Improvements in Instruments for Measuring or Setting-out Distances or Angles.
Year: 1890

page_text: I ate of Application, 18th Mar., 1890
Complete Specification Left, 13th Dec., 1890-Accepted. 7th Feb.. 1891
PROVISIONAL SPECIFICATION.
Improvements in Instruments for Measuring or Setting-out Distances
or Angles.
We, ARCHIBALD BARR, Professor of Engineering in the University of Glasgows
and WILLIAM STROUD, Professor of Physics, Yorkshire College, Leeds do hereby
declare the nature of this invention to be as follows:-
This invention relates to optical squares and to that class of range finder in which
two observers, one at each end of a base line employ simultaneously each an
angular instrument, the one to set out a constant angle (say a right angle) by
adjusting his position with reference to the object (whose range is required) and
the second observer while the latter measures the range by adjusting his instrument
to the angle between the first instrument and the distant object.
Our invention relates in the first place to an optical square or instrument for
setting out a constant angle (say 90°); in the second place to the instrument for
measuring the variable angle referred to, and by this means indicating the distance
of an object; in the third place where a base line of constant length is used, to
optical means for observing whether the distance between the two observers is
correct without using any connecting string or other actual connection between the
instruments or observer; and in the fourth place to means for dispensing altogether
with a fixed base without sacrificing the advantages of a direct reading instrument.
(1) The constant angle (say 90°) is set out by means of an optical square
consisting of a prism of glass or other transparent substance having two reflecting
faces (which may be silvered or unsilvered) inclined at 45° to one another and two
transmitting faces so cut that the rays pass through them (on entrance and
emergence respectively) normally or nearly so.
This
One method of making the prism is to construct the angles as follows:-
90° between the two transmitting faces, 1124 between each transmitting and the
contiguous reflecting face, and 45° between the two reflecting faces.
arrangement is equivalent to an ordinary optical square with the advantage
that it is not liable to derangement. The lines joining two objects seen in
coincidence when one is looked at direct over the prism and the other _as
reflected in the prism should be at right angles to one another. In order
to adjust this angle very accurately to 90° we attach when necessary to one of the
transmitting faces (preferably the one which is at right angles to that towards
which the eye is directed) a prism of glass or other transparent material of small
angle and rotate it in its own plane upon the face until the adjustment of the angle
is correct, and then cement or otherwise fix it in that position.
(2) For ascertaining the distance of the object, a second observer is provided with
an instrument containing a prism similar to that already described for setting off
the constant angle, and also a prism of glass or other transparent material of small
angle, hereinafter called the deflecting prism for measuring small deviations from
the right angle. This prism is placed in the path of the rays from either the
distant object (whose range is required) or a mark upon the instrument of the
other observer which is to be arranged so as to be seen in coincidence with the distant
object.
The deflecting prism is provided with means for rotation about an axis perpen-
licular to one of its faces, or nearly so, (or as an alternative method the prism may
be rotated about the thin edge of the prism or an axis parallel thereto) for the
purpose of altering the direction from which the rays traversing it appear to proceed.
By this means, small deviations from the right angle can be measured and the
mount of rotation of such a prism may be used with a base line of constant length
for reading off the distance directly upon a graduated circular scale attached to the

page_text: deflecting prism without the intervention of any multiplying gear of any kind and
further the length of scale can be made as much as is desired by simply increasing
the diameter of the circular scale. Such scales will extend round a semi-circle at
most and two such scales may be graduated on opposite semi-circles so as to correspond
to two different lengths of base."
(3) When a base line of constant length is used, the usual connecting string may
he dispensed with by adopting the following device :-One observer carries a staff
or other object (held say vertically) on which two prominent marks are made, ay
5 feet apart; the second observer is provided with a prism of small angle, which is
held with the thin edge horizontal when the staff is to be held vertically and so as
to be in front of half the pupil of the eye, the space in front of the other half being
either uncovered or provided with a small plate of glass or prism of small but
not necessarily the same angle with its thin edge parallel to but oppositely
placed with reference to the first prism, or in some cases the prisms may be of
different angles and have their thin edges similarly directed or placed. In this
way the observer sees two images of the staff and marks the one above the other,
and by moving to or from the other observer, he can make the image of one
mark seen through one half of the pupil coincide with the image of the other mark
seen through the other half. When this adjustment is accomplished the distance
between the two observers will depend only on the distance between the marks
on the staff and the angle or angles of the prism or prisms. When necessary a
mirror or reflecting prism may be used in connection with the above device to
obviate the necessity of the observer turning his head when he desires to verify
his distance from the other observer. Instead of placing the prisms directly before
the pupil they may be used in connection with some telescopic arrangement.
