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Rh act to complicate further the design of an efficient sighting system. No description of such a sight, therefore, is possible within the limits of this article. For reasonable accuracy of fire, the mounting must be level. Anti-aircraft mountings are generally fitted with training arcs; in some cases a second set of indicators is fitted to enable horizontal ranges and quadrant angles to be used. A sight- ing system which also embodies the principle of the independent line of sight has been tried in the British service.

The panoramic sight is used witji field ordnance for indirect lay- ing for direction, from a position out of view of the target. The layer is enabled to take advantage of any auxiliary aiming-mark and to lay without exposing himself. The sight may be used also for direct laying.

A typical panoramic sight is shown in fig. I. It is a prismatic telescope mounted in a vertical tube having a rotating hood at the upper, and a horizontal tube at the lower end; the hood is mounted on a horizontal graduated dial-plate with which it may be

revolved through a complete circle, by worm-gearing; the worm-spindle is mounted in an eccentric to enable the worm to be thrown out of gear for quick setting; on the periphery of the dial-plate are graduations read by a pointer fixed to the vertical tube; while the ends of the worm-spindle are fitted with micrometer-heads to read smaller divisions. A prism mounted within the hood can be rotated about a horizontal axis by means of a worm-spindle in mesh with a toothed arc on the prism holder; this permits of a wide selection of aiming-points in the vertical plane. The optical ar- rangements are shown in fig. 2. They consist in a reflecting prism mounted within a holder in the hood; in the vertical tube, an erecting prism controlled by differential gear to move at half the rate of the dial-plate and hood ; a fixed object-glass below the erecting prism ; and a reflect- ing prism below the object-glass to reflect light along the hori- zontal tube; in the horizontal tube, a glass diaphragm en- graved with horizontal and ver- tical lines, and in rear of this, eye-lenses to give a magnified image. The rays of light from the object enter through a plain glass window and are reflected downwards by the prism in the hood; they pass through the erecting prism, the purpose of which is to give an erect image, whatever the rotation of the hood, and are focused by the object-glass; they then pass to the lower prism and are reflected along the horizontal tube; the eye-piece magnifies the image

FIG. 3

which is seen on the cross-lines of the diaphragm, the latter being illuminated by a plain window let into the horizontal tube.

Other forms of dial sight are, the simple graduated and flexible dial fitted with an open sighting-bar; and the collimator, which exists in several forms. In principle an eye-lens and ground-glass window are mounted at the ends of a tube, an arrow or cross on the window being at the principal focus of the lens. With one eye applied near the lens, the cross will be seen, but there is no vision beyond the cross due to the ground-glass; the other eye sees the aiming-point. The gun is traversed until the cross appears superimposed on the aiming-point. Both eyes are used, and are viewing approximately parallel rays. Unlike ordinary open sights, there are not three points at varying distances to be brought simultaneously into alignment, but the eyes view two points under practically the same conditions.

Sights for Rifles and Machine-Guns. The improved ballistics of rifles generally, on the one hand, and the shortening of the sight radius due to shortening of the barrel, on the other, have made the question of better sights a very urgent one. Attempts to retain the old sight radius by bringing the rear sight closer to the eye resulted in the discovery that it was impossible effectively to use an open sight nearer than twelve to fourteen inches from the eye (owing to the inability to focus on rear sight, front sight, and target at the same time) and led to the reintroduction of the aperture or peep sight, in itself an old invention. In this the rear sight is placed close to the eye, and no attempt is made to focus on the aperture. The

eye looks through the aperture and instinctively centres the front sight. Any blur around the aperture will not impair the accuracy as it is equal on all sides of the opening. If the eye is now focused on the target, the front sight is at sufficient distance to afford very good definition. Another advantage is that the accuracy of the sight is not affected by slight changes of light. _ Aperture sights are of two general types: the disc aperture con- sisting of a large disc, which shuts off all view of the target except that seen through a peep hole of from -03 to -06 in. in diameter; and the Lyman type which has a thin rim with peep hole about -oio in. in diameter. Disc sights, which are used principally for target shooting, are unsuitable on account of their limited field for moving- targets, or where quick aiming is required.

