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54 War and the commune fighting and was twice wounded in Dec. 1870, at the battle of Patay, and again in April 1871 before Paris. He was made a major (chef d'escadron) in Jan. 1883, lieutenant-colonel in 1890, colonel in 1895, general of brigade in 1900, and general of division in 1903. In the period between 1900 and his retirement, Gen. Percin was a very active reformer and innovator in the tactics of the artillery arm. The typical field-artillery tactics of 1914, based on time shrapnel covering fire, and on the intimate liaison of infantry and artillery, were largely due to his work, and after his retirement he continued a very active student and critic of artillery operations. His marked personality, and his political opinions as a radical, however, made him many enemies. At the outbreak of the World War he was recalled to service, but only as commander of the Lille region, and he was involved in the controversies connected with the evacuation of Lille. Later he was employed for a short period as inspector-general of artillery units. In Jan. 1915 he was placed in the reserves. He was given the Grand Cross of the Legion of Honour in June 1917.

PERCIVAL, JOHN (1834-1918), English divine, was born in Westmorland Sept. 27 1834, the son of William Percival, of a yeoman family. He was educated at Appleby and Queen's College, Oxford, where he took his degree in 1858. In 1860 he was ordained, and went to Rugby as an assistant master. In 1863 he went to Clifton College as first headmaster, remaining there for 15 years. He was elected president of Trinity College, Oxford, in 1878, and while in this position took much interest in the foundation of Somerville College for women. In 1887 he became headmaster of Rugby, and in 1895 was appointed to the bishopric of Hereford. His broad churchmanship placed him in opposition to the dominant tendency in the Church of England, and he was also a strong and militant Liberal in politics, being an ardent advocate of the disestablishment of the Church in Wales. He died at Oxford Dec. 3 1918.

'''PEREZ CALEBS. BENITO''' (1843-1920), Spanish novelist (see 21.139), died Jan. 4 1920. The final series of his Episodios Nacionales contained Espana sin rey (1908); Espaiia Trdgica (1909) ; Amadeo /. (1910) ; La Primer a Reptblica and De Cartago & Sagunto (1911); and Canovas (1912). He also published various plays and novels, including El Caballero encantado (1909), and Santa Juana de Castilla (1918).

See L. Olmet and A. Carraffa, Los Grandes Espanoles, vol. i., Galdos (1912).

PERIODICALS: see NEWSPAPERS.

PERISCOPE. An optical instrument used in land warfare and in submarine navigation, enabling an observer to see in all directions while remaining under cover or submerged. Essen- tially it consists in an optical system of lenses and mirrors, or mirrors alone, the upper part of which projects from cover, or from the deck of a submarine, while the observer looks into the lower end, receiving an image of the surrounding country or sea by reflection down a tube.

The use of reflecting mirrors for the purpose of observing from cover is no novelty, and during the trench warfare of the Crimean War 1854-5 a device was patented which scarcely dif- fers from the simple mirror periscope of the World War. From the beginning of the 2oth century, however, the practical intro- duction of submarine navigation brought about the develop- ment of new elaborate periscopes of great length and provided with an optical system of lenses, which were built into the structure of the submarine. At the same time, on land, the new necessities imposed on field artillery by the growing use of covered positions led to the development of scissors-telescopes (see RANGEFINDERS) and panorama-telescopic sights (see SIGHTS), in which the optical system was arranged with the tube of the telescope vertical and the object-glass and eyepiece systems at right angles to the axis of the tube. And in the World War, while optical instruments of this kind were elaborated and improved, the periscope as such came into use for the infantry garrisoning trenches. Manufactured in large quantities it soon became an essential part of infantry as well as of artillery and machine-gun equipment. In the present article, periscopes for land service and those forming part of the equipment of sub- marines will be described in turn.

(1) Land-service Periscopes vary much both in design and size, some being only a few inches long while others are as much as 80 ft. in length. The simplest form of periscope, and that most generally used by troops, consisted of a tube, rectangular in section, provided with two mirrors, the upper of which, inclined at an angle of 45 to the axis of the tube, reflected the image of the foreground verti- cally downwards to a second mirror, also inclined to the axis at 45 into which the observer looked. But in order to obtain an adequate field of view, the mirrors, and therefore the box, had to be made somewhat large, and in the close-quarters conditions of trench warfare even the few inches by which they projected over the parapet or other cover made them sufficiently obvious to draw fire. Less conspicuous periscopes were therefore designed, and these, in order to take in enough of the foreground, had to be provided with a magnifying as well as a reflecting system. In the British service half of the stereoscopic scissors-telescope used in range- finders was frequently employed as a periscope. Its lower end was fitted with a ball-and-socket joint to enable it to be laid in any direction, and be- neath this is a screw which can be screwed by means of a small lever into a piece of wood embed- ded in the side of a trench.

In an ingenious periscope designed by Messrs. R. & J. Beck of London (fig. i) the upper prism is supported above the telescopic system on a flat strip of metal which can be slid through side sup- ports on the body of the pcrisco[>e. When in use, the prism is supported some inches above the body and is the only part that can be seen by the enemy. If it is shot away, it Ccin be replaced in a few seconds. When the periscope is not in use, the prism is lowered and protects the upper lens in the body.

Small German periscopes were usually I metre or J a metre in length and had two eyepieces giving magnifications 10 and 15 diameters. The optical system is shown in fig. 2. They could be either held in the hand or attached to a direction stand.

A neat rainguard made of sheet metal, to the same curve as the body of the periscope and al- most 8 inches long, is attached to the upper prism box by two spring straps. When in use, it is held at right angles to the periscope above the upper window by a bayonet catch ; when not in use, it is lowered and sprung round the body of the periscope just below the upper prism box.

Many periscopes of considerable length and special design have been used, to en- able observations being made in compara- tive safety from behind large objects, e.g. houses, trees, etc., or from folds in the ground. Of these the most remarkable is the German Giant Periscope, two speci- mens of which exhibited in the collection of trophies in the Imperial War Museum, Crystal Palace, have excited considerable popular interest. This periscope is con- siderably larger than any others, and was designed for observing over obstacles of between 9 and 26 metres in height. It can be rapidly erected or taken down and transported on its carriage. It consists mainly of 3 parts, viz. a steel telescopic mast and upper and lower optical sys- tems which are attached to it.

The telescopic mast is carried in trun- nions on the carriage, and travels closed and in a horizontal position. Gears are provided for elevating, levelling, aligning the upper and lower optical systems, ad- justing the inclination of the reflector and rotating the mast around a vertical axis so that observations may be made and azi- muth angles taken in all directions. The

FIG. I