![]() This, of course, leads to a reduced value of the actual resolving power of the electron microscope from what theoretically might be possible. In electron optics, in order to keep “lens” aberrations to a minimum, limitations are imposed on the size of the aperture. In practice, however, any microscope’s resolving power depends not on the wave length of the observing “light” alone, but on the lens system and on the aperture of the objective lens. Since an Angstrom unit is one ten-millionth of a millimeter in length, this theoretical resolution comes to. Theoretically, the electron microscope should be able to resolve objects as small as 0.05 Angstrom units. But the wave length of the associated electrons in the beam of a 50 KV electron microscope is many hundreds of thousands of times smaller than the wave length of visible light. Because it is impossible to form a correct optical image of objects smaller than about half the wave length of the observing light, it is impossible to see anything smaller than about two-tenths of a micron (a micron, one one-thousandth of a millimeter, is equal to 0.00004 inch) with the conventional light microscope. ![]() Why is there a limitation to what can be seen with the light microscope? Both theory and experiment have shown that it is the wave length of the light which sets an absolute and unalterable lower limit to what can be seen. Mitochondria membranes are seen to have an organized structure suggesting a relation to enzyme systems. In the case of a completely black area, the specimen under study has completely retarded the electron beam and has, in this way, prevented it from reaching the exposed photographic film. In other words, the specimen is cut so thin that those portions which appear white in the electron micrograph have not retarded or deflected the focused beam of electrons at all, while those portions of cells, such as the nucleus, which appear darker in the electron micrographs seem so by virtue of the density of the material originally present in that area. In electron microscopy, a beam of electrons is substituted for the conventional microscope’s beam of light magnetic “lenses” take the place of glass and most of what is visible is seen because of the scattering of electrons by the material of the specimen. Details in the image are visible because of the light absorbed or scattered by the object being viewed. In the light microscope, magnification of the image is achieved by transmitting light from a source through glass lenses. A comparison of the light and electron microscope will be helpful in making this clearer. For, while the electron microscope can magnify an object many thousands of times more than can a light microscope, the instrument makes certain demands of the microscopist. This development of "ultra-microtomy"-or thin-section techniques-grew out of the need to overcome some of the limitations posed by the very nature of the electron microscope. but the application of this "supermicroscope" to biology has recently taken a giant stride ahead with the refinement of techniques for preparing specimens for study at high magnification. The electron microscope is not a new research instrument (the first successful ones were produced in the early 1930’s). Investigation of the macromolecular structure of cells and their components has become increasingly rewarding with the development of electron microscopy-a biophysical technique which, in a few short years, has answered many old questions and raised an equal number of new ones. Today, one of the most exciting research frontiers in biology is the once-invisible region beyond the limit of the conventional microscope-the level at which proteins, carbohydrates, fats, minerals and water are organized in the pattern we call life. NUCLEUS: Electron microscopy has added little to knowledge of the nucleus, but "pores" found in the membrane probably permit exchange between the nucleus and cytoplasm. GOLGI COMPLEX: Previously seen as an ill-defined structure near the nucleus, the Golgi complex is now described as a system of thin membranes forming large vesicles. A system of tiny canals bounded by membranes, granules and vacuoles all make up major portions of cytoplasm. CYTOPLASM: With electron microscope, structural components of cytoplasm can be seen. MITOCHONDRIA: Grublike black bodies seen with the light microscope are now known to have a definite membrane structure. ![]() ![]() CELL SURFACE: Two new features are revealed-finger-like projections, or villi, covering the surface and the surface membrane, resembling two thick lines. ![]() ILLUSTRATIONS contrast the structure of an intestinal cell as known from light vs. ![]()
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