From Classic Encyclopedia 1911
"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. Essentially 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 differs from the simple mirror periscope of the World War. From the beginning of the 20th century, however, the practical introduction of submarine navigation brought about the development 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 submarines will be described in turn.
(i) 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 vertically 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 ether 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 rangefinders 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 beneath this is a screw which can be screwed by means of a small lever into a piece of wood embedded in the side of a trench.
In an ingenious periscope designed by Messrs.
R. & J. Beck of London (fig. 1) the upper prism is supported above the telescopic system on a flat strip of metal which can be slid through side supports on the body of the periscope. 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 can 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 1 metre or 2 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 almost 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 enable observations being made in comparative 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 specimens of which exhibited in the collection of trophies in the Imperial War Museum, Crystal Palace, have excited considerable popular interest. This periscope is considerably 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 systems which are attached to it.
The telescopic mast is carried in trunnions on the carriage, and travels closed and in a horizontal position. Gears are provided for elevating, levelling, aligning the upper and lower optical systems, adjusting the inclination of the reflector and rotating the mast around a vertical axis so that observations may be made and azimuth angles taken in all directions. The telescopic mast consists of 8 tubes. The lower one is attached to the carriage, and the upper one is pulled out as far as it will go and retained in position by catches before the mast is raised. The other six are connected to each other and to the lowest one by wire cables and pulleys in such a way that when the cable which connects the two lowest tubes is wound in by means of a winch, each of the tubes except the fixed one will rise within the next one through the same distance. When erect, the mast is steadied by means of three guy ropes.
The details of the optical systems are as follows: The rays from a distant object after passing through a protecting window A (fig. 5) are reflected by a mirror B down the centre of the conical casing which contains the upper optical system and is attached to the top of the mast. The two achromatic lenses, C and D, bring the rays to a focus on the plane surface of the large lens, E, forming an image there. Immediately above this plane surface and almost touching it is a system of wires which enables angular distances from the centre of the field to be read at the eyepiece below. The mirror can be elevated and depressed by means of a flexible shaft which passes up the centre of the mast and actuates gear attached to the mirror frame. From the large lens, E, the rays pass through the open air for a considerable distance, depending upon how much the mast has been raised, to the lower optical system. Here they pass through the lenses and prism shown into one of the eyepieces, F. By moving the lens G up and down the image can be formed in the correct position for the eyepiece at all extensions of the mast.
There are three eyepieces which are mounted on a revolving sleeve in such a way that any one of them can be quickly brought into use, to give the magnification suitable to the height of the mast. (Low power from 3 to 8; medium from 5 to 14; high from 7 to 21.) Each eyepiece is provided with a dummy eyepiece which comes opposite to the eye which is not observing and permits of it being kept open. This lessens eyestrain. Coloured anti-glare glasses are provided.
(2) Submarine Periscopes. - When a submarine is completely submerged the occupants are not able to see through the water except under very exceptional conditions. In the Mediterranean on a sunny day it is possible to see for several yards through the water at about 25 ft. below the surface. In the North Sea, and usually, it might be said that once the boat is submerged, direct observation through the water is impossible. In the very earliest submarines a cupola was built on the top of the hull, which was kept just above the surface when it was desired to take observations. To re- duce resistance, these cupolas were made telescopic in the French submarines "Gymnote "and the " Gustave Zede, " but the arrangement proved unsatisfactory. An optical tube replaced this cupola in the "Gustave Zede," and comprised a short tube (on top of the submarine) with a lens to close the top end, which was kept just above the surface when running submerged. Horizontal rays of light entering at the top were reflected by a prism down the tube and focussed on to a sheet of paper in front of the helmsman inside the submarine. This gave him a limited view of what lay directly ahead. The word " periscope" was first applied to this instrument.
The modern submarine periscope consists essentially of a long tube, the top of which is just above the water when diving, while the lower end passes through a stuffing box on the shell of the boat into the control-room. The top is closed by a pressure-tight window, inside of which is a prism which reflects the light rays vertically down the tube to a prism at the bottom end, where they are reflected in a horizontal direction and focussed in an eyepiece attached to the bottom of the tube. Thus the commander can see what is happening on the surface when navigating the submarine some 20 ft. or more below it.
