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Geometrical Optics for Electrons Quite Similar to the Optics of Light

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Geometrical Optics for Electrons Quite Similar to the Optics of Light THE ELECTRON MICROSCOPE A NEW ToOL FOR BACTERIOLOGICAL RESEARCH L. MARTON Research Laboratories, RCA Manufacturing Company Received for publication August 1, 1940 The science of bacteriology could hardly exist without the microscope, and it is almost providential that pathogenic bacteria are within the limits of visibility of the present-day microscope. However, the limits of microscopical observation have been severely felt since early in the development of bacteriological research, and many attempts have been made to extend the range of- observation. These attempts brought the realization that the sizes of micro-organisms extended far beyond the limits of visibility of light microscopes, and therefore the need has been constantly felt for a better instrument which would give more detail. Such an instrument is provided in the electron micro- scope which, in its present-day development, extends the obser- vation range by a factor of about 50 to 100, with possible further extensions in the future. Electron microscopy is based on the discovery of geometrical optics for electrons quite similar to the optics of light. To understand the term "geometrical optics" let us first consider the action of an electric or magnetic field on an electron beam. It is well known that an electron beam is deflected by such fields, and we can therefore compare their action on the beam to the action of a refractive medium on a light beam. A lens is nothing but a refractive medium of special symmetry-in this particular case of rotational symmetry. If we create an electric or mag- netic field of rotational symmetry, such a field acts on an electron beam as a lens, i.e., the electron beam is concentrated or made divergent in the same way that the light beam is acted upon 397 398 L. MARTON by a glass lens. It has been proved mathematically that the laws of geometrical optics can be fully applied to such systems, and experimentally that we can obtain electronic images which can be made visible, for instance on a fluorescent screen. An image can be formed of any self-emitting object, as would be the case with light if we observed the image of the source itself, or we can illuminate an object in the same way as we illuminate ELECTRON El ECTRON LIGHT SOURCE SOURCE SOURCE CONDE NSER COIt MAGNETIC l C 0 H D E N S E It i CONDENSER U L E N.S . OBJECTIVE COIL OBJECTIVE MAGNETIC OBJECTIVE U LENS !\ PROJECTION I'' COIL /i\ INTERMEDIATE IMAGE FLUORESCENT PROJECTOR SCREEN OR /\ O E NRVAT ON L SECOND STAGE SCREEN MAGNIFIED IMAGE (PHOTOGRAPHIC PLATE) PEI. 1. SCHEMATIC DiGRA.AU or THE ELCRON MICROSCOPE IN COMPARISON WITH THE LIGT MCROSCOPE one in a light microscope, and observe the image of the object with the help of the illuminating beam. After this discovery, the obvious next step was the building of compound optical systems, and, in particular, a system corre- sponding to the compound microscope. Such an instrument is built up of different elements for which we can use the same nomenclature as in a light microscope (fig. 1). The light source of the light microscope is replaced by an electron source, the electron beam being concentrated on the specimen by means of the first field in the same way that the light beam is concentrated THE ELECTRON MICROSCOPE 399 by means of a condenser lens. The field is created by a coil through which current flows and produces inside the coil the necessary magnetic field of rotational symmetry. Two more similar coils are used in place of the objective and eyepiece lenses of the light microscope. We may call the first one the objective coil and the second one the projection coil, because the image is not viewed by applying the eye to an eyepiece, but by projecting it on a fluorescent screen. The function of both lenses is exactly the same as in the familiar light optics. The objective coil produces a first stage magnification which is re-enlarged by the projection lens. The above described microscope system has some very special properties. In the first place, the electrons travel without hindrance only in vacuum, and therefore the whole microscope must be pumped out to a high degree. This means that the specimens and photographic plates must also be in the vacuum. The operation of an instrument of this kind is necessarily different from the operation of a light microscope. Focusing, for instance, is done in a quite different way. In light optics the optical components of the system are fixed by the construction, and focus- ing is done by changing the distance between the specimens and the optical system. In electron microscopes the focal length of a lens is given by the strength of the applied