View source for Periscope

[[File:Periscope simple.svg|thumb|left|Principle of the periscope. The periscope on the left uses mirrors whereas the right uses prisms.<br>
'''a''' Mirrors<br>
'''b''' Prisms<br>
'''c''' Observer's eye]]

[[File:Periskop linsen.svg|thumb|right|Principle of the lens periscope.
The two periscopes differ in the way they erect the image. The left one uses an erecting prism whereas the right uses an erecting lens and a second image plane.<br>
'''a''' [[Objective lens]]<br>
'''b''' [[Field lens]]<br>
'''c''' Image erecting lens<br>
'''d''' [[Ocular lens]]<br>
'''e''' Lens of the observer's eye<br>
'''f''' Right-angled prism<br>
'''g''' Image-erecting prism]]

A '''periscope''' is an instrument for observation over, around or through an object, obstacle or condition that prevents direct line-of-sight observation from an observer's current position.
In its simplest form, it consists of an outer case with [[mirror]]s at each end set parallel to each other at a 45-degree angle.
This form of periscope, with the addition of two simple lenses, served for observation purposes in the trenches during [[World War I]]. Military personnel also use periscopes in some [[gun turret]]s and in [[armoured vehicle]]s.

More complex periscopes, using [[Prism (optics)|prism]]s and/or advanced fiber optics instead of mirrors, and providing magnification, operate on [[submarine]]s and in various fields of science. The overall design of the classical submarine periscope is very simple: two telescopes pointed into each other. If the two telescopes have different individual magnification, the difference between them causes an overall magnification or reduction.

==Early examples==
[[File:Periscope rifle Gallipoli 1915.jpg|thumb|right|[[Australian Light Horse]] troops using a [[periscope rifle]], [[Gallipoli Campaign|Gallipoli]], 1915. Photograph by [[Ernest Brooks (photographer)|Ernest Brooks]].]]
[[File:Bundesarchiv Bild 101I-198-1363-29A, Russland, Artillerie-Beobachtung.jpg|thumb|A team of German [[artillery observer]]s using  periscope binoculars, 1943]]

[[Johannes Gutenberg]], known for his contribution to printing technology, marketed a kind of periscope in the 1430s to enable pilgrims to see over the heads of the crowd at the [[Anniversary#Latin-derived numerical names|vigintennial]] religious festival{{which?|date=October 2014}} at Aachen. [[Johannes Hevelius]] described an early  periscope with lenses in 1647 in his work ''[[Selenographia, sive Lunae descriptio]]'' [Selenography, or an account of the Moon]. Hevelius saw military applications for his invention.

In 1854 [[Hippolyte Marié-Davy]] invented the first naval periscope, consisting of a vertical tube with two small mirrors fixed at each end at 45°. [[Simon Lake]] used periscopes in his submarines in 1902. [[Howard Grubb|Sir Howard Grubb]] perfected the device in World War I.<ref>
[http://inventors.about.com/library/inventors/blperiscope.htm The History of the Periscope – Inventors – About.com]</ref> [[Morgan Robertson]] (1861–1915) claimed<ref>Morgan Robertson, ''The Schenectady Gazette'', Friday morning, March 26, 1913, p.19
</ref> to have tried to patent the periscope: he described a submarine using a periscope in his fictional works.

Periscopes, in some cases fixed to [[Periscope rifle|rifles]], served in [[World War I]] (1914-1918) to enable soldiers to see over the tops of [[Trench warfare|trenches]], thus avoiding exposure to enemy fire (especially from snipers).<ref name="FIRST">
''First World War'' – Willmott, H.P.; [[Dorling Kindersley]], 2003, Page 111
</ref>

During [[World War II]] (1939-1945), artillery observers and officers used specifically-manufactured periscope binoculars with different mountings. Some of them also allowed estimating the distance to a target, as they were designed as [[stereoscopic rangefinder]]s.{{citation needed|date=February 2014}}

==Armored vehicle periscopes==
[[Tank]]s use periscopes extensively: they enable drivers or tank commanders to inspect their situation without leaving the safety of the tank. An important development, the [[Rudolf Gundlach|Gundlach rotary periscope]], incorporated a rotating top; this allowed a tank commander to obtain a 360-degree field of view without moving his seat.  This design, patented by [[Rudolf Gundlach]] in 1936, first saw use in the [[Polish Army|Polish]] [[7-TP]] light tank (produced from 1935 to 1939). As a part of Polish–British pre-[[World War II]] military cooperation, the patent was sold to Vickers-Armstrong for use in [[British Army|British]] tanks, including the ''[[Crusader tank|Crusader]]'', ''[[Churchill tank|Churchill]]'', ''[[Valentine tank|Valentine]]'',  and ''[[Cromwell tank|Cromwell]]'' models.  The technology was also transferred{{according to whom|date=July 2014}} to the [[United States Army|American Army]] for use in its tanks, including the ''[[M4 Sherman|Sherman]]''. The [[Red Army|USSR]] later copied the design and used it extensively in its tanks (including the [[T-34]] and [[T-70]]); [[Wehrmacht|Germany]] also made and used copies.<ref>
Not Only Enigma... Major Rudolf Gundlach (1892-1957) and His Invention), Warsaw-London, 1999</ref> 
The  Embedded Image Periscope (EIP) designed and patented by Kent Periscopes provides standard unity vision periscope functionality for normal daytime viewing of the vehicle surroundings plus the ability to display digital images from a range of on-vehicle sensors and cameras (including thermal and low light) such that the resulting image appears "embedded" internally within the unit and projected at a comfortable viewing position.

