Souchier prismatic rangefinder

The Souchier rangefinder consisted of a glass pentagonal prism, 1 cm thick and 5,5 cm wide, weighting approximately 30 g. The five lateral faces were smooth, while the two bases were matt. The angles made by the faces measured respectively : A = 67° 30’ ; B = 90° ; C = 177° 50’ ; D = 69° 40’ ; E = 130°. The prism was held by a wooden frame, that enveloped it completely, leaving uncovered only the two little opening AC (object glass) and DG' (eyepiece). The eyepiece was composed by two inclined facets (DC and CG'), that could be closed by means of a matt cursor. On the upper base was engraved a table, protected by a celluloid transparent envelope.
The face AB was turned to the target, while the facets DC and CG' were led towards the watcher’s eye. A beam of parallel rays coming from the face AC was deflected of 90° and, going out, it was split in two. The facet DC indeed was tilted to the face CG'B with such an angle that the rays going through DC and CG' formed an angle of 1° 5’. This angle was the set parallax, which constituted the structural constant of the rangefinder. Keeping the prism horizontally, a man, who watched alternately through the facets CG' and DC, saw two images of the target, placed in two different direction P' and P", at right angles to the target itself and forming an angle of 1° 5’. This happened whatever was the distance from the target and inclination of the prism.
The rangefinder could be used in two different way: moving back or moving forward in the direction of a marker. In either way the observer should have the target on his flank and held the rangefinder horizontally by its right angle. If the target was on his right, he should hold the rangefinder with the left hand, if it was on his left, with the right hand. The rangefinder should be held between the thumb and the first finger in such a way, that the observer was able to see the marker in the arch formed by the fingers. If the images were not perpendicular or clear, he should turn the device in order to straighten them and remove the iridescence.
	Moving back. Supposing that the seeking range was AP, an observer, placed in A, kept the target A on his right. Looking through the facet DC, he saw the image of the target in the direction P", forming the angle PAZ equal to 91° 5’. He choose an object (a tree, a stone) standing in Z in the direction AB not more than 150 m away or sent a scout with a landmark in that direction. Through it he located the direction AP", where he could see the image of the target, looking directly over the prism. Then he placed a marker in A, and moved back along the direction AZ until he saw through the facets CG' the image P' coincident with the marker Z.
Supposing that he was in B, since the angle PAZ was 90°, the angle BPA would be 1° 5’. Therefore in the triangle ABP : AP = AB / sin 1° 5’ = AB / 0.02 = 50 AB. To obtain the seeking range, it was sufficient to measure the base AB and multiply it for 50, which was the coefficient of the rangefinder. Since actually it was not always possible to obtain an angle of 1° 5’, the coefficient might be different from 50, for instance 52, as in the diagram. In order to make easier the calculation of the range, a table was engraved on the upper base of the prism, reporting the product of this coefficient with the numbers from 8 to 60. To avoid to confuse the two images, at first the observer placed the cursor on A (avancer), covering the useless facet, and obtained the coincidence between P" and Z. Then to obtain the second coincidence between P' and Z, he moved the cursor on R (reculer). Moving forward. Starting from B, the observer placed the cursor on R (reculer) and obtained the coincidence between P' and Z, then he moved the cursor on A (avancer), and obtained the coincidence between P" and Z. Being the triangle ABP the same, the measure of the base AB could be easily calculated by the observer himself, while he was advancing from B to A. In this way he would determine the seeking range. The first way was regarded as more precise, since it was easier to keep the direction between the marker and the observer place, but the second way was quicker, since the observer had not to come back in the first place in order to measure the base. The Souchier rangefinder was tested both by the French École normale de Tir at Châlons, and the Russian Officers Rifle School at Oranienbaum. It was judged strong, light, extremely reliable and portable, accurate and easy to use. The images of the target obtained with it showed clarity superior to every other pocket rangefinder. It enabled to measure almost any range in less that three minutes with a margin for error of less of 25 m every 1000 m, similar to that of the best rangefinders of the time. It was also very cheap, costing only 16.50 leva in 1893, when the other prismatic rangefinder of the time cost at least 100 leva. The Russian Commission, however, noticed that at distances greater than 1500 paces, the image of the target lost its clearness and to obtain a perfect coincidence with the marker, the observer should have excellent eyesight. In order to obviate this disadvantage, the Russian captain Eroguin invented a device to adapt the rangefinder to the field glass adopted by the Army by means of the plate F. It weighed only 25 g and cost about 4 leva. With this simple device, the rangefinder not only doubled its accuracy at ranges from 2000 to 3000 paces, but could also measure ranges from 5 km to 8 km, being utilizable also by the artillery. It is interesting to notice that sometimes the western sources called the rangefinder adopted by the Russian and Bulgarian Army “Suchet” (Handbook of the Bulgarian Army, p. 36, Streffleurs militärische Zeitschrift, 50 (1895), p. 49) or “Souchet” (Rivista di Artiglieria e Genio, IX/4 (1892), pp. 298-303), but this was only a transliteration mistake of the cyrillic word Суше. SOURCES : - “Prisme-télémètre à réflexion totale”. La Nature, 876 (15 mars 1890), pp. 237-238; - Fraenkel. Jérôme : “Russie. Le prisme-télémètre Souchier”. Revue d’Artillerie, 41 (Octobre 1892-Mars 1893), pp. 301-314.

Souchier prismatic rangefinder
capt. Eroguin adaptator