Souchier prismatic rangefinder
Souchier rangefinder consisted of a glass
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°
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°
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.
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°
that he was in B, since the angle PAZ was 90°, the angle BPA would be 1°
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°
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.
Souchier rangefinder was tested both by the French École normale de Tir at Châlons, and 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
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.
- “Prisme-télémètre à réflexion totale”.
- Fraenkel. Jérôme : “Russie. Le prisme-télémètre Souchier”. Revue d’Artillerie, 41 (Octobre 1892-Mars 1893), pp. 301-314.