A periscope is a device 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° angle. This form of periscopic device, 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 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.
Early examples Johannes Hevelius described an early periscope (which he called a "polemoscope") 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. Mikhail Lomonosov invented an "optical tube" which was similar to a periscope. In 1834, it was used in a submarine, designed by Karl Andreevich Schilder. 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. Periscopes, in some cases fixed to rifles, served in World War I (1914–1918) to enable soldiers to see over the tops of trenches, thus avoiding exposure to enemy fire (especially from snipers). The periscope rifle also saw use during the war – this was an infantry rifle sighted by means of a periscope, so the shooter could aim and fire the weapon from a safe position below the trench parapet. 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 and armoured vehicles use periscopes: they enable drivers, tank commanders, and other vehicle occupants to inspect their situation through the vehicle roof. Prior to periscopes, direct vision slits were cut in the armour for occupants to see out. Periscopes permit view outside of the vehicle without needing to cut these weaker vision openings in the front and side armour, better protecting the vehicle and occupants. A protectoscope is a related periscopic vision device designed to provide a window in armoured plate, similar to a direct vision slit. A compact periscope inside the protectoscope allows the vision slit to be blanked off with spaced armoured plate. This prevents a potential ingress point for small arms fire, with only a small difference in vision height, but still requires the armour to be cut. In the context of armoured fighting vehicles, such as tanks, a periscopic vision device may also be referred to as an episcope. In this context a periscope refers to a device that can rotate to provide a wider field of view (or is fixed into an assembly that can), while an episcope is fixed into position. Periscopes may also be referred to by slang, e.g. "shufti-scope".
Gundlach and Vickers 360-degree periscopes An important development, the Gundlach rotary periscope, incorporated a rotating top with a selectable additional prism which reversed the view. This allowed a tank commander to obtain a 360-degree field of view without moving his seat, including rear vision by engaging the extra prism. 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 where it saw further development for use in British tanks, including the Crusader, Churchill, Valentine, and Cromwell models as the Vickers Tank Periscope MK.IV. The Gundlach-Vickers technology was shared with the American Army for use in its tanks including the Sherman, built to meet joint British and US requirements. This saw post-war controversy through legal action: "After the Second World War and a long court battle, in 1947 he, Rudolf Gundlach, received a large payment for his periscope patent from some of its producers." The USSR also copied the design and used it extensively in its tanks, including the T-34 and T-70. The copies were based on Lend-Lease British vehicles, and many parts remain interchangeable. Germany also made and used copies.
Periscopic gunsights Periscopic sights were also introduced during the Second World War. In British use, the Vickers periscope was provided with sighting lines, enabling front and rear prisms to be directly aligned to gain an accurate direction. On later tanks such as the Churchill and Cromwell, a similarly marked episcope provided a backup sighting mechanism aligned with a vane sight on the turret roof. Later, US-built Sherman tanks and British Centurion and Charioteer tanks replaced the main telescopic sight with a true periscopic sight in the primary role. The periscopic sight was linked to the gun itself, allowing elevation to be captured (rotation being fixed as part of rotating turret). The sights formed part of the overall periscope, providing the gunner with greater overall vision than previously possible with the telescopic sight. The FV4201 Chieftain used the TESS (Telescopic Sighting System) developed in the early 1980s that was later sold as surplus for use on the RAF Phantom aircraft.
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