AN IMPROVED RANGE FINDER
Year: 1885

page_text: "
I, ALEXANDER MELVILLE CLARK, of the firm of A. M. and Wm. Clark, of
53, Chancery Lane, in the County of Middlesex, Fellow of the Institute of Patent
Agents, do hereby declare the nature of the said Invention for "AN IMPROVED
RANGE FINDER (a communication to me from abroad by Joseph Mayer, of
Bruneck (Tyrol) in the Empire of Austria, Lieutenant in the Austrian Army) and
in what manner the same is to be performed, to be particularly described and
ascertained in and by the following statement :-
The construction of the range-finder which forms the object of this Invention, is
based upon the following consideration :-When an observer, after having estimated
(on the object whose distance is to be determined, or in the immediate neighbour-
hood of said object) a dimension perpendicular to the line to be measured, and so
chosen that its length may be judged with approximate accuracy, places between
the said object and his eye (at a given invariable distance from his eye and
parallel to the estimated transverse dimension) a transparent scale of an exactly
known length, two similar triangles are included between the visual rays to the
two ends of said cross dimension. Of one of these triangles the base is ascertained
by reading off on the scale the number of divisions included between the said
visual rays, and the altitude is known; while of the other triangle the base is
known and consequently its altitude i.e. the distance to be determined, can easily
be calculated by proportion.
In order to facilitate the solution of said proportion, and to avoid as far as
possible the errors which are always likely to arise when relatively very small and
very great dimensions are to be compared with each other, the length of the
transparent scale is so chosen that when said scale is held before the eye, at a given
distance, it appears under the same visual angle as a given greater dimension
would appear under, when viewed from a known greater distance.
In the annexed drawing, figure 1 shows a simple instrument constructed
according to the Invention and serving to determine by steps or paces the distance
of an object, the cross dimension of which has been estimated by steps or paces,
man's breadths, or meters.
Figure 2 illustrates the use of this instrument.

page_text: The instrument consists of a small metal plate having the form of an equilateral
triangle provided on each side with a scale in the form of a rectilinear projection
designated a b c respectively.
When this instrument is held before the eye at a distance of 56·25 centimetres,
the lengths of said projections respectively correspond to the visual angles under
which the dimensions of 4 metres, 4 steps or paces, and 4 man's breadthis would
respectively appear when viewed from the distance of 100 paces, and said
projections are therefore marked with the letters M, S, and M. B. According as
the cross dimension of the object whose distance is to be determined, or a cross
dimension in the immediate vicinity of said object, has been estimated by metres,
steps or man's breadths, one or the other of said three projections is used when
determining the distance. Each projection is divided, by means of notches, into
16 parts of equal length, and in order to render the scale thus formed more
perspicuous, the notches marking 1/8th 1/4th and 1/2 the total length of the scale,
are made of different depth. In order that the instrument may always be held
before the eye at the distance of 56.25 centimetres, for which it has been constructed,
it is provided with a bow pivoted to its centre, to which is attached a cord of the
proper length.
As clearly shown in figure 2 the cross dimension x y or x'y' of the object whose
distance is to be determined, having been estimated by one of the above mentioned
units of length, the instrument is so placed before the eye o at the distance given
by the cord, that one end m of the scale a, b or c which corresponds to the same
unit of length, covers one end x or x of the estimated cross dimension x y or x¹ y¹;
the operator then observes at which notch the visual ray to the other end, y or y¹,
of the estimated cross-dimension intersects the scale.
It results from what has been stated above that the respective lengths m m¹ of
the scales a b, c is to their distance m o from the eye, as 4 meters, 4 paces or
4 man's breadths, are to 100 paces, respectively. Consequently, the two 'following
proportions can be constructed since the fractions m n = and m n1 =
mm 1
4
oxom xy: mn, and ox: om : : x ¹ y' : mn 1.
By substitution of the known powers we have:
ox: 100 xy; and ox¹: 100 : : x¹ y¹ .
mm1
8
:
Hence the distances to be determined are ox= 100 xy, and ox¹ = 200 x¹ y¹.
By analogous examination of all possible cases, the following general rules are
found for the use of an instrument of the described construction. The estimated
cross-dimension is to be multiplied by 100 and the resulting product
1st is to be divided by 4, when the cross-dimension is covered by the whole
length of the scale.
2nd is to be divided by 2 when the cross-dimension is covered by 12 of the
length of the scale.
3rd indicates the distance when the cross-dimension is covered by 1/6th of the
length of the scale.
4th is to be multiplied by 2 when the cross-dimension is covered by 1/8th of the
length of the scale.
5th is to be multiplied by 4 when the cross-dimension is covered by 1/18th of the
length of the scale.
To supersede these calculations, a table containing the distances which correspond
to different cross-dimensions, and observed portions of the scale covering said
dimensions, is provided with each instrument.
Having now particularly described and ascertained the nature of the said
invention and in what manner the same is to be performed I declare that what
I claim is
1st A range finder, consisting of a transparent scale of known length and
destined to be held at a given, invariable distance before the eye in such a manner
as to cover a previously estimated cross-dimension of or near to the object whose

page_text: distance is to be found, whereupon the operator observes how many parts of the
scale coincide with the estimated cross-dimension and calculates the distance to be
found by means of the given distance of the scale from the eye, the estimated
cross-dimension and the known portion of the scale covering said cross-dimension,
substantially as set forth.
2nd In a range finder of the described kind, for the purpose of simplifying the
calculation and reducing the possible errors, a scale having such a length that it
appears under the same visual angle as a known greater dimension would appear
under when viewed from a known greater distance, substantially as described.
3rd The described construction of the range finder, having the form of a
triangular metal plate, provided with rectilinear projections a, b, c, on its sides
said projections being divided by notches of different depths into 16 parts, and
their lengths being so chosen that when held before the eye at a given distance,
they appear under the visual angle of 4 meters, 4 paces and 4 man's breadths
respectively, viewed from the distance of 100 meters, substantially as shown and
described.