Range-Finders

In the coincidence type two partial views of the target are seen by one eye in juxtaposition, one above and the other below a fine "separating line," as in fig. T. The process of "taking a range" consists of operating a device that brings the two partial images into alignment; the range scale then indicates the range.

In a stereoscopic range-finder the two images are presented separately to the two eyes of the range-taker so that the target is seen as in a stereoscope. In the field of view of each eye-piece there is a mark, the two marks being also seen stereoscopically as one. The range is taken by operating an optical device which alters the apparent distance of either the target or the mark until the target and the mark appear to be at the same distance, when the range may be read from the range scale.

Coincidence Range-finders.

As the base is so very small compared with the ranges to be measured, great refinement is essential in the means adopted. If partial images of an object such as a mast are presented to the eye above and below a fine separating line, a want of alignment due to 12 seconds of angle may be detected under ordinary conditions by the unaided eye. In a laboratory this might be reduced to 3 seconds. In this type of observation "the resolving power," as usually defined with regard to telescopic observations, does not apply, nor does the physiological limit of acuity of vision, stated to be 6o seconds.

If the telescopic magnification be 24, a parallax angle of -I second between the entering beams is detectable. Fig. 2.

If B be the base length and R the range, both in yards, and 0 be the apex angle in radians, then, for the small angle concerned Formula (3) enables us to find the minimum error that is just detectable under good conditions at any range R, with a coin cidence range-finder of base B and magni fication 24.

Equation (3) shows that ranges may be determined within i oo yards with a 9-ft. base up to those of r i,000 yards range, with 15-ft. base up to 14,00o yards and with 3o-ft. base up to 20,000 yards and with ioo-ft. base up to 36,00o yards.

For the 9-ft. range-finder, if 3,000 yards is the shortest range to be meas ured, then the largest apex angle is K-16 radian, about 3 minutes which com prises the whole of the scale correspond ing with ranges from 3,00o yards to in finity. At 20,000 yards the apex angle is

yewo radian; about 30 seconds or The difference in angle between 20,000 yards and 20,500 yards is 4 second or the angle subtended by a halfpenny at 41 miles. This will indicate the accuracy of the optical devices to be used.

The optical devices used for effecting coincidence are usually the translation al deflecting prism system invented by Barr and Stroud in 1888 or the rotating pair of prisms used by Barr and Stroud in some early types, but chiefly adopted by the continental makers. Other devices have been used, but only these two have survived. Fig. 3 shows the essential parts of a coincidence range-finder.

The eyepiece prisms are shown as crossed mirrors for simplicity.

Assuming that the left-hand ray enters the left window at right angles to the base and is directed by the optical system so as to appear in the centre of the field of view, the right-hand ray will enter the right-hand window at an angle depending on the range and the base length. After passing through the object glass the convergent beam of rays is bent through a constant angle by the thin deflecting prism. By sliding the prism longitudinally the image formed by this beam may be so displaced that the eye will see the upper and lower images in alignment. The range may then be read from the range scale, which is rigidly attached to and moves with the prism. Equation (I) shows that the scale is a reciprocal one. The scale may be read through a second eye piece, or through a special window, or may even be made visible in the main eyepiece at one side of the field of view. It may be noted that there can be no backlash in this arrangement.

The translational prism must be placed in the convergent rays of the telescopic system. The rotating prisms may be placed out side it in the parallel beam. The rotating prisms are mounted in two rings, geared together so as to rotate in opposite directions.