periscope(redirected from Vision block)
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periscope(pĕr`ĭskōp) [Gr.,=view around], instrument to enable a person to see objects not in his direct line of vision or concealed by some intervening body. Its essential parts are a tube, prisms, lenses, mirrors, and an eyepiece. The image is received in one mirror and reflected through the tube with its lenses to a mirror visible to the viewer. Periscopes used in submarines are so arranged that they can be turned to permit a view of the entire horizon, with built-in rangefinders and typically six times magnification. Submarine periscopes are of noncorrosive metal, have tubes up to 30 ft (9.1 m) long and about 6 in. (15 cm) in diameter (only a small section projects above the water), and may be withdrawn into the submarine. Many smaller types of periscopes are used in trenches and tanks. With the development of fiber optics, periscopes (known as cystoscopes or endoscopes) have become useful in medicine.
an optical instrument for viewing from such military shelters as trenches and dugouts, from tanks, and from submarines. Many periscopes permit measurement of horizontal and vertical angles on the terrain and also range-finding. The design and optical characteristics of a periscope are determined by its purpose, place of installation, and the depth from which observations are to be made.
The simplest periscope is the vertical type, consisting of a vertical sighting tube and two mirrors. Set at 45° angles to the axis of the tube, the mirrors form an optical system that refracts light rays coming from the observed object and directs them toward the observer’s eye. Periscopes in which right-angle prisms are installed in the tube instead of mirrors are common, as are telescopic lens systems and rotating systems, which can be used to obtain an erect magnified image. The field of vision of a periscope with low magnification (up to 1.5 power) is about 40°; it usually decreases with greater magnification. Certain types of periscopes give panoramic vision.