348 



Mr. Gr. Forbes. 



[Feb. 22, 



angles along each tube, and are then reflected at the two middle 

 prisms into the two telescopes of the binocular fixed to the base, in 

 directions parallel to the original rays, intercepted by the outer prisms. 

 It is the measurement of the angle between these rays that tells the 

 distance of the object looked at. This angle is measured by two 

 vertical lines, one in each telescope, seen by the two eyes. One of 

 the lines is fixed, the other moved by a micrometer screw until the 

 two lines appear as one, while the object is seen distinctly. This 

 gives the distance accurately to 2 per cent, even at 3000 yards. But 

 now stereoscopic vision comes in and gives far greater accuracy. The 

 line seems to stand out solid in space, and the slightest turn to a 

 micrometer screw may cause the line to appear nearer or farther than 

 the object looked at, and when the line appears to be at exactly the 

 same distance, the micrometer reading gives the distance with an 

 accuracy far greater than that attainable by observing the duplication 

 of images on the retina.* 



(2.) The Adie Telemeter. — This was the first short-base telemeter or 

 range-finder ever supplied, but its inaccuracies were great. These in- 

 accuracies were in part avoided in the designs of Barr and Stroud, and 

 of Zeiss. These three are the only instruments of the kind which have 

 ever been obtainable. In all of them a double base is used, placed at 

 right angles to the direction of the object, having two plane reflectors at 

 its two outer ends, and two plane reflectors at its middle point parallel 

 to the others — all of these plane reflecting surfaces being inclined at 45° 

 to the base. In all three instruments the two object glasses are close to 

 the outer reflectors, the eye-piece or eye-pieces of the telescopes being at 

 the middle of the base. The three instruments differ in the means of 

 measuring the angle of parallax. The angle to be measured is very 

 small, and if any one of the reflectors be twisted in the plane of vision 

 through a small angle, the error produced, is double this angle. This, 

 necessitates exact parallelism between the two reflecting surfaces in. 

 each half of the base. Such rigidity cannot be attained in a portable' 

 instrument. Nor can the warping action of the sun's rays be pre- 

 vented from affecting the parallelism without diminished portability. 



(3.) Double Reflection at each end of the Base. — I have succeeded in 

 overcoming the necessity for extreme accuracy in the rigidity of the 

 base by replacing the single reflectors by a double reflection at 

 each end of each half of the base. The angle between the two 

 surfaces producing the double reflection is maintained unalterable 

 by grinding and polishing these surfaces on a special glass prism. 

 I have experimented on many forms of prism, but fig. 1 is ex- 

 extremely convenient. Light enters the first prism at the surface AB. 

 It is reflected at the two surfaces BC, DE, which are silvered, and am 



* [It has been found convenient in practice to replace the lines by balloons pho- 

 tographed on glass.] 



