Range-Finders

If these prisms have their thin edges vertical and similarly directed, they produce a maximum deviation in the horizontal plane in one sense, and when each is rotated 18o° they produce a maximum deviation in the other sense, in neither case produc ing any deviation in the vertical plane. If they are of equal angle of deviation they produce no deviation in any direction when each is rotated 90° from the original position, and it will be evi dent that as the prisms are rotated equally and in opposite direc tions from their original positions they produce a variable devia tion in the horizontal plane, while they produce no deviation in the vertical plane. The rotations of such a pair of prisms in a range-finder comprising object glasses and eyepiece prisms, as illustrated in figure 3, therefore provides a means of measuring the angle 0, and a scale of ranges may be associated with the mechanism producing the equal and opposite rotations of the prisms. With suitable modifications such a pair of prisms may be inserted between one of the objectives and the eyepiece prisms, or one of the prisms may be inserted in each of the beams, say, before they enter the end reflectors. This latter method of work ing was adopted in the first 15-foot range-finders made by Barr and Stroud for the Russian navy and the Italian navy in 1908.

Adjustments.

Two main adjustments are provided for. The halving adjustment is to ensure that the images in the two half fields are neither duplicated nor that any part is missing at the separating line. This may be effected by suitably swinging about a horizontal axis a parallel plate of glass placed between one object glass and the eyepiece prism. The infinity adjustment is provided to permit of any small index error being removed. For this purpose the images of an infinitely distant object such as the moon or a star are set in coincidence and the scale is then set independently to read infinity. During ordinary range-taking the scale and prism are moved together.

Eyepiece prisms or "centre-prism-combinations" of many dif ferent varieties have been produced so as to present various effects in the field of view. (See Barr and Stroud's Patent Specifications from 1888 onwards.) For instance, both partial images may be erect, as shown in fig. 3, or the lower image may be erect and the upper image inverted. The latter type is often used in field range-finders. Again one side of the range-finder may produce nearly the whole field of view, while the other side is made to produce a narrow strip across the centre. Again, in this narrow strip, the view may be inverted. One type of eyepiece prisms from a German range-finder is shown in fig. 4. It gives an inverted strip field and shows the range scale at the side of the field of view. In all coincidence range-finders the fineness of the dividing line is important.

Stereoscopic Range-finders.

Externally these resemble the coincidence type very closely. The compli cated eyepiece prism system is, however, replaced by two fairly simple prisms, each with its own eyepiece. The adjustments provided are also, in general, similar: but an additional adjustment is required to ensure that the stereoscopic marks are in register with each other. The optical de vices for producing stereoscopic movement may be similar to those in the coincidence type. For example, the Levallois stereo scopic height-finder, an instrument of French make, makes use of one fixed and also of one translational prism.

There does not seem to be much difference between the ac curacy of the two types when used by experts in each. The chief disadvantages of the stereoscopic range-finder in service use are that fewer men can be found with a sufficiently acute stereoscopic sense and that this stereoscopic sense is liable to occasional large errors and to com plete derangement if the operator be sub jected to fatigue or mental strain. For these reasons they have always been re garded with suspicion in the British service.

Other adjuncts to monostatic range finders are astigmatisers and internal ad justers. Astigmatisers as used in coinci dence range-finders are thin cylindrical lenses which may be inserted in the tele scopic systems at will. Their effect is to draw the images into vertical streaks.

They are useful for taking the range of ill-defined objects, very small objects and of lights. Fig. 5 shows the effect on a star and fig. 6 the effect on a group of lights.

Internal adjusters, of which there are several types, are pro vided in some of the larger range-finders. They consist of an arrangement of optical parts whereby the infinity adjustment can be carried out within the range-finder itself at any time. This method cannot, however, replace adjustment on a natural "in finity," such as a star or the moon, or on an object at a known range. Small range-finders are provided with a "lath" on which are two marks separated by a distance equal to the base length of the range-finder, constituting an artificial infinity.

General Construction.

The main body is a tube, which may be double with an air space in order to insulate the interior from heat. A general expansion due to temperature produces little effect. A change of 50° F may change the base length by :16% and can be detected by a 3o-foot range-finder only at ranges below 3,00o yards. Differential expansion due to the use of different materials in the various parts is taken into account in the design of the details. As the end reflectors are optical squares a very slight bend of the main tube will produce little or no effect on the range adjustment due to angular movement of the reflectors.