field, and therefore the focusing is done by varying the optical constants of the lens similarly to the way in which the focal length of the human eye is varied by changing the curvature of the lens. The distance between the object and the optical system therefore remains constant, and the magnification can be changed continuously from the lowest to the highest magnification, instead of the step- by-step variation obtained in a light microscope by exchanging eyepieces. The great advantage of an electron optical system lies in its highly increased resolving power. The practical microscopist knows that the resolving power, i.e., the smallest distance sepa- rately shown by an optical system, is about one-half of a wave- length for the best light optical systems. Since the fundamental discovery of DeBroglie, we know that the electron behaves for 400 L. MARTON some applications as a corpuscle, and for other applications as a wave, the wave-length of which depends on the speed of the elec- tron; and, for the speeds generally used in electron microscopy, it is about one 100,000th of the wave-length of visible light. Under identical conditions, this would mean 100,000 times better resolving power, or 100,000 times higher useful magnification for the electron microscope than forthe light microscope. This, however, cannot be realized, the reasons for the limitations being manifold. In the first place, the lenses of electron optics exhibit all of the optical defects known to light optics, plus a few more, and at the present state of our knowledge we are not able to correct very far the electron optical aberrations. For instance, spherical aberration is about 1,000 times worse than in light microscopy. We do not have any means of correcting the chromatic defect. The result is that the numerical apertures of electron optical systems are much reduced over the numerical apertures of light optical systems, and as the resolving power is not only propor- tional to the wavelength but also inversely proportional to the numerical aperture, we come to the conclusion (represented in fig. 2), that for the present-day practical apertures of about 0.001, we should expect the best resolving power to be about 10 Ang- strom Units, or a corresponding useful magnification of about 200,000 times. Incidentally, the use of extremely small apertures has another advantage: as in light microscopy, the smaller the aperture the greater is the depth of focus, and the resulting depth of focus for the herein described electron microscope is at least ten times greater than the depth of focus of the highest power light microscopes. Another peculiarity of electron microscopy is the mechanism of image formation. In light microscopes we see the image due to differences in absorption, or refraction in the specimen. In electron microscopes the image formation is not due to either of these effects, but to scattering of the electrons. As mentioned before, electrons do not travel in straight lines except in vacuum, and even a few molecules suffice to deflect an electron beam. This means that we do not have any material transparent to THE ELECTRON MICROSCOPE 401 electrons in the same sense as to light, and therefore we only have transmission of an electron beam when the substance through which the beam passes is extremely thin. This means that, first, the glass slides of light microscopy must be replaced by extremely thin films as specimen holders, and, second, that a A.U. 102 54 0 0 p41 to a~ 10 6r4 A 1 /4 k-O% -2 I lo- 1 1002n 103 041.U. 105 103 10 volts Electrons FIG. 2. RESOLVING POWER IN FUNCTION OF THE W)AN-ELENGTH FOR DIFFERENT NUMERICAL APERTURES what we observe in an electron microscope image are only differ- ences in thickness and in density of a specimen. The practical application of these principles can be best ex- plained by describing~~~~~Wavelegtan RCA electron microscope (fig. 3). The electrons are generated by an "electron gun," similar in construc- tion to an X-ray tube. The main tube contains the various 402 L. MARTON optical and mechanical elements of the microscope-three coils which act as lenses, the "object chamber," and the "photographic chamber." The lenses, as mentioned above, are coils consisting of a large number of turns of copper wire, which are housed in FIG. 3. FRONT VIEW OF THE RCA ELECTRON MICROSCOPE iron enclosures. The function of the iron enclosures is to con- centrate the field between the gaps in the enclosure, and to give the necessary shape to the field. The object chamber has two important functions (fig. 4). The first is to introduce and withdraw the specimen to and from THE ELECTRON MICROSCOPE 403 the main tube without breaking the vacuum in the microscope, and the second is to permit motion of the specimen in front of the objective, replacing the mechanical stage of a light micro- scope.
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