ETYM Pref. peri- + -scope.
An optical instrument that provides a view of an otherwise obstructed field.
Optical instrument designed for observation from a concealed position such as from a submerged submarine. In its basic form it consists of a tube with parallel mirrors at each end, inclined at 45ş to its axis. The periscope attained prominence in naval and military operations of World War I.
Although most often thought of as a submarine observation device, periscopes were widely used in the trenches during World War I to allow observation without exposing the observer, and special versions were also developed to be attached to rifles. + prikaži više

ETYM Greek, viewing afar, farseeing; tele far, far off + skopos a watcher, akin to skopein to view: cf. French télescope. Related to Telegraph, and -scope.
An instrument that magnifies the image of distant objects; SYN. scope.
Optical instrument that magnifies images of faint and distant objects; any device for collecting and focusing light and other forms of electromagnetic radiation. It is a major research tool in astronomy and is used to sight over land and sea; small telescopes can be attached to cameras and rifles. A telescope with a large aperture, or opening, can distinguish finer detail and fainter objects than one with a small aperture. The refracting telescope uses lenses, and the reflecting telescope uses mirrors. A third type, the catadioptric telescope, is a combination of lenses and mirrors. See also radio telescope.
Refractor.
In a refractor, light is collected by a lens called the object glass or objective, which focuses light down a tube, forming an image magnified by an eyepiece. Invention of the refractor is attributed to a Dutch optician, Hans Lippershey 1608. Hearing of the invention in 1609, Galileo quickly constructed one for himself and went on to produce a succession of such instruments which he used from 1610 onward for astronomical observations. The largest refracting telescope in the world, at Yerkes Observatory, Wisconsin, has an aperture of 102 cm/40 in.
Reflector.
In a reflector, light is collected and focused by a concave mirror. The first reflector was built about 1670 by Isaac Newton. Large mirrors are cheaper to make and easier to mount than large lenses, so all the largest telescopes are reflectors. The largest reflector with a single mirror, 6 m/236 in, is at Zelenchukskaya, Russia. Telescopes with larger apertures composed of numerous smaller segments have been built, such as the Keck Telescope on Mauna Kea. A multiple-mirror telescope was installed on Mount Hopkins, Arizona, 1979. It consists of six mirrors of 1.8 m/72 in aperture, which perform like a single 4.5 m/176 in mirror. Schmidt telescopes are used for taking wide-field photographs of the sky. They have a main mirror plus a thin lens at the front of the tube to increase the field of view.
The liquid-mirror telescope is a reflecting telescope constructed with a rotating mercury mirror; charge-coupled devices are used to compensate for the fact that a liquid mirror cannot be tilted. In 1994 a small number of liquid-mirror telescopes were being used in research.
Telescopes in space.
Large telescopes can now be placed in orbit above the distorting effects of the Earth's atmosphere. Telescopes in space have been used to study infrared, ultraviolet, and X-ray radiation that does not penetrate the atmosphere but carries much information about the births, lives, and deaths of stars and galaxies. The 2.4-m/94-in Hubble Space Telescope, launched 1990, can see the sky more clearly than can any telescope on Earth. + prikaži više