.4n Ordinary Microscope
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Focuses light with glass lenses
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C o w , r r \ ~of the RCA Rcreorch Lo6orolorirs
HE scientist today stands a t the threshold of a vast new world of chemistry, opened up by the electron microscope. In principle the electron microscope focuses electrons with magnetic lenses, just as an ordinary microscope focuses light with glars lenses. The shorter the wave length, the greater the resolving power. The wave lengths of photons, or visible light. are from 4000 t o 8000 A.; electrons have a wave length of about 0.01 A. Although cathade-ray images were obtained as early as 18iF (Goldstein), i t was not until 1926 that the lens properties of magnetic and electric fields were demonstrated (Busch). This was shortly before Davisson and Germer' showed that electrons behaved like light waves. Pioneers followed in quick succession: Rupp (1928), Zworykin (1929), Davisson of the Bell Telephone Laboratories (1931), and Brueche and his collaborators in Berlin (1932). using electrostatic lenses. The 6rst compound magnetic lens microscopes, progenitors of the 1941 streamlined RCA model, were developed in Germany by Knoll and Ruska (1932); and a t the University of Brussels by Marton, who was also the 6rst t o use his instrument for biological studies (1934). Several years later a t the University of Toronto Burton. Hillier, and Prebus constructed a similar instrument. I n one study with this machine colloidal carbon was magnified 102,000 diameters, revealing it to be spheroidal, averaging 28 millimicrons in diameterabout half the value indicated by ultraminoscopic counting methods. Interested in electron optics for many years, the RCA laboratories a t Camden, New Jersey, supported the development of a research model of the electron microscope by L. Marton under Associate Director V. K. Zworykin. Recently, a streamlined model suitable for routine or r,e.ese&h laboratories has been developed there by J. Hillier. This instrument is now bring made for sale a t $9500. The first purchaser was the American Cyanamid Company, who, in six weeks' time, obtained inleresting results of fundamental importance. I n the electron microscope, electrons are generated by a tungsten filament, accelerated t o 3 0 , 0 0 ~ 0 , 0 0 0volts by a strong electric field, and then magnetically focused upon the object to be examined. This corresponds t o gathering light with the condenser lens of a microscope The object, such as a typhus hacillus, is mounted in a nitrocellulose film a millionth of an inch thick, supported on a fine wire cloth. Although the entire apparatus must be evacuated t o lo-' mm.. pictures may be taken in rapid succession, since the RCA instrument is equipped with air-lack chambers which admit air t o only a small section of the apparatus when cfianging samples. The electrons p-assthrough the sample and are guided by the magnetic field of the "objective" t o a fluorescent screen, where an image of the object, magnified one hundred-fold, is produced. This image is then further magnified by a projection.coil, similar t o that used in television, t o produce a second image on a fluorescent screen or photographic plate with magnifications up t o 20,000. The picture recorded by these electrons has been named an elcctron micrograph as distinguished from a photograph obtained with photons. The electron micrographis usually so clear that it may
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GEKMEK,"Optical experiments with electrons. Parts I and 11." J. CHEM.EDIJC., 5, 1041, 1265 (1928).
The Electron Microscope Focuses eleetmns with magnetio lenses
bc photographically enlarged t o as much as 100,000 diameters. beyond which there is no improvement in definition. Conservatively speaking, a sample which is enlarged 100,000 diameters under the electron microscope would be as clearly resolved as a 1500-diameter magnification by an ordinary optical micrmope. More simply, the ordinary microscope can detect particles some 1500 A. in diameter, but the electron microscope goes dawn t o 30 A. Since atoms are only one-tenth this size, chemists can nearlv-but unfortnnateh not auit-oeer into a world which first eiisted in the minds oi ~emocritus:Lucretius. and 1)alton. Giant mulecules can even now be p h o t v ~ ~ u p h ~ d . bur them issomt distance yet to go bcforc individual oroms sill be revealed. During recent months the RCA Laboratories, in collaboration with Dr. Stuart Mudd of the University of Pennsylvania, chairman of a committee of the National Research Council on the electron microscope, have examined the structure of germ cells which appear as mere specks in an o r d i i r y microscope. They have found, for example, that streptococcus g e m are bound in chains by rigid and continuous outer membranes, and have examined the internal structure of whooping cough germs and the structure of the flagellae of typhoid germs. Dr. H. Morton and Thomas F. Anderson have discovered that tellurite salts within diphtheria bacilli are reduced t o crystals of tellurium, the crystals perforating the ceU walls in some cases. Dr. Anderson ilso took the electron micrograph shown here of the tobacco mosaic virus prepared by Dr. W. M. Stanley of the Rockefeller Institote. In this picture note the interesting spiral internal structure of the bacterium. Adhering t o its tip may he seen the rod-shaped particles of the tobocco mosaic virus which, being crystallizable, has all the properties of a pure chemical substance. Neither these nor the small specks of normal rabbit serum can be seen under an ordinary microscope. Micrographs were also taken showing the molecular structure of soap crystals. Many industrial companies are examining their products under the RCA electron microscope. Rouge a t 15,000 diameters exhibits jagged edges; carbon particles in a gas flame and crystals in cigar smokeare easily identified by their distinctive shapes; synthetic fibers and plastics reveal the secrets of their molecular structure t o the electrm microscope. At the Kodak Research Laboratories the development of the ~hotoxraohicimane is being studied. Under such hiah mamification, the so-called "grainGf silver turns out t o he more like a clump of seaweed than a solid speck. Such studies will undoubtedly lead t o the production of better he-grain films. In Germany. Ruska a t the Siemens and Halske A. G., and Brueche in Berlin describe: studies of mine dusts for health protection; micrographs showing that the forces holding adsorbed red gold sols t o tobacco mosaic virus below pH 4.5 are so strong that the gold cannot be aggregated t o the blue form, as it normally is upon the addition of sodium chloride; micrographs of individual molecules of hemocvanin from the blood of mollusks and of edestin particles from vegetable ails. The new field of electron microscopy, realm of molecular studies, lies unexplored. We are indebted t o Dr. Thomas F. Anderson, RCA Electron Microscope Fellow of the National Research Council for 1940 t o 1941. and to Dr. W. M. Stanley of the Rockefeller Institute, for -H. N. ALYEA supplying the accompanying micrograph. ~
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ELECTRON SOURCE
MAGNETIC CONDENSER
00JECTIVE
INTERMEDIATE IMAGE PROJECTOR
S E C O N D STAGE M4GNIfIED IMAGE
