Gresham, William S.

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 The Knowledge Bank at The Ohio State University Ohio State Engineer Title:  New Worlds Through the Electron Microscope Creators:  Gresham, William S.  Issue Date:  1943-02  Publisher:  Ohio State University, College of Engineering  Citation:  Ohio State Engineer, vol. 26, no. 3 (February, 1943), 9-10, 19.  URI:  http://hdl.handle.net/1811/35905    —Courtesy: Westinghouse Electric mSorting machine-mass spectograph segregates gas molecules according to their massesNEW WORLDS THROUGH THEELECTRON MICROSCOPEWILLIAM S. GRESHAM, Engr.lll"The greatest extension of humanvision since 1677 when Anthony vanLeeuwenhook first focused spermatozoaunder his crude lens is the electron mi-croscope."That statement appearing in Life Magazinefrankly signifies the importance of the develop-ment of the electron microscopy. The essence ofmicroscopy is to reproduce the object in greaterdetail. Magnifications ten-fold greater than withthe ordinary light microscope are possible with1 this new instrument. Microscopic details smallerthan one-half the wave length of visible lightare beyond the limits of human sight. However,in the electron microscope designed and built by> Radio Corporation of America at Camden, NewJersey, a wave length one hundred thousand timesshorter than that of light brings these detailsinto sharp definition.i- On the basis of conventional diagrams in op-tics, it is reasonable to say that infinite magnifi-cations should be possible. This appears feasible,for in these diagrams light is represented bystraight lines. However, light has the charac-teristics of a wave motion and can thus bendaround very minute particles. This phenomenonbecomes very important as the particles becomeextremely small.The development of the electron microscopebegan in 1931 when M. Knoll and E. Ruska con-structed one which consisted of two lenses. Im-provements were later made by E. Ruska and L.C. Marton in Belgium. These were followed byother improvements which refined the instrumentand eliminated many of the time- and labor-con-suming operations.Scientists had for many years cherished thehope that some form of radiation would be dis-covered which would have a wave length muchshorter than that of light, and would give greater' magnifications in microscopy. During the lastdecade of the nineteenth century both electronsand x-rays were discovered, and these were atonce suggested as possible solutions of the prob-lem. On account of their effects on photographicplates and their power to penetrate matter, x-rayswere from the first thought to be a form of light,but all attempts to make them capable of refract-ing failed.Light has become known within the last fewyears as a type of energy. A good example of thisF e b r u a r y , 1 9 4 3is the photo-electric cell. When the beam oflight of this cell is broken, another circuit ismade and any of a variety of mechanisms cansubsequently be operated. With light being atype of energy and having a wave form, physicistswondered if a beam of electrons would have thesame dual properties as that of light. A beamof electrons consists of a stream of small neg-atively charged particles having a mass aboutone two-thousandths that of the hydrogen atom.As a ray of light passes through an opticallens, the ray is always refracted. This phenome-non makes possible the ordinary optical micro-scope. A beam of electrons cannot pass throughan optical lens; consequently, another methodmust be used to refract them. It was alreadyknown that magnetic fields had a decided effecton protons and electrons and that the desiredeffect could be accurately controlled. Therefore,magnetic fields can accomplish the same resultswith electron beams that optical lenses do withlight.The reason that the electrons are acceleratedto high speeds is the fact that the wave lengthof the electrons becomes shorter as their speedincreases. Their short wave length makes pos-sible the high magnifications.The tungsten cathode is in the shape of ahairpin and is mounted at the lower end of a rod.This may be raised or lowered within a cathodeshield by means of an adjusting screw and a bel-lows arrangement. The possible adjustment isadvantageous for two reasons: first, the efficiencyof the cathode is increased, and, second, thecathode may be drawn up into the shield so thatonly the electrons from the tip might be able toescape. The electrons leaving the cathode areaccelerated downwards and pass through theanode, the deflection plate, and the condensinglens. The anode and deflection plate are usefulin controlling the beam of electrons, while thecondensing lens reduces the cross section of thebeam. As the electrons hit the particles of thespecimen, the electrons are scattered or absorbedin proportion to the thickness and atomic prop-erties