the observer. In the later stereoscopic range-finders, marks in the eye-pieces are adjusted so as to appear to be at a fixed distance when seen stereoscopically. The view containing the target appears to approach or recede from the observer when the working head is turned; and the range is read off from an external range scale or drum.
Stereoscopic range-finders suffer from the disadvantages that very few people are able to see stereoscopically with sufficient exactness to obtain good results and the degree of accuracy obtained by a range-taker appears to vary from time to time. Under the same atmospheric conditions, stereoscopic range-finders do not appear to give such good results as coinci- dence ones. Owing to the marks on the diaphragms in the eye-pieces being more distinct than the image of the target, there is difficulty in deciding when they are at the same apparent distance. On the other hand, owing to both eyes being used, the target should be seen more distinctly than through a coinci- dence range-finder. Ranges of targets with the sky as a back- ground, e.g. aircraft, masts of ships, and trees on a crest line, are comparatively easy to take with a stereoscopic range-finder; but those of comparatively indistinct objects, objects with near backgrounds and objects in a shadow, e.g. a gun under a tree, are very much more difficult to take.
Directors. The director is an instrument employed chiefly for the measurement of azimuth angles and angles of sight. There are numerous patterns of these instruments. Some are of simple form ; while others are complicated and are provided with quick and slow motion movements for their azimuth and altitude movements and for laying on a gun or target without the setting of their azimuth scales being altered. Some are used as directors pure and simple, whilst others are provided with powerful telescopes for observing fire. British directors are graduated in degrees and minutes, whereas those of nearly all other powers are graduated in milliemes. Instruments graduated in milliemes can be made much more compact than those graduated in degrees and minutes, as the main azimuth plate has only 64 divisions instead of 360. The azimuth scales of some German directors are graduated in i6ths of a degree and i6ths of a degree are also sometimes used for angle-of-sight scales.
The German director for field artillery is a good example of modern types of this instrument. Powerful telescopes like the stereoscopic or scissors telescope are often provided with fittings which enable them to be used as directors as well as for observing fire. Azimuth angles can be measured by means of an azimuth scale, and angles of sight by means of a small bubble and scale. In one pattern of German stereoscopic telescope, the angle of sight is measured by means of a device which is com- pactly pla'ced between the two arms of the telescope, above its elevating gear.
The Field Plotter is an instrument used for obtaining the gun range when the range-finder is not near the gun. It consists of two similar nickel-plated steel plates, having azimuth scales and a scale of yards (upon which the distance of the gun from the range-finder can be set) engraved upon them, and two range arms which are pivoted at the centres of the azimuth scales and connected by means of a slider. The two plates are slid along one another until the distance between the gun and the range-finder is opposite the reader. The slider is then moved until an arrow upon it reads on the arm the distance from the range-finder to the target. The arm is then moved until another arrow reads the angle, recorded by the director, between the target and the gun. The plotter is then turned over, and the range from the gun to the target and the angle at the gun between the target and the director are read off. When fire has been opened, it is necessary to' observe the results, so that corrections for both range and direction may be made. Also if time fuzes are used, the angular height above the target at which they burst the shell must be observed and corrected if necessary. Various instruments, e.g. graticuled binoculars, stereoscopic telescopes and periscopes, are used for this purpose.
The binoculars used in the British service are of the prismatic type having a magnification of six diameters and are fitted with
a glass diaphragm in front of the right eye-piece on which are engraved the vertical lines 5 apart; their heights above the horizontal line being alternately 5 and i. The central line is longer, extending 15 above, and 5 below the horizontal line. When observing fire, the point of intersection of the centre vertical line with the horizontal line is laid on the target ; and, when the burst of the shell is observed, its angular distance to the right or left of the target and above it can be ascertained by noting its position with reference to the lines on the diaphragm. Graticules used by other continental powers vary very much in design and often consist of a very large number of short lines at definite distances from the axis of the telescope.
The stereoscopic or scissors telescope was very largely used in the World War, especially by the Germans. The British pattern has a magnification of 10 diameters and a field of view of 4. The two arms can be used either vertically for observation from below cover, or in line with one another for observation from behind vertical cover.
FOR COAST DEFENCE
Coast defence range-finding instruments are usually either range-finders or position-finders. A range-finder measures ranges from itself to the target and may also record the bearing of the target. A position-finder measures ranges and bearings to the target from the point for which it has been adjusted. This would usually be a gun which may be many hundred yards away from the position-finder. The position-finder sometimes also records the range and bearing of the target from its own position. Coast defence range-finding instruments may be divided into classes, depending upon the nature of their bases, as follows:
1. Depression instruments, having vertical bases (either range- finders or position-finders) the accuracy of which depends upon their height above the sea-level. They measure ranges with equal accuracy in all directions; and, if sufficient height is available, they are most reliable instruments. It is usually considered that, to obtajn ranges with an error not exceeding I %, loo ft. of height is required for every 5,000 yd. of range. All that the range-taker has to do is to keep a cross wire in the telescope of his instrument laid on the water line of the target.
2. Instruments having a fixed horizontal base (usually position- finders). Each usually consists of two instruments installed at the ends of a base which may be several thousand yards in length, the length required depending upon the range and arc of fire of the gun for which the instrument has been installed. The instrument at one end of the base is known as a transmitter and usually consists of a telescope mounted above an azimuth circle. It measures the bearing of the target and transmits it to the instrument at the other end of the base which is known as the receiving instrument. The bearings may be transmitted by telephone; or, as the telescope is traversed, electric impulses may cause some portion of the receiving instrument, e.g. a metal arm or a dial, to move automatically. The receiving instrument is usually an accurate, solidly constructed plotter, made to a certain scale, e.g. 500 yd. to an inch. In its simplest form it would consist of a base plate upon which two straight arms were pivoted, the relative positions of the pivots being, to the scale of the instru- ment, the same as those of the transmitter and receiving instrument. The first arm would be kept set to the azimuth angles received from the transmitter, and the second arm would be kept in line with the target by means of a telescope attached to it. The point where the two arms intersect would represent the relative position of the target. The arms being graduated in ranges and moving over azimuth scales, the range and bearing of the target from either end of the base could be read off. By means of a third arm, pivoted to the base plate in the position representing the position of the gun, the range and bearing of the target from the gun could be ascertained if the arm were brought above or below the point of intersection of the two other arms. Horizontal-base range-finding instruments are often designed to allow of several alternative bases being used, the pivots of the arms in the receiving instrument being set for the base which is most suitable for the conditions prevailing at the time. Thus for very long ranges a very long base would be used ; but, if the visibility were bad, a much shorter one might be preferable. Horizontal-base instruments have certain disadvantages. For a definite range, their accuracy varies with the position of the target. It is greatest when the target is opposite the centre of the base, decreases as the target moves to the right or left, and disappears altogether when the target is in line with the base. It is somewhat difficult for the receiving instrument to indicate to the transmitter the target whose range is to be found. There are difficulties in passing azimuth angles from the transmitter to the receiving instrument. If they are telephoned, delay and errors are involved, and owing to the fact that they may