(4) For the instrument with a variable base we propose to allow for the change
in base by arranging the prism or prisms (mentioned under the third part of our
invention) inside the tube of a small telescope and o disposed as to be capable of
longitudinal motion within the same between the objective and the eye piece.
Parallel to this telescope and conveniently at a distance from it equal to the
distance between the eyes will be attached a tube (say on the right-hand) containing
a reflecting prism at the end near the eye as above described under the first part
of our invention and a deflecting prism as described under the second part of our
invention, the latter capable of rotation as described. In this case however the
scale instead of being linear so as to read off directly for a constant base, will be
superficial so that by shifting either pointer or scale suitably in a longitudinal
direction, allowance may be made for change in base. The arrangement in fact is
equivalent to having an infinite number of semi-circular scales corresponding to
every variety of base within certain limits. If the base is changed the scale is
arranged to be changed so as to suit the new base. This may be done thus:-The
scalc may be made upon the outside of a cylindrical tube, or inside a cylindrical
tube of transparent or translucent material or it may be made say of paper cut to
the form of a rectangle one side of which (say A B) is equal to the length of
traverse of the prism or prisms which move longitudinally in the small telescope,
the outer side of the rectangle (A C) being of such a length that it may be just
long enough to meet and form a complete circle when the paper after graduation is
finally cemented inside or outside a glass tube about 2" in diameter.
As only a semi-circle can be utilized for the purpose of any scale one half this
piece of rectangular paper would be ungraduated. It will however be convenient
to graduate the remaining half so as to correspond to bases of say half the length
and to have a mark upon the staff of the assistant observer bisecting the previously
mentioned marks and to adjust this bisecting mark in coincidence with either of the
other two marks in the telescope tube when this portion of the scale is employed.
Either the scale or the scale pointer may be capable of longitudinal motion and
either capable of rotation and attached to the deflecting prism so as to rotate with
it. Let us suppose the scale pointer fixed to the main tube of the instrument.
The glass tube carrying the scale will also have firmly attached to it at the end

page_text: farther from the eye the deflecting prism, so that the glass tube and prism can be
rotated together inside a metal tube provided with a suitable window extending
nearly the whole length of the tube so as to admit light through the glass for
reading the scale.
This metal tube will be capable of longitudinal motion inside the right hand tube
which supports the reflecting prism mentioned above as well as the pointer and will
be so connected to the prism or prisms which move longitudinally inside the small
telescope that the metal tube and prism or prisms may be moved simultaneously by
a mechanical arrangement which may be similar to the ordinary focussing arrange-
ment in an opera glass. The operation of determining or allowing for the change
in base will consist in observing the two marks on the staff or other object mentioned
under the third part of our invention and carried by the assistant observer by using
say the left eye to look through the small telescope. The images of these marks
may be adjusted to appear in coincidence by moving the prism or prisms referred to
nearer to or further from the object of the small telescope by using the mechanical
arrangement just referred to and which may consist of a quick pitched screw situated
between the telescope tube and the right-hand tube and actuating simultaneously the
prism or prisms inside the telescope tube and the metal tube referred to which
carries the glass tube (upon which the scale is fixed) in such a way that the glass
tube can only be rotated within the metal tube but cannot be moved longitudinally
within the same. The operation of measuring the range will consist in superposing
a prominent mark or line carried on the instrument of the assistant observer on the
object whose range is required, an operation which is effected by rotating the glass
tube carrying the deflecting prism about its axis.
The assistant observer having previously adjusted his position so that the angle
subtended by the distant object and a mark or line on the chief observer's instru-
ment shall be 90°. When the scale is suitably graduated and the pointer suitably
placed the reading of the range may be at once effected.