The greater the depth of submergence the less the disturbance made by the submarine on the surface of the water, and the greater the immunity from gun-fire, ramming, etc.; also in a sea-way the deeper the submarine the more readily is it con trolled. For these reasons the length of the peri- (FIG. 6 scope has steadily increased, and the dimensions of the upper end have as steadily decreased. In creased length necessitated an increase in the diameter of the main tube to limit the amplitude of the vibrations caused by being pushed through the water. A typical instrument in the British navy was 30 ft. long, with a 5.9 in. diameter main tube, and the top 3 ft. of the upper tube 2 in. diameter. For the German " U " boats Messrs. Zeiss made a periscope 7 metres long, main tune 150 mm. (5.9 in.), and about 2 ft. 6 in. of the top tube 30 mm. (1.2 in.) diameter.
The main tube must be accurately machined as it has to be readily trained in its stuffing-box as well as be water-tight in all positions, through a considerable range of vertical travel. The modern practice is to take rapid observations rather than to keep the periscope above the water all the time. To facilitate this mechanical lifting, gear is provided which is readily controlled, and can raise or lower the periscope at a speed approaching 25 ft. per minute.
The field of view is usually about 40° at a magnification of 1 5. It is therefore necessary to train the periscope round when taking observations on different bearings. This can be done in two ways, either by rotating the optical train inside the main tube, or, as is more usually the case, rotating the whole periscope. With the increase in weight and size the effort required has increased, and power training has sometimes been necessary. Where possible, however, by refinements in workmanship, etc., efforts are made to keep the torque required so low as to be within the power of the operator. Usually a revolving scale round the edge of the field shows the direction of the view. If the whole instrument moves, the operator also moves round with it in the boat, and knows at once in which direction he is looking.
Owing to proximity to the magnetic compass the whole of the tube must be non-magnetic. High-strength bronze was used in the earlier practice in the British navy. A special nickelchrome steel was manufactured and machined by Messrs. Krupp for use for the outer tube of the German navy periscope used before the war, and a similar steel was developed and used in the British service, but it is costly and more difficult to machine to the required accuracy than is the case with bronze.
The use of aircraft for anti-submarine work led to the demand for a periscope which could be used for looking overhead. In the skysearching periscope the upper prism can be rotated by mechanism inside the periscope, so that aerial observations can be readily made before the submarine " breaks surface." To enable a distant ship or other object to be examined more closely it is possible in some periscopes to change the magnification from a normal power of 1.5 to a power of 6. This, and the sky searching previously mentioned, means increased internal gearing and a larger upper tube. As a rule every submarine has at least two periscopes, one unifocal with a small upper tube and the other bifocal and sky-searching with a larger upper tube.
Whilst in the British service sky searching up to right overhead was arranged for, German periscopes as a whole are limited to 20° above the horizontal. In each case zo° depression is allowed for to follow the roll of the submarine.
For special purposes other features are added, such as rangefinding attachments, etc. A " night " periscope for use at dusk has been developed. It is much shorter than the typical instruments described, so that the maximum brightness of image is obtained.
The periscope when installed in the submarine is used for two purposes: (a) general observation for submerged navigation; (b) for correctly aligning the submarine when firing a torpedo at a target. In connexion with (a) the principal requirement is clearness of field. Continuous use of a periscope is very trying for the observer's eyes, and for use in bright weather light-filter screens are provided to reduce the glare. It has also been found that in foggy and misty weather suitable colour screens are of assistance. These screens are usually embodied in the eyepiece. For purposes of torpedo attack the periscope is used as a range-finder to determine the distance the target is away, and also in connexion with tables to determine the correct time to fire the torpedo, allowing for the speed of the enemy, course, etc. Officers of submarines have devised various mechanical devices to avoid calculations, and these have been added to the periscope. Although two periscopes are provided when attacking, one only would be shown for short periods to get check observation so as to prevent the wash of the upper tube revealing the proximity of the submarine.