==Naval use==
Periscopes allow a submarine, when submerged at a relatively shallow depth, to search visually for nearby targets and threats on the surface of the water and in the air. When not in use, a submarine's periscope retracts into the hull. A submarine commander in tactical conditions must exercise discretion when using his periscope, since it creates a visible wake (and may also become detectable by radar {{Citation needed|date=July 2014}} ), giving away the sub's position.

[[File:Submarine periscope.jpg|thumb|left|Officer at periscope in control room of a U.S. Navy submarine in World War II. The officer pictured is Captain Raymond W. Alexander, Sr. and the photo was taken in 1942.]]
[[File:Attack periscope.png|thumb|left|Submarine monocular attack periscope.]]

The Frenchman Marie Davey built a simple, fixed naval periscope using mirrors in 1854.  Thomas H. Doughty of the US Navy later invented a prismatic version for use in the [[American Civil War]] of 1861-1865.

Submarines adopted periscopes early.  Captain [[Arthur Krebs]] adapted two on the experimental French submarine ''[[Gymnote]]'' in 1888 and 1889. The Spanish inventor Isaac Peral equipped his submarine ''[[Peral Submarine|Peral]]'' (developed in 1886 but launched on September 8, 1888) with a fixed, non-retractable periscope that used a combination of prisms to relay the image to the submariner. (Peral also developed a primitive gyroscope for submarine navigation and pioneered the ability to fire live torpedoes while submerged.<ref>[http://pedrocurto.com/1.html El Arma Submarina Española]</ref>{{Verify credibility|date=March 2014}})
The invention of the collapsible periscope for use in submarine warfare is usually credited{{By whom|date=January 2014}} to [[Simon Lake]] in 1902. Lake called his device the ''omniscope'' or ''skalomniscope''.  There is also a report{{Citation needed|date=January 2014}} that an Italian, Triulzi, demonstrated such a device in 1901, calling it a ''cleptoscope''.

[[File:Torpedoed Japanese destroyer HD-SN-99-02974.JPEG|thumb|A torpedoed Japanese destroyer, photographed through the periscope of U.S.S. ''Nautilus'', June 1942]]

{{As of|2014}} modern submarine periscopes incorporate lenses for magnification and function as telescopes.  They typically employ [[Prism (optics)|prism]]s and [[total internal reflection]] instead of mirrors, because prisms, which do not require coatings on the reflecting surface, are much more rugged than mirrors.  They may have additional optical capabilities such as [[range finding|range-finding]] and targeting.  The mechanical systems of submarine periscopes typically use hydraulics and need to be quite sturdy to withstand the drag through water.  The periscope chassis may also support a radio or radar antenna.

Submarines traditionally had two periscopes; a navigation or observation periscope and a targeting, or commander's, periscope. Navies originally mounted these periscopes in the [[conning tower]], one forward of the other in the narrow hulls of diesel-electric submarines. In the much wider hulls of {{as of|2014 | alt = recent}} US Navy submarines the two operate side-by-side.  The observation scope, used to scan the sea surface and sky, typically had a wide field of view and no magnification or low-power magnification. The targeting or "attack" periscope, by comparison, had a narrower field of view and higher magnification. In World War II and earlier submarines it was the only means of gathering target data to accurately fire a [[torpedo]], since [[sonar]] was not yet sufficiently advanced for this purpose (ranging with sonar required emission of an electronic "ping" that gave away the location of the submarine) and most torpedoes were unguided.

21st-century submarines do not necessarily have periscopes.  The [[United States Navy]]'s [[Virginia class submarine|''Virginia''-class submarines]] and the [[Royal Navy]]'s [[Astute class submarine]]s instead use [[photonics mast]]s, pioneered by the [[Royal Navy]]'s [[HMS Trenchant (S91)|HMS ''Trenchant'']], which lift an electronic imaging sensor-set above the water.  Signals from the sensor-set travel electronically to workstations in the submarine's control center.  While the cables carrying the signal must penetrate the submarine's hull, they use a much smaller and more easily sealed—and therefore less expensive and safer—hull opening than those required by periscopes. Eliminating the telescoping tube running through the conning tower also allows greater freedom in designing the pressure hull and in placing internal equipment.

==See also==
* [[Coincidence rangefinder]]
* [[Relay lens]]
* [[Rangefinder]]
* [[Vickers Tank Periscope MK.IV]]

==References==
{{Reflist|30em}}

==External links==
{{Commons}}
* [http://www.maritime.org/fleetsub/pscope/index.htm ''The Fleet Type Submarine Online: Submarine Periscope Manual''] United States Navy '''Navpers 16165''', June 1979
* [http://www.phy.ntnu.edu.tw/ntnujava/index.php?topic=1348.0 ''Simulation of a Periscope''] at [http://www.phy.ntnu.edu.tw/ntnujava/index.php NTNUJAVA Virtual Physics Laboratory]

{{Wikipedia|Periscope}}
[[Category:Optical devices]]
[[Category:British inventions]]
[[Category:1902 introductions]]
[[Category:Submarine components]]