A suitable reflector may be provided in connection with the telescope so that it
may not be necessary for the observer to turn his vision towards the assistant
observer or and perhaps better the chief observer may look directly at the assistant
observer's mark or line and see the distant object by reflection in the reflecting
prism.
Dated this 17th day of March 1890.
BOTTOMLEY & LIDDLE,
154, St. Vincent Street, Glasgow, Applicants' Agents.
COMPLETE SPECIFICATION.
Improvements in Instruments for Measuring or Setting-out Distances
or Angles.
We, ARCHIBALD BARR, Professor of Engineering in the University of Glasgow,
and WILLIAM STROUD, Professor of Physics, Yorkshire College, Leeds, do hereby
declare the nature of this invention and in what manner the same is to be
performed, to be particularly described and ascertained in and by the following
statement:
This invention relates to optical squares and to that class of range finders in
which two observers, one at each end of a base line employ simultaneously each
an angular instrument, the one to set out a constant angle (say a right angle) by
adjusting his position with reference to the object (whose range is required) and
the second observer, while the latter measures the range by adjusting his instrument
to the angle between the first instrument and the distant object; and the object of
our invention is to provide instruments which shall not be liable to be put out of
optical adjustment through accident or use.
Our invention relates in the first place to an optical square or instrument for

page_text: setting out a constant angle (say 90°); in the second place to the instrument for
measuring the variable angle referred to, and by this means indicating the distance
of an object; in the third place where a base line of constant length is used, to
optical means for observing whether the distance between the two observers is
correct without using any connecting string or other actual connection between the
instruments or observers; and in the fourth place to means for dispensing
altogether with a fixed base without sacrificing the advantages of a direct reading
instrument.
(1) The constant angle (say 90°) is set out by means of an optical square
consisting of a prism of glass or other transparent substance having two silvered
reflecting faces inclined at 45° to one another and two transmitting faces so cut that
the rays pass through them (on entrance and emergence respectively) normally or
nearly so.
One method of making the prism is to construct the angles as follows:-90°
between the two transmitting faces, 11240 between each transmitting and the
contiguous reflecting face, and 45° between the two reflecting faces. This arrange-
ment is equivalent to an ordinary optical square, with the advantage that it is not
liable to derangement. The lines joining two objects seen in coincidence when one
is looked at directly over the prism and the other as reflected in the prism should
be at right angles to one another. In order to adjust this angle very accurately
to 90° we attach when necessary to one of the transmitting faces, or place in one of
the beams of light by which the two objects are seen, a prism of glass or other
transparent material of small angle and rotate it in its own plane until the adjust-
ment of the angle is correct and then cement or otherwise fix it in that position.
Any other form of doubly reflecting prism which will set out a constant angle
such as a right angle may be used in place of the reflecting prism above described.
(2) For ascertaining the distance of the object a second observer is provided
with an instrument carrying a doubly reflecting prism and also a prism of glass or
other transparent material of small angle, hereinafter called the deflecting prism for
measuring small deviations from the right angle. This prism is placed in the path of
the rays from either the distant object (whose range is required), or a mark upon
the instrument of the other observer, which is to be arranged so as to be seen in
coincidence with the distant object.
The deflecting prism is provided with means for rotation about an axis
perpendicular to one of its faces, or nearly so, (or as an alternative method the
prism may be rotated about the thin edge of the prism or an axis parallel thereto)
for the purpose of altering the direction from which the rays traversing it appear to
proceed. By this means, small deviations from the right angle can be measured
and the amount of rotation of such a prism may be used with a base line of constant
length for reading off the distance directly upon a graduated circular scale attached
to the deflecting prism without the intervention of any multiplying gear of any kind,
and further the length of scale can be made as much as is desired by simply
increasing the diameter of the circular ring upon which it is cut. Such scale will
extend round a semi-circle at most, and two such scales may be graduated on
opposite semi-circles so as to correspond to two different lengths of base.