of the particles. This is exactly what hap-pens when light hits the object in the visualmicroscope.The absorbed electrons, upon leaving the object,pass through the aperture of the objective lensP a g e 9—Courtesy: Engineering Experiment Station NewsClay minerals through electron microscope; upperKaoliniie; lower Halloysite.and are focused by its strong magnetic field uponthe first image screen. The electrons from anydesired part of the object may be directed throughan opening in the first screen into the second orprojection lens. The electrons are again focusedby the magnetic field of this lens to form a muchgreater magnified image on the second screen.The image at this point may either be seen onthe flourescent screen or photographed by re-placing the screen with a camera. A high magni-fication is obtained in each lens by having a largeratio between image distance and object distance.P a g e 10The total magnification is, of course the productof the magnifications produced by each lens.In order to see the characteristics of the speci-men, it is necessary to suspend it on an electronoptically transparent film. A very satisfactorymembrane can be made with a solution collodionand amyl-acetate. Drops of this solution are al-lowed to fall on a surface of water. The amyl-acetate evaporates quickly and leaves a film ofcollodion on tne water. The object is then placedon the film, which is about one millionth of a milli-meter in thickness. The film and object are thenplaced between two small disks of very fine wiremesh which serves to support it.Since the magnifications are so great, it wouldrequire a good deal of patience to adjust the speci-men until the desired part is focused. Therefore,a periscopic device has been built into the instru-ment.When looking at the final image, the operatorcan, by moving his eyes sideways, look into theperiscope and see the intermediate image. Thisdevice aids the operator a great deal in his studyof the specimen.The air must be exhausted from the microscopebecause the electrons would bump into the com-paratively large molecules of air and be stopped.Therefore, the main body of the microscope isexhausted, and the specimen is entered andremoved by the airlock so that it does not inter-fere with the vacuum in the main body of theinstrument. The airlock is similar to the escapechamber of the submarine. After the specimenhas been placed in position, the air is exhaustedfrom the lock and it is connected to the rest ofthe microscope. Before removing the specimen,the lock is disconnected from the main body ofthe instrument, and it is then opened. The timetaken by the pump to evacuate the chamber isabout two minutes. Likewise, the plateholder fortaking photographs is placed in an airlock situ-ated at the bottom of the instrument.What is actually seen in the microscope is aflourescent screen which is about 3!/2 x 4 ^ inchesin size. The image that is seen on this screen isproduced by the electrons hitting it. For eachelectron that hits the screen there is a tiny flash.Consequently, the light parts of the image repre-sent the less dense parts of the object throughwhich the electrons have passed.The electron microscope is opening a new world,a world that has never been seen before. There-fore, research workers using the instrument willhave to advance slowly from the known to theunknown. Whooping cough germs which are atthe limit of the visual microscope can be enlarged(Continued on page 19)T h e O h i o S t a t e E n g i n e e rELECTRON MICROSCOPE(Continued from page 10)with the electron microscope to the size of foot-balls and their internal structure studied.So-called filtrable viruses that cause a series ofdiseases probably have been seen for the firsttime, but they have not yet been identified. Theyare called filtrable diseases because they passthrough filters, and their presence can only beproven by producing the disease with the filtrate.Its industrial applications will probably be justas important as the medical ones. Industries mak-ing cement or making rubber are anxious to findout what lies beyond the visual microscope. Asmatter of fact, any industry that refers to photo-micrographs of specimens will be extremely inter-ested in this new instrument.The electron microscope should prove anotherboon to mankind. With magnification of 100,000diameters as compared to 6,000 diameters withthe light microscope, its possibilities are enor-mous. Just as Anthony van Leeuwenhook gazedinto his simple microscope to see a new world, sowill those who gaze into the electron microscopesee a world new in the realm of microscopy.F e b r u a r y , 1 9 4 3 P a g e 19

New Worlds Through the Electron Microscope

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New Worlds Through the Electron Microscope

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