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 mirrors 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 turrets and in armoured vehicles.
More complex periscopes, using prisms and/or advanced fiber optics instead of mirrors, and providing magnification, operate on submarines 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.
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 vigintennial religious festival[which?] 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. 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.
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 rangefinders.
Armored vehicle periscopes
Tanks use periscopes extensively: they enable drivers or tank commanders to inspect their situation without leaving the safety of the tank. An important development, the 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 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 tanks, including the Crusader, Churchill, Valentine, and Cromwell models. The technology was also transferred[according to whom?] to the American Army for use in its tanks, including the Sherman. The USSR later copied the design and used it extensively in its tanks (including the T-34 and T-70); Germany also made and used copies.
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.
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 ), giving away the sub's position.
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 (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.[unreliable source?]) The invention of the collapsible periscope for use in submarine warfare is usually credited[by whom?] to Simon Lake in 1902. Lake 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.
As of 2009[update] modern submarine periscopes incorporate lenses for magnification and function as telescopes. They typically employ prisms 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 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.