(3) When a base line of constant length is used, the usual connecting string may
be dispensed with by adopting the following device :-One observer carries a staff
or other object (held say vertically) on which two prominent marks are made, say
5 feet apart; the second observer is provided with a prism of small angle, which is
held with the thin edge horizontal (when the staff is to be held vertically) and so
as to be in front of half the pupil of the eye, the space in front of the other half
being either uncovered or provided with a small plate of glass or prism of small but
not necessarily the same angle with its thin edge parallel to but oppositely placed
with reference to the first prism, or in some cases the prisms may be of different
angles and have their thin edges similarly directed or placed. In this way the
observer sees two images of the staff and marks, the one above the other, and by

page_text: moving to or from the other observer he can make the image of one mark seen
through one half of the pupil coincide with the image of the other mark scen
through the other half. When this adjustment is accomplished the distance between
the two observers will depend only on the distance between the marks on the staff
and the angle or angles of the prism or prisms. When necessary a mirror or
reflecting prism may be used in connection with the above device to obviate the
necessity of the observer turning his head when he desires to verify his distance
from the other observer. Instead of placing the prisms directly before the pupil,
they may be used in connection with some telescopic arrangement.
(4) For the instrument with a variable base, we propose to allow for the change
in base by use of a telescopic instrument constructed in accordance with our Patent
No. 11,025, of 1889. Parallel to this telescopic instrument and conveniently at a
distance from it equal to the distance between the eyes, will be attached a tube (say
on the right hand) containing a reflecting prism at the end near the eye as above
described under the first part of our invention, and a deflecting prism as described
under the second part of our invention, the latter capable of rotation as described.
In this case however the scale instead of being linear so as to read off directly for
a constant base, will be superficial so that by shifting either pointer or scale suitably
in a longitudinal direction, allowance may be made for change in base. The
arrangement in fact is equivalent to having an infinite number of semi-circular
scales corresponding to every variety of base within certain limits; and if the base
is changed the scale is changed so as to suit the new base.
In order that our invention may be properly understood and readily carried into
effect, we have hereunto appended an explanatory sheet of drawings in the several
figures of which corresponding parts are marked with the same letters of reference.
Figure 1 shows one mode of applying our invention for telemetric operations
when two observers, one at each end of a base line A B employ simultaneously each
an angular instrument, the one B, whom we shall call the assistant observer) to set
out a constant angle (say of 90°) by adjusting his position with reference to the
chief observer and the object O, while the chief observer at A measures the
range A O by adjusting his instrument to the angle OA B.
The instrument carried by the assistant observer consists of an optical square or
instrument for setting out a constant angle (say 90°). We prefer to construct it as
shown in Figures 2 and 3, Figure 2 being a sectional plan upon the line of
Figure 3, and Figure 3 a sectional elevation upon the line y y Figure 2, where C
represents a prism of glass, having two silvered reflecting faces DE and FG,
inclined at 45° or nearly so to one another and two transmitting faces DH and H G
at 90° or nearly so to each other, and each making an angle of 112° or nearly so
with the contiguous reflecting faces. Any other form of doubly reflecting prism
which will set out a constant angle such as a right angle may be used in place of
the prism above described. This prism takes the place of, and is used in the same
manner as, the two mirrors in an ordinary optical square. The lower (or upper)
plane of the prism is placed at the level of the eye-hole I, so that the observer looks
partly through, and partly under (or over) the prism. This prism is held in a
suitable metal case, and when used in connection with another instrument in the
manner described above for the determination of distances this metal case also
carries a prominent mark at J for the chief observer to align upon the distant
object.
On account of the difficulty of grinding the prism to indicate exactly 90° or other
desired angle, it may be necessary to adjust the angle, and this may be done by
placing in one of the beams of light from either the mark or the distant object a
refracting prism of small angle K. This prism may be chosen of the required
angle, or a prism of larger angle may be used and be rotated about an axis
perpendicular to its own plane until the desired correction is effected, and fixed in
that position. This prism may be cemented upon one of the transmitting faces of
the reflecting prism, as shown at K' in Figure 2ª.

