These lenses magnify the image by 4x to 100x. Most light microscopes have three or four objective lenses on a rotating turret. After passing through the specimen on the stage, the light enters an objective lens. A condenser located below the stage has lenses that focus the light on the specimen and a diaphragm that regulates contrast. Most compound microscopes today have an illuminator built into the base. Some prefer this method for its relative simplicity, and its speed is an asset in hospitals, where a biopsied tissue may need to be examined rapidly and the diagnosis reported to the surgeon while the patient is in the operating room. Another is the frozen section method, in which a tissue is frozen with compressed carbon dioxide and sectioned with a special cold microtome, eliminating the time-consuming process of paraffin embedding. One variation is to embed the tissue in special plastics (resins),Īllowing for thinner sectioning. Other methods of histotechnique have been developed for special purposes. A widely used stain combination called hematoxylin and eosin, for example, typically colors cell nuclei violet and the cytoplasm pink. The colors of a prepared tissue are not natural colors, but they make the tissue's structural details more visible. The slices of tissue, called histological sections, are typically thinner than a single cell. In brief, classical histotechnique involves preserving a specimen in a fixative, such as formalin, to prevent decay embedding it in a block of paraffin and slicing it very thinly with an instrument called a microtome removing the paraffin with a solvent and then staining the tissue, usually with two or more dyes. The advancement of light microscopy also required methods for preserving plant and animal tissues and making their cellular details more visible, methods collectively called histotechnique (from histo, meaning "tissue"). The most significant improvement in microscope optics was achieved in the nineteenth century, when business partners Carl Zeiss (1816 –1888) and Ernst Abbe (1840 –1905) added the substage condenser and developed superior lenses that greatly reduced chromatic and spherical aberration, while permitting vastly improved resolution and higher magnification. Until 1800, compound microscopes designed by Hooke and others were limited to magnifications of 30x to 50x, and their images exhibited blurry edges (spherical aberration) and rainbowlike distortions (chromatic aberration). He thus became the first to see individual cells, including bacteria, protozoans, muscle cells, and sperm.Įnglishman Robert Hooke (1635 –1703) further refined the compound microscope, adding such features as a stage to hold the specimen, an illuminator, and coarse and fine focus controls. While others were making lenses by such methods as squashing molten glass between pieces of wood, Leeuwenhoek made them by carefully grinding and polishing solid glass. His microscopes were collapsing tubes used like a telescope in reverse, and produced magnifications up to nine times (9x).Īntony van Leeuwenhoek (1632 –1723) invented a simple (one-lens) microscope around 1670 that magnified up to 200x and achieved twice the resolution of the best compound microscopes of his day, mainly because he crafted better lenses. Light microscopes date at least to 1595, when Zacharias Jansen (1580 –1638) of Holland invented a compound light microscope, one that used two lenses, with the second lens further magnifying the image produced by the first.
The history of microscopy has revolved largely around technological advances that have produced better resolution. Resolution is the ability to distinguish two objects as separate entities, rather than seeing them blurred together as a single smudge. The usefulness of any microscope is that it produces better resolution than the eye. Mere magnification without added detail is scientifically useless, just as endlessly enlarging a small photograph may not reveal any more detail, but only larger blurs. A light microscope (LM) is an instrument that uses visible light and magnifying lenses to examine small objects not visible to the naked eye, or in finer detail than the naked eye allows.
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