350 



Mr. G. Forbes. 



[Feb. 22, 



in practice. Rotation of one-half of the base about the line of the 

 base produces no error if the axes of the two prisms in each half-base 

 be parallel, and if not parallel the hinge can easily be made suffi- 

 ciently true to prevent an appreciable error arising. 



(6.) Accuracy Required. — The angle which has to be measured is the 

 angle between two rays from the distant object to the two ends of the 

 base, and the distance of the object is d = bja if b is length of base; 

 and the angle corresponding to any distance is cc = bjd. 



The smallest angle perceptible to the naked eye has been sup- 

 posed to be 30" . A magnifying power of 12 is the highest which 

 can be used in practice without a tripod or other stand. Hence 

 30"/12 = 2"*5 is the smallest angle which can be observed, or an angle 

 = 0-0000125. Now an infantry range-finder iriust be capable of dis- 

 tinguishing between 2940 and 3000 yards (2 per cent.). Hence the 

 angle &/2940— 6/3000 must be at least equal to -0000125, which gives b 

 not less than 1*838 yards. Following this argument, I have employed 

 a base of 2 yards' length in the form of a steel tube of square section, 

 0-5 mm. thick, with a hinge in the middle, so that in travelling the 

 6-feet base folds up to 3 feet 3 inches. 



As a matter of fact I find that, owing to the adoption of stereo- 

 scopic vision, with which the limit of visibility is far less than the 30" 

 assumed above, the accuracy of the instrument in the hands of a prac- 

 tised observer on a well-defined object is far greater than 2 per cent, at 

 3000 yards — the limit of accuracy which I had laid down. 



(7.) The Binocular. — When the base is stretched out and the binocu- 

 lar attached, the distant object can be viewed by stereoscopic vision. 

 The eyes have little or no power of estimating absolute distance by 

 this means, but have the greatest accuracy in the comparison of the 

 muscular effort required to converge the eyes on objects at different 

 distances. Two marks are fixed in the focal planes of the binocular, 

 and it has been found best to make this mark in the form of a balloon 

 photographed with a tail-rope hanging down to the centre of the field 

 of view. The two balloons are almost identical, but on the left side 

 of the left balloon the letter L is marked, and on the right side of the 

 right balloon the letter E is marked. This assists in the focussing of 

 each eye- piece separately, and indicates when both eyes are operative. 

 If the distance between oculars is wrong, or if the line of the oculars 

 be not horizontal, one of the letters, R or L, disappears. 



One of the balloons is movable by a micrometer screw towards or 

 from the other, so that they can be seen stereoscopically with any con- 

 vergence of the optic axes of the eyes, and therefore at any apparent 

 distance by turning the micrometer. The observation consists in 

 turning the micrometer until the balloon appears to be at the same 

 distance as the object. The micrometer head is divided and marked 

 with distances inversely as the angular movement from infinity. 



