RANGE-FINDERS are used to enable the fire of guns and rifles to be directed with maximum effect. They are indispensable to warships (see GUNNERY: Naval), coast forts and anti-aircraft guns. Artillery in the field generally relies on accurate maps when such are available, but in open and unmapped country a range finder is very useful. Infantry require a small and light instru ment. In ships weight is unimportant, there are limits to size, while ranges may be 20 miles or more. Coast fort ranges are similar to those of ships, but practically unlimited space is available.
The instruments which have been designed to meet these varied conditions may be divided into three types : (a) the self-contained or monostatic, also called short-base or one-man; (b) the long horizontal base system, consisting of two azimuth (horizontal angle measuring) instruments and a plotter; and (c) the de pression range-finder, a single instrument depending for base on its height above the sea.
In each type the base is the base of a triangle, of which the apex is the target. Adjustment of one or both of the angles at the base solves the triangle and usually sets the indicator of the range scale. In type (a) one base angle is a right angle and, since an exceedingly delicate means of adjusting the other is possible, the base may be small. In type (b) adjustment is necessarily coarse, hence a very long base is required, but there are great advantages. In type (c) one angle is again a right angle, while the depression angle can be adjusted with an accuracy inter mediate between the other two types. Ships and field army ar tillery and infantry use only monostatic range-finders, while coast forts may have all three types. Anti-aircraft guns use height finders which will be described later. (K. F. D.) (a) MONOSTATIC RANGE-FINDERS Short-base or monostatic range-finders are those which are entirely self-contained, the instrument itself constituting the base. From a scientific point of view they are perhaps the most inter esting of the three types and have been brought very near to human limits of perfection. This is largely due to the work of the British inventors, Messrs. Barr and Stroud, from i888 onwards. Range-finders vary in length from about 2-ft. to the ioo-ft.
coast defence range-finders.
In the earliest engagements of the World War the largest in use in the British navy had a base length of only 9 ft., but range-finders up to 3o-ft. base and more had been made by Messrs. Barr & Stroud and were in use before the war. One of r5-ft. base was in the Russian navy from 1908. The 9-ft. range-finders were called upon to measure ranges up to 20,000 yards (II or 12 miles) though originally designed to meet the requirements when the maximum range contemplated was very much less. Modern naval range-finders meeting increased range requirements vary from ft. to 4o-ft. bases.
Many forms of monostatic range-finders have been constructed, commencing with Magellan, an optician, in 1775. The next in importance was Adie, 186o. In 1888, Professors Barr and Stroud produced the first of the long series of range-finding instruments that bear their name.' Two types only are in general use, the "coincidence" and the "stereoscopic." The best known as those of Barr and Stroud, Zeiss and Goerz. The two types are distinguished ,chiefly by the mode of presentation of the images of the target to the eyes of the observer. As usually constructed the range-finder consists ex ternally of a tube carrying internally at each end a reflector in the form of an "optical square." This is either a "pentagonal" prism or a pair of mirrors inclined to each other at an angle of The advantage of this type of reflector over a single mirror is that an incident ray is always reflected (within narrow limits) through a constant angle ; in this case 90°. The optical distance between the end reflectors con stitutes the base length. The end reflectors receive rays of light from the target and reflect them • towards the centre of the base, where "eyepiece prisms" are placed to direct them outwards through a single eyepiece, in the coincidence type, or through two separate eyepieces, in the stereoscopic type. Between the end reflectors and the eyepiece prisms are placed object glasses which, with the eyepiece or eyepieces constitute the range-finder tele scopes. In addition, there is an optical device to enable the range to be ascertained and provision is made for effecting adjustments.