenbday t,hinking, mentioned earlier as the desire to study living biological specimens, has resulted in the developnient in England of a point x-ray focus shadow micmscope. V. E. Cosslett described these recent experiments, perhaps first carried out by von Ardenne in Germany some 12 years ago. The important aspect is the ability t o use anesthetized biological specimens a t atmospheric pressure and in normal fluid environment. Exposure times of about 1 minute with 1-micron resolution are practical. Variations of the electronic system (operating in vacuum) consisting of the electron gun and focusing mgnetic lens may be employed, depending on the desired intensity of the focused electron spot on the x-ray forming target (metal windox). Relatively thick (25 microns) specimens must be used for purposes of contrast; 10-kv. x-rays are normally used. The iutensity output of the electron gun, which is focused on the x-ray tube target, will probably limit the resolution. hlong similar lines, L. llarton, Kational Bureau of Standards,
ANALYTICAL CHEMISTRY is applying field electron emitters instead of the usual thermionic gun in order to increase the electron intensity focused on the x-ray target. The resultant higher x-ray intensity promises to permit 0.01-second exposures with resolutions on the order of 300 A. RI. E . Haine, Associated Electrical Industries, Aldermaston, Berkshire, England, discussed recent investigat,ions on high voltage electron guns, primarily of the biaped type. Filament temperature, current density, accelerating voltage, and geometry Jvere considered theoreticnlly and experimentally. Biased guns were shown to be operable so t h a t the observed brightness (final vien-ing screen) was equal t o the theoretical brightness. Geometrical aberrations and space charge effects are apparently unimportant. The advantage of the biased gun over the unbiased gun lies in the greater localization of the thermionic emission point. Two other topics of discussion uf interest to electron niicroscopists were specimen contrrmination and interference plienomena in the images of platelike crystals. The first aspect was discussed by S. G. Ellis, who postulated that surface migration of hydrocarbons which presumably decompose might be an esplanation. M. E. Haine, on the other hand, described evidence to suggest that condensation from the vapor state was responsible for specimen contamination, because the effects are lenoticeable as the temperature of the specimen rises. In any case, finely divided metals on the specimen or in the column are undesirable. The second aspect was illustrated by James Hillier, RCA Laboratories, Princeton, N. J. Electron micrographs of thin overlapping platelike crystals of clays occasionally exhibit two differently spaced interference fringes. The larger, high contrast spacings may result from overlapping crystals with an air wedge in between so as to permit Bragg reflections. The close, low contrast spacings are more difficult to explain, although a Moire pattern due to overlapping, parallel, but crystallographically rotated crystals was offered as an explanation. Theoretical aspects of image formation in the electron niicroscope were discussed by H. Boersch, Physikalisch-Technische Bundenstalt, Braunschweig, Germany. He presented a statistical analysis showing why the contrast of objects of size comparable with the resolution limit may be inadequate to permit their resolution. This analysis depends upon the small photographic density changes perceptible t o the eye, and the correlation of these density changes with the ratio of clectrons diffracted by the object to the total number of electruns representing the background. The elimination of chromatic a.berration, a serious factor in the resolution of thick objects, by the use of so-called electron filters was also described by Hoersch. These filters are really totally reflecting mirrors consisting of highly negatively charged plates below the object which reflect back those inelastically scattered electrons which, because of their wide angle of scattering, would only serve to lower the object contrast in the final image. E. Ruska, Siemens and Halske A.G., Berlin-Siemansstadt, Gerniany, who as a student of M. Knoll (now a t Princeton UniverFity) helped to build the firat magnetic electron microscope some 20 years ago, described his electron optical bench and recent German developments in permanent magnet lenses. A variety of pole piece designs with their specific advantages from the standpoint of lens defects were illustrated. The very recent and exceedingly interesting point projection niicroscopes were discussed by E. W. Muller, Kaiser-Wilhelni Institut, Berlin-Dahlem, Germany. Furthermore, he actually demonstrated a field electron microscope. This instrument having a circular 2-inch screen equipped with a glass projection lens >vasused to demonstrate the resolution and thermal migration of single barium atoms. I t was undoubtedly the first time that most members of the audience had seen single metallic atomp. Fortunately, the tungsten tip onto which the specimen atoms or moleculeE are adsorbed is a single crystal exhikiting various dif-
.
V O L U M E 23, NO. 12, D E C E M B E R 1 9 5 1
1887
ferent crystallographic faces. The molecules or atoms of the object migrate t o different faces on the crystal tip, depending upoii the temperature t o which it is heated (up t o 1800" C.). Thus it is possible to speculate on the nature of short-range interatomic forces. A motion picture film record of the fluorescent screen image of thermal migrations of various metals also proved t o be fascinating. Interpretation of these phenomena constitutes a real challenge. An w e n more recent development, the field ion microscope,
using protons, operated similarly was also desrrihed. Better resolution, on the order of 3 A,, shelving the atomic lattice spacings of t,he tungsten crystal point was illustrated by means of photographs. The point is, in this instrument, made positive so that the adsorbed hydrogen atoms are liberated as protons whch owing to scattering give rise to a silhouette image of the object also adsorbed on the tungsten point. The high vacuum recjuirements and the necesPary thermal stability of the subject nxitter were outlined.
Electron Microscope Society of America F. A.
u m r
General Aniline & F i l m Corp., Easton, Pa.
