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A periscope is an instrument for observation from a concealed position. In its simplest form, it is a tube in each end of which are mirrors set parallel to each other at 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. Periscopes are also used in some gun turrets and armored vehicles.

More complex periscopes, using prism instead of mirrors, and providing magnification, are used on submarines. 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

British trench periscope, Cape Helles 1915
Johann Gutenberg, better known for his contribution to printing technology, marketed a periscope in the 1430s to enable pilgrims to see over the heads of the crowd at the vigintennial religious festival at Aachenmarker. Simon Lake used periscopes in his submarines in 1902. Sir Howard Grubb perfected the device in World War I. Morgan Robertson (1861-1915) claimed to have tried to patent the periscope: he described a submarine using a periscope in his fictional works.

Periscopes, in some cases fixed to rifles, were used in World War I to enable soldiers to see over of the tops of trenches, so that they would not be exposed to enemy fire (especially from snipers).

Periscopes are extensively used in tanks, enabling drivers or tank's commanders to inspect the situation without leaving the safety of the tank. An important development, Gundlach's periscope, had a rotating top, allowing a tank commander to obtain 360 degree view without moving.The design was first used in the Polish 7-TP light tank. Shortly before the war it was given to the British and was used in most tanks of WWII, including the British Crusader, Churchill, Valentine, and Cromwell and the American Sherman. The design was later copied and used extensively in tanks of the USSR (including the T-34 and T-70) and Germany.Periscopes proved useful in trench warfare, as seen in the illustrations, representative of action at Gallipoli.

Naval use

Periscopes allow a submarine, submerged at a shallow depth, to search for targets and threats in the surrounding sea and air. When not in use, the periscope is retracted into the hull. A submarine commander in tactical conditions must exercise discretion when using his periscope, since it creates an observable wake and may be detectable to radar, giving away the sub's position.

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

The invention of the collapsible periscope for use in submarine warfare is usually credited to Simon Lake in 1902, who called his device the omniscope or skalomniscope. There is also a report that an Italian, Triulzi, demonstrated such a device in 1901 calling it a cleptoscope.

In another early example of naval use of periscopes, Captain Arthur Krebs adapted two on the experimental French submarine Gymnote in 1888 and 1889. Perhaps the earliest example comes from the Spanish inventor Isaac Peral on his submarine in 1888 Peral - developed in 1886 but launched on September 8, 1888. Peral's fixed, non-retractable periscope used a combination of prisms to rely the image to the submariner, but his submarine pioeneered the ability to fire live torpedoes while submerged. Peral also developed a primitive gyroscope for his submarine navigation.

A torpedoed Japanese destroyer, photographed through periscope of U.S.S.
Wahoo or U.S.S.
Nautilus, June 1942.

A modern submarine periscope incorporates lenses for magnification and functions as a telescope. It typically employs prism and total internal reflection instead of mirrors, because prisms, which do not require coatings on the reflecting surface, are much more rugged than mirrors. It may have additional optical capabilities such as range-finding and targeting. The mechanical systems of submarine periscopes typically use hydraulic power and need to be quite sturdy to withstand the drag through water. The periscope chassis may also be used to support a radio or radar antenna.

Submarines traditionally had two periscopes: a navigation or observation periscope and a targeting, or commander's, periscope. Early 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 US Navy submarines, the two operate side-by-side. The observation scope was used to scan the sea surface and sky and 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.

However, 21st century submarines do not necessarily have periscopes. The United States Navy's Virginia-class submarines instead use photonics masts, 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 placing internal equipment.

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