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The most widely used microscopes are optical microscopes, which use visible light to create a magnified image of an object. The simplest form of optical microscope is the double-convex lens with a short focal length. These lenses can magnify an object by up to 15 times. In general, however, a compound microscope is used, which has multiple lenses to provide more magnification than a single convex lens could alone. Some optical microscopes can magnify an object by 2,000 times or more.

The compound microscope consists essentially of two lens systems, the objective and the ocular, mounted at opposite ends of a closed tube. The objective lens is composed of several lens elements that form an enlarged real image of the object being examined. The microscope lenses are set up so that the real image formed by the objective lies at the focal point of the ocular; the observer looking through the ocular sees an enlarged virtual image of the real image. The total magnification of the microscope is determined by the focal lengths of the two lens systems.

The accessory equipment of an optical microscope includes a firm stand with a flat stage for holding the material to be examined, and some means for moving the microscope tube towards and away from the stage so that the specimen can be brought into focus. Ordinarily, specimens for microscopic examination are transparent and are viewed by using light that passes through the specimen. The specimens are usually mounted on thin, rectangular glass slides. The stage has a small hole through which light passes. Underneath the stage there is a mirror that reflects light through the specimen, or a special electric light source that directs light through the specimen.

In photomicrography, which is the process of taking photographs through a microscope, a camera is mounted directly above the microscope's eyepiece. Normally the camera does not contain a lens, because the microscope itself acts as the lens system. The term microphotography, sometimes used instead of photomicrography, is usually applied to the technique of duplicating and reducing a picture or a document to a miniature size for storage.

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Microscopes used for research commonly have a number of refinements to enable a complete study of the specimens. Because the image of a specimen is highly magnified and inverted, manipulating the specimen by hand is very difficult. As a result, the stages of high-powered research microscopes are mounted so that they can be moved by means of micrometer screws; in some microscopes, the stage can also be rotated. In addition, all research microscopes are equipped with three or more objectives, mounted on a revolving head, so that the magnifying power of the microscope can be varied.


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