read off against a pointer from the scale seen with the left eye. For
night use, means are provided for illuminating the scale. The range
to lights may be ascertained by the use of the astigmatizer, an optical
device by which a point of light is drawn out into a vertical streak.
Fig. 2.—Barr and Stroud.
A beam of light from the objective falls on each reflector (fig. 2), and passing through the object-glasses, each is received by an arrangement of prisms about the centre of the tube, and reflected through the right eye-piece. Two partial images are thus seen. The images could be united by the rotation of one of the reflectors, but owing to the small base used the necessary movement would be so extremely small that it would be practically impossible to measure it. The difficulty has been surmounted by utilizing fixed reflectors and effecting coincidence by means of a prism of small angle. The deflecting prism is situated in the line of the beam of light from the reflector at the right-hand end of the tube. Its multiplying action is of great delicacy. The angle available for subdivision, to measure ranges between infinity and 250 yds., is only one-third of a degree. In a travel of 6 in. the prism renders accurate measurements possible within the required limits. To bring images of distant objectives into coincidence, the prism must be moved towards the eye-piece, and for near objectives in the opposite direction. The range scale is attached to the prism. A consequent advantage is that the accuracy of the instrument is not affected by back lash arising from wear, or irregularity in the actuating mechanism. When once installed, the instrument is always ready for use. Should adjustment be required it is readily and easily applied. It is not within the sphere of this article to enter into the detail of the adjusting mechanism. For further particulars the reader is referred to the Proceedings of the I nxlitutian of Mechanical Engineers, 30th January 1896. The working of the range-finder is so simple that its use is quickly learnt by any man who can read, and with little instruction and practice he can “take a range" in 8 to 12 seconds. Besides its principal purpose, in connexion with gunnery, there are minor uses in navigation and nautical surveying to which the range-finder can be applied.
With the high speeds of modern war-vessels, guns and their objective approach each other so quickly that unless ranges can be communicated from the instrument to the guns with rapidity and accuracy the range-finder is deprived of much of its value. In Connexion with the naval range-finder an apparatus is provided, which though not part of the range-finder is sufficiently important to claim passing notice. The apparatus consists of a transmitting and a receiving instrument of clockwork mechanism electrically controlled. In appearance they resemble the ordinary engine-room telegraph, on the dials of which ranges take the place of orders. The transmitter can communicate with a number of receiving instruments, disposed as required in different parts of the ship.
2. Before the introduction of the Marindin range-finder described below, the British army in the field used the “mekometer." The instruments used by the cavalry and infantry are smaller and lighter than those of the artillery pattern, but the principle involved is identical.
The mekometer is practically a box sextant. Two instruments are used simultaneously at the ends of a base of fixed length. One sextant, called the right-angle instrument, is fitted with index and horizon glasses permanently inclined at 45°. It consequently measures a right angle. In the other sextant, called the reading instrument, a graduated drum takes the place of the usual index arm and scale. The drum is graduated spirally with a scale of ranges. Both reading and right-angle instruments are fitted with a vane of gun metal with a white strip down the centre to facilitate observations. Telescopes of low power can be fitted to the instruments, and two cords of 50 (or 2512) yds. are provided with which to measure the base.
| Meko- meter. |
|---|
Two observers attach the ends of the cord of fixed length (usually 50 yds.) to their instruments and separate until it is taut. The observer with the right-angle instrument moves into such a position that coincidence of image will be given between the objective and the vane of the instrument at the other end of the base, i.e. he makes ABC a right angle (fig. 3).
When the right angle is established, the observer at C turns the graduated drum of tge reading instrument until the image of the vane of the right angle instrument coincides with the direction of the objective. The range AC is then read on the drum. The ranges on the drum are measures of the angle BAC when the base BC is 50 yds.
 |
Fig. 3. |
The mekometer is open to the objection which is common to all range-finders requiring more than one observer. There is always a danger that observers may cause coincidence on different objectives or on different parts of the same objective, and thus inaccuracy in the recorded range must result. The instruments are expected to give an accuracy of less than 2 % at 2000 yds. For ranges over that distance, i.e. for usual artillery ranges, it is desirable to use a double base (100 yds. in length), in which case the range registered on the drum must be doubled. This operation, although slight, is a distinct disadvantage, since it adds to the time of taking a range and is a possible source of error. For fleld artillery, however, a range-finder is only an auxiliary adjunct. The true range can be found by a process of trial and error (see Artillery) in as short a time as the mekometer observers take to report it. It must further be remembered that as shrapnel is the principal projectile of field artillery, not only the correct elevation but also the true len th of time fuse has to be found. This the range-finder cannot do. Hence it is that the range-finder for field artillery, although a valuable auxiliary, is not of the same importance as in purely defensive positions, such as batteries for harbour defence, and land forts.
The Marindin range-finder was from 1908 gradually introduced in the infantry to replace the mekometer. It was the invention of Captain A. H. Marindin, of the Black Watch (Royal Highlanders).
The principle of the instrument is that of coincidence, as in the Gautier Christie, Le Cyre, Souchier, and Barr and Stroud. But it differs from the last mentioned in that the right prism is made movable, and this movement (necessarily extremely small) is a function of the recorded range.
| Marindin Range- Finder. |
|---|
The steel tube, forming the base of the instrument, which carries the prisms, is supported inside an aluminium outer tube in such a way that no direct shock is communicated to it. The appearance of the outside of the instrument, together with the names of the various parts, is shown in fig. 4.
Fig. 4.—Marindin Range-Finder.
The instrument can be used in two main positions, viz. horizontally, for ranging on upright objects, or vertically, for ranging on horizontal targets.
Fig. 5.
For instance, in the diagram (fig. 5) of a road running uphill, the instrument could be held in any of the three positions indicated, and would give good ranges, but probably the best range would be obtained if held as at c. If it is required to use the instrument
