Stroobants prismatic
rangefinder
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The
Stroobants rangefinder was
adopted by the Belgian Army for infantry use and was employed by the
Bulgarian fortress-siege artillery between the end of the 19th and the beginning
of the 20th century. It was the smallest and cheapest rangefinder of its
time, but it was considered accurate and very
practical, as it allowed determining the required distance in 2-3 minutes. It
was 44mm long, 18mm wide, 12mm high and weighed only 23 g. To
prevent damage to the exposed faces of the prisms, which might
be dented by objects in the pocket, and to prevent the faces from
becoming dirty or losing their polish, the rangefinder was equipped with a
metal box with a lid, which was also used to transport the tables.
On
one side of the box, the prisms were marked with A and B, on the other
side there were the numbers indicating the coefficient, which should be added to the base. Prism A was used for establishing a right
angle, prism B an angle greater
than 90° (91° Use of the Stroobants rangefinder |
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To
determine the distance of a target placed in O, the observer stood at P
with his back to PO, brought the
rangefinder between the thumb, index finger and middle finger of his right
hand towards prism B, making sure
that the index finger did not go beyond the middle of prism A. Then he placed one side of prism A 3-4 cm from his right eye so that
the other side was perpendicular to the line PO and looked into prism A,
aiming with his left eye. Thanks to the double reflection of the rays coming
from O on the smooth surfaces of
the prisms, the observer saw the image of the object in front of him on a
line perpendicular to PO. |
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To
determine the distance of a target placed in O, the observer stood at P
with his back to PO, brought the
rangefinder between the thumb, index finger and middle finger of his right
hand towards prism B, making sure
that the index finger did not go beyond the middle of prism A. Then he placed one side of prism A 3-4 cm from his right eye so that
the other side was perpendicular to the line PO and looked into prism A,
aiming with his left eye. Thanks to the double reflection of the rays coming
from O on the smooth surfaces of
the prisms, the observer saw the image of the object in front of him on a
line perpendicular to PO. Then the observer simultaneously looks into the
prism and under the prism and guided a aiming post (Y) in the direction of the object if
possible at most 100 m. The further away this point was, the easier it was
for him, when he starts moving forward, to stay in the direction of the image
exactly on the perpendicular line of PO.
In this way, the angle OPY is
constructed. The observer took the
rangefinder from prism A and looked
into prism B: in this case, the
image of the object appeared to him to the right of the line marked by the aiming
posts, because the angle given by prism B
is greater than the right angle. He went in the direction of the aiming posts,
looking at the same time into prism B
and moving exactly along the line PY,
and noticed that the image gradually approached the direction of the aiming
posts until it coincided with the line PY.
At this point, the observer was at the vertex S of the triangle OPC,
he measured the distance of the line PC,
called base. He multiplied the base by the coefficient engraved on the copper
box: the resulting number showed the distance to the target. The distance could also be calculated by working backwards instead of
forwards, using prism B first. The
angle OCY was constructed by
guiding a aiming post Y to coincide
with the image of the object; at point C,
the observer placed another aiming post against the right eye, then moved
back along the line CY, looking
through prism B until he achieved
coincidence again. In this way, he obtained the vertex P and, measuring the distance PC,
through the coefficient, he determined the distance. In
fact, the observer usually preferred to use the second method, but without aiming
posts: he looked at a very distant mark and its coincidence with the image
obtained from point C, looking through the prism B. This mark replaces the aiming
post Y; if the correspondence was
not exact, the observer moved left and right, forwards or backwards, until it
was obtained. Then he sent to point C a soldier holding the rifle or sabre
vertically; then he moved away proceeding as above. Since
it was impossible to obtain exact prismatic angles in the construction of the
instrument and since the refraction of the glasses varied slightly, a
separate table was made for each rangefinder and the
coefficient thus found experimentally was engraved on the rangefinder case. |
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SOURCES : - Ръководство за занятията въкреспостата артилерия. Част IV. Подготовка и служба на наблюдателите,
Sofia 1902, pp. 43-47; - MILITARY
INFORMATION DIVISION : “Field range finders
in European Armies (Austria-Hungary, Belgium, England, France, Germany,
Italy, and Russia)”. The Military Schools of Europe and other papers select for publication,
Washington 1896, pp. 93-94. |
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