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  3. The Electron Microscope The general layout of the illumination system and lenses of the electron microscope corresponds to th

The Electron Microscope The general layout of the illumination system and lenses of the electron microscope corresponds to th

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问题    The Electron Microscope
   The general layout of the illumination system and lenses of the electron microscope corresponds to the layout of the light microscope. The electrons are accelerated by a high-voltage potential (usually 40, 000 to 100, 000 volts), and pass through a condenser lens system usually composed of two magnetic lenses. The system concentrates the beam on to the specimen, and the objective lens provides the primary magnification. The final image in the electron microscope must be projected on to a phosphor-coated screen so that it can be seen. For this reason, the lenses that are the equivalent of the eyepiece in an optical microscope are called "projector" lenses.
   Normally, the electron microscope is upside-down when compared with the light microscope, with the electron gun at the top of the column and the fluorescent screen at the bottom. The screen is viewed through a window let into the front. The column of the microscope is held under high vacuum to prevent the electrons passing through it from striking air molecules and being scattered.
   The strength of an electron lens depends on the current passing through the coil that produces the magnetic field. The strength of the lens can be varied by altering the current. In the electron microscope, therefore, the lenses are fixed, and adjustments are made to magnification and focus by altering the current passing through the lens coils. The condenser lens focuses the beam of electrons on to the specimen and affects the amount of illumination on the screen; the objective lens focuses the image; and the projector lenses alter the magnification.
   Magnetic lenses suffer from the same defects (chromatic and spherical aberration) in the same way as glass lenses. But the same methods of correction cannot be used, because there is no "negative" electron lens.
   A very small lens aperture is employed to correct spherical aberration, but this severely limits the final resolution. Chromatic aberration is reduced by using electrons of a single wavelength. To produce such electrons, the accelerating voltage must be kept very steady because the wavelength of the beam is related to the accelerating voltage.
   Electron-microscope lenses suffer from the further aberration of astigmatism, which affects light-microscope lenses to a far lesser degree. Astigmatism is caused by the lens having two focal planes for axes at right angles to each other .
   Nothing can be done about astigmatism in an optical microscope. But in an electron microscope it can be corrected. Astigmatism in the electron microscope arises from two sources: from the lenses themselves, and from dirty apertures ( which are only 25 to 50 microns in diameter) in the objective lens. In both cases the astigmatism can be corrected by a skilled operator. In effect, the electron microscope achieves its superior resolution more in spite of, than because of, its lenses.
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答案 电子显微镜 电子显微镜的照明系统和透镜系统的总体布局与光学显微镜的布局是相似的。电子通过高压电动势(一般为4万一10万伏)来加速其运动。电子束通过一般由两个磁透镜组成的聚光透镜系统。聚焦透镜系统把电子束集中到试样上，由物镜进行初步放大，电子显微镜的最后图像必须要投射到荧光屏上，这样才能看得见。因此，这些相当于光学显微镜的目镜称之为投映透镜。 在正常情况下，电子显微镜与光学显微镜比较，是上下颠倒的，电子枪在镜筒的顶端，而荧光屏则在底部。在镜筒前部开了个窗洞，通过它可以看到荧光屏。镜筒保持高度真空以防止在其中通过的电子撞击空气中的分子，从而被冲散。 电子透镜的聚焦强度取决于通过磁透镜线圈的电流，电流通过线圈产生磁场。透镜的聚焦能力能够通过改变电流进行调节。因此，在电子显微镜内，透镜是固定的，对放大率和焦距的调节是通过改变磁透镜线圈内的电流而进行的。聚焦透镜把电子束聚焦在试样上，并会影响荧光屏的照明度，物镜聚焦成像，而投映透镜则改变放大率。 电子显微镜(磁)透镜具有与玻璃透镜同样的缺点，即产生色散和球面像差，但是却不能用于对玻璃透镜同样的方法加以纠正，因为没有“负”电子透镜。 使用很少的透镜孔径来纠正球面像差，但这样做严重地影响了最终的图像清晰度。色散可以利用同一波长的电子来加以减少。要产生这样的电子，加速电压必须保持稳定，因为电子束的波长是与加速电子的电压有关的。 电子显微镜磁透镜还进一步受像散的影响，像散对光学显微镜而言则影响很小，像散的形成，是由于透镜对互成直角的两根轴具有两个焦平面。 光学显微镜的像散是无法纠正的，但电子显微镜的像散则可以纠正。电子显微镜的像散是由两个原因造成的，一是由于透镜本身的原因，二是由于物镜的孔径不干净(孔径直径只有25—50微米)。这两种情况都可以由一位熟练工人来纠正。实际上，电子显微镜之所以具有高清晰度，与其说是由于它的透镜的缘故，还不如说是由于与透镜无关的其他原因。
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