The Modern Range-Finder

Abstract

THE War Office has published a pamphlet on the modern range-finder, written by Prof. Cheshire, and, as is to be expected from an author of such technical knowledge, it is a clear and thorough exposition of a difficult and attractive subject When it is considered that all that a range-finder has to do is to enable the observer to utilise the angle of convergence upon a distant object of the widely spaced eyes of the instrument in order to find the distance of the object the problem of range-finding may appear to be very simple, and so in principle it is. This is not the difficulty. The real difficulty is to make an instrument which shall be portable, handy, and quick in use, and also shall attain the ultimate possible limit of accuracy. That which is not only attainable, but attained every day is something so perfect as to exceed the utmost that an inventor might have dared to hope for. Some form of reflecting device is needed at each end to bring the two sets of optical beams together into a single eyepiece. Any structure that supports the mirrors or prisms is liable to bend under its own weight or on account of differential heating. Simple reflectors at the ends would double any such angular displacement, and the kind of accuracy required would be unattainable. Double-reflection prisms, however, may be tilted without affecting the apparent direction of the object, as may be noticed when using the ordinary camera lucida. However, such oblique reflection would require prisms of inconvenient size; accordingly pentagonal prisms are used, which, however, require to have their reflecting faces silvered, as they are within the critical angle. As these prisms turn the beam through an invariable angle, slight flexure such as is here contemplated does not matter. The prismatic devices near the eyepiece designed to bring the two beams in two parts of the field into view together and into perfect alignment, where the object is at a very great distance, must not only do this, but the line of demarcation between the fields should be sharp throughout its extent. This is essential to accuracy. These fields may both appear erect, or one may appear inverted either laterally or vertically. Where there is convergence of the beams the alignment is disturbed, and the optical means by which it is corrected, as by a sliding prism, are connected up with a scale, so that the distance may be read directly. In the Bsrr and Stroud range-finder, which is more particularly described and illustrated, this scale is seen by the other eye through a separate eyepiece. It is satisfactory to find that in the essential of sharpness of the line of demarcation the Barr and Stroud instrument is superior to two German forms. It is quite impossible in the limits of space here available even to indicate the nature of the highly ingenious three-dimension reflecting devices which serve to bring the two converging beams into sharply separated parts of the field, and in the Barr and Stroud instrument at the same time to throw them up at an angle of 60°, so that the observer lying on the ground or in other comfortable position may look down at a convenient angle instead of wearing out his neck by looking horizontally. In one form of instrument made by Zeiss the telescopic magnification of the two beams is different, so that the images seen in juxtaposition are of different