In the Lyman type the entire target and its surroundings may be seen while aiming; it may be used in poor lights where open sights cannot be used, and moving objects may be quickly aimed at. The sights on the British 1914 rifle (U.S. Rifle Model of 1917), and the Browning automatic rifle are of this type (fig. 3). The aperture (a) is o- 1 in. in diameter, and is made through a disc of about twice the diameter of the aperture. The battle-sight 1 is vertical and in use when the leaf sight is flat and vice versa ; the sizes of aperture and ring are the same in both. This sight was found very satisfactory during the World War. It has no lateral adjustment, service expe- rience having shown that the sights were satisfactory without this refinement. An aperture o- 1 in. in diameter is thought by many to be too large, -070 in. to -080 in. being the size most favoured. The U.S. 1903 Rifle (Springfield), and the now abandoned Ross rifle are the only other military rifles with an aperture sight. In the case of the Springfield, which has also sight notches of the usual type on the slide, it is found that, although the aperture is very small (0^05 in.), and too far from the eye, it is always preferred to the V. for fine shooting. The superiority of the aperture sight over the open sight is in fact now generally admitted. It is almost universally used by rifle experts, and the delay in its general adoption for military use is due to the impracticability of effectively and cheaply applying it to rifles now in existence.

Optical rifle fore sights designed for use with aperture rear sights have recently been placed on the market. These sights consist of a dioptric convex lens of magnification of about 2j diameters, with a spot or ring in the centre for use as a bead. The lens when viewed 3 or 4 ft. from the eye gives an upright magnified image of distant objects. An aperture rear sight, preferably of the disc type, with an aperture of 0-03 to 0^04 in. diameter, or a lens rear sight should be used. Better definition as well as magnifying power is claimed for these sights.

Luminous sights for night use have been experimented with and used to a limited extent on machine-guns and rifles for military pur- poses. These sights, as a rule, consist simply of metal sheaths which slip over the front sight and rear sight and have suitable containers for the luminous material used. For periscope sights see RIFLES AND LIGHT MACHINE-GUNS.

Telescope sights are often applied to sporting rifles and military rifles used for special purposes. These sights consist of a telescope with a reticule for aiming which takes the place of a front sight. The magnification is usually from 2-5 to 6 diameters; those from 2-5 to 3 power are considered the best for general purposes as they give a larger field and can be used in poor light. Adjustments for range are made either by moving the crosshairs only, as in most German rifle telescopes, or by means of holding brackets with adjusting screws which give both vertical and lateral adjustment; the latter type being more accurate. The advantages claimed for telescopic rifle sights are that they permit more accurate aim by magnifying the errors of holding the rifle, also that they allow objects to be seen more distinctly, particularly in a poor light. The latter is probably their greatest advantage ; their superiority to a good aperture sight not being as great as is popularly supposed.

The Vickers and Browning machine-gun sight embodies the prin- cipal small-arms-sight developments of recent years. The slide, which is made to travel obliquely in the leaf to compensate for drift, has a revolving disc with apertures of various sizes which may be used for different ranges and light conditions. A fine adjustment for ele- vation and a windage adjustment are provided, also an open battle- sight. The fore sight is a blade, open or hooded.

The Lewis gun has a simple tangent aperture sight without drift or windage adjustments, but with a vertical adjusting screw for fine adjustments in elevation. The sights used for direct fire on Continental European machine-guns are all of the open type and are usually without correction for either drift or windage.

Tubular sights, which consist simply of a straight piece of tubing, are used to some extent on ta_nk machine-guns, as these sights only require a very small opening in the armour plates for their effective use. The latest tendency, however, is to provide tank machine- guns with a telescopic sight of special construction.

Anti-aircraft machine-gun sights, which compensate for the drift and trajectory of the bullet and offset the speed and direction in

1 A battle-sight is one which requires no adjustment for distance, or rather is permanently set to a certain range, inside which the bullet in its flight never rises more than a man's height above the line of sight.