THE ninth annual meeting of this society was held a t the Institute, Philadelphia, Pa., on Sovember 8, 9, aiid 10, 1951. The attendance of about 300 people set a nex record which clearly defines the growth of this society. r
1 Franklin
The success of the meeting must be largely credited t o the program chairman, T. G. Rochow, American Cyananlid Co., Stamford, Conn., and the chairman of the committee on locai arrangements, D. D. Pendleton, Jr., Radio Corp. of .4merica, Camden, N. J. The meeting had been deliberately planned to follow the Symposium on Electron Physics sponsored by the Sational Bureau of Standards earlier during the same week. -4s a consequence, a new record for attendance by foreign electron microscupists was established, from the standpoint of 60th number of people and number of foreign countries represented. Five countries in the European area and Uruguay were represented; their personnel attended the meeting and Friday evening banquet as guests of the Electron Microscope Society of America. It developed that the banquet was highlighted by these foreign electron microscopists, who gave brief but entertaining afterdinner talks. The incumbent president, Robley C. Williams, University of California, Berkeley, Calif., presided, and in the customary manner introduced to the society the next year's president, R. D. Heidenreich, Bell Telephone Laboratories. hfurray Hill, N. J. The program of this meeting is abstracted below; only those 41~ papers of general interest t o the readers of A N ~ ~ r r r c . CHEarrsTRY have been considered. The first session dealt largely with instrumentation. Empirical Treatment of the Influence of Morphology and Accelerating Voltage on Electron Diffraction Patterns. S. G. ELLIS, RCA Laborataries Division, Princeton, N. J. The first portion of this paper is described in the review of the Symposium on Electron Physics. The second portion was devoted to a discussion of the precision of electron diffraction with reference to the RCA EMU instrument. The correlation between diffraction ring sharpness and accuracy of measurement was outlined. Thin. well dispersed specimens permit accuracies of about 0.1%. Intermediate and Diffraction Lens for RCA Electron Microscope Type EMU. BENJAMINM. SIEOEL,Department of Engineering Physics, Cornell University, Ithaca, N. Y . The paper describes a useful technique for relating diffraction patterns with specific areas mithin the specimen. The usual modifications of removing lens pole pieces is eliminated. The ability to focus the image focal plane in the objective lens at the object plane of the projector lens permits the formation of the specimen diffraction pattern. The removable aperture is a desirable feature because it conveniently permits a reduction in field size giving use to the diffraction pattern. Circuit diagrams for conjunctive n iring of projector with intermediate lens were explained.
This paper also described a technique for relating diffraction patwith specific small areas within the specimen. Instrument modifirtitions such as removal of projector pole piece and insertion of a 120-micron condenser aperture are required. The distortion normally encountered can be minimized by placing a camera between the objective and projector lens, thereby eliminating the projector lens. ierns
An Objective Aperture Alignment Device. hI.mK E. GETTSER, Sloan-Kettering Institute for Cancer Research, Sew York, N . T.
The value of this technique is based upon the ability to see on the final viewing screen the motion of the objective aperture caused by motion of the stage control knobs. The friction ring between the pole piece sections makes contact with a dummy specimen holder, 30 that stage movement causes movement of the aperture under the iriction ring. After alignment, the regular specimen holder replacer the dummy probe. Reliability of Internal Standards for Calibrating Electron MicroH. L. TTATSOX, Edsel B. Ford Institute for Medical scopes. JOHN Research, Henry Ford Hospital, Detroit, Mich.. AND WILLI.\ML. GRCBE,Research Laboratories Division, General Motors COIp . , Detroit 2, Mich. The limitations of using Dow Latex 550 G spheres were outlined. Bacterial growth at the expense of the latex under nonsterile storage conditions is important. Collodion grating replicas, calibrated before use by means of spectroscopic methods, appear to be the best internal magnification calibration standards. The calculated grating dimensions are checked to about +2%. Calibration of the grating replicas directly on the electron microscope specimen screen, to be subsequently used for specimen support, minimizes possible errors due to manipulation of the grating replica. Experimental Improvements in the Performance of an Electron HILLIER,RCri Laboratories Division. PrinceMicroscope. JAMES ton. N.J. Several new instrument modifications were outlined. The use of a small (5-micron) centerable condenser aperture reduces thermal loading of the specimen and decreases the exposure time to several seconds a t high magnification. The use of a double objective pole piece permits useful instrument magnification up to about 50,000 diameters and reduces the effects of stray magnetic fields and lack of coincident voltage and magnetic alignment. Vibration in the specimen can be virtually eliminated by tightening a screw through the stage against the specimen holder. Examination of the final image through a low power stereoscopic microscope permits more precise focus and detection of image defects. Critical Studies on the Contrast at the Resolving Limit of the Electron Microscope. JAMES HILLIER. RCA Laboratories Division, Princeton, N. J. T h c resolution of structure a t the lower limits (below 25 . I . ) I t was pointed
rrre;;tly depends upon the contrast in the image.
out that photographic emulsions normally used in electron micrography are much too sensitive to the electron 'to permit minute variations in electron beam intensity distribution in the image. Theoretically the photographic grains should be much less sensitive Electron Diffraction from Small Areas of Electron Microscope to the electron, so that intensity variations in the image comparable Specimens Using an RCA Electron Microscope. J. D. BOADWAY, in magnitude to the just perceptible density variations (about 1%) Hhawinigan Chemirale, Ltd., Shawinigan Falls, Quebec, Canada. on the photographic plate would be recorded.