page_text: The chief observer's instrument is represented in Figures 4 and 5, Figure 4 being
a sectional plan upon the line. Figure 5, and Figure 5 an elevation of the instru
ment. Lis a reflecting prism similar to that just described, and if nece--ary
similarly corrected; M is a refracting prism giving a small angle of deviation and
preferably constructed of crown and fint glass so as to be achromatic. This
prisin M is held in a piece N capable of rotation relatively to the case P' supporting
the reflecting prism; Q is a ring forming part of the piece N and graduated as
described below to indicate ranges by reference to an index R, Figure 5 marked
upon the case P S is a portion of the case P which serves to cover and protect the
scale, except in the region of the index; the rings T and U constitute & imechanism
(sufficiently shown on drawing) for supporting the piece N upon the case P. A
mark V, (preferably white upon a black background V*) supported upon the prism
case l' is to be seen by the assistant observer super-imposed upon the distant object.
The instruments may be provided with telescopes to increase the accuracy of
observation.
When a string or other material connection is used to fix the base length, two
wires or chains W W, Figures 2 and 3 connected to a hook X may be attached to
cach instrument, so that the pull of the string may enable each observer more
rapidly to bring the mark upon the other instrument into the field of view.
The mode of operation of the refracting prism M, Figure 4, and of the correcting
prism K, Figure 2, is as follows:-If such a prism be placed in front of the eye, an
object seen through it will appear to describe a circle as the prisın is rotated in its
own plane. Thus, in our instruments when the prism is in the position shown by
the full lines in Figure 6, the object ()' will be seen in coincidence with the mark V,
whereas when the prism is rotated through 180° as shown by the dotted lines, the
object will be seen in coincidence with the mark V. For positions of the prism
intermediate between these extremes, objects intermediate in azimuth between 01
and O will be seen in coincidence with V, the instrument being suitably directed
in altitude in each case.
The scale may be graduated by trial to correspond to a particular base, or the
following method which will be understood from Figure 7 may be used. Suppose
that when the refracting prism has its thick edge towards the assistant observer (as
shown by the dotted lines in Figure 6) an object at 500 yards is seen by cach
observer in coincidence with the other's mark, and that when the prism is turned
through 180° an object at an infinite distance is seen in coincidence with the marks.
Set out a scale of reciprocals along the diameter YZ of a circle, Figure 7. Draw
lines through the graduations perpendicular to the diameter; the points where
these lines cut the circle will show the graduations corresponding to 1000 yards,
2000 yards etc.
We may graduate the other half of the scale ring so as to correspond to a
different base, or three such scales may be employed (each occupying not more
than 120°) and a separate index on the piece P, provided for each.
Instead of using a string or other material connection we may provide optical
means of indicating the distance between the two observers. This may be accom-
plished by employing a telescopic instrument having a moveable deflecting prism
according to our l'atent No. 11025 of 1889 carried by one of the observers, and two
marks at a fixed distance apart carried by the other observer. Scales may be
provided upon the scale ring Q, Figure 4, to correspond to a variety of bases. Or
a telescopic instrument of the kind referred to may be arranged parallel to the chief
observer's instrument, and the scale ring Q, Figure 4, may be lengthened to form a
scale drum and graduated with curved lines in such a manner that when the
deflecting prism of the telescopic instrument is moved so that the marks carried by
the assistant observer are seen in coincidence from the chief observer's station, an
index will be moved along the scale drum so that the curved lines where they are
intersected by the index during the rotation of the drum, shall form a scale
appropriate to the distance between the observers as base.

page_text: Having now particularly described and ascertained the nature of our said inven-
tion and in what manner the same is to be performed, we declare that what we
claim is :-
1. Adjusting optical squares or other instruments for setting out constant angles
by means of a deflecting prism of the required angle, or a prism of greater angle,
rotated about an axis perpendicular to its own plane till the required adjustment is
effected, and fixing the prism in this position.
2. An instrument for use in telemetric observations substantially as described
with reference to Figures 4 and 5.
3. A range tinder or telemeter consisting of two instruments substantially as
described, one for setting out a constant angle, the other adjustable to a variable
angle by the rotation of a deflecting prism in one of the beams of light, such prism
carrying a scale upon which the distance of an object may be directly read.
Dated this 17th day of December 1800.
BOTTOMLEY & LIDDLE,
154, St. Vincent Street, Glasgow, Applicants' Agents.
BOTTOMLEY & LIDDLE,
154, St. Vincent Street, Glasgow, Applicants' Agents.