Improvements in Telemeters or Instruments for Ascertaining the Distances of Objects.
Year: 1889

page_text: Date of Application, 9th July, 1889
Complete Specification Left, 8th May, 1890-Accepted, 5th July, 1890
TRUVIDIUNAL DPECIFICATION.
Improvements in Telemeters or Instruments for Ascertaining the
Distances of Objects.
We ARCHIBALD BARR, Professor of Engineering and WILLIAM STROUD Professor
of Physics of the Yorkshire College of the Victoria University, Leeds in the County
of York do hereby declare the nature of this invention to be as follows:
:
The object of our Invention is to provide an instrument whereby distances may be
readily determined, without direct measurement, as by the use of a surveyor's chain or
rods. .
An instrument constructed in accordance with our Invention consists of a telescope
having a prism of small angle (preferably achromatic) moveable along the tube, and
covering half its section, and hereinafter referred to as the "deflecting prisin," the
reinaining half being left vacant or preferably filled by a piece of plate glass of
thickness equal to the prism, or by a second prism of small angle, with its edge
oppositely placed with reference to the first.
Connected with the prisan or prisis is a scale for indicating the distance of the
object observed. A staff is also provided with two prominent marks upon it at a
known distance apart, and observations of the distance of this staff are readily effected
by the observer directing the telescope so as to see the marks on the staff, the line
joining the marks being perpendicular or nearly so to the axis of the telescope. By
moving the deflecting prism or prisms to and fro along the tube by some suitable and
convenient mechanism, the observer adjusts the image of one mark formed by rays
traversing the aforesaid prism, with the image of the other mark formed by rays
which have traversed the remaining half section of the tube, or the second prism, until
the two marks appear in coincidence or alignment. The scale attached to the prism
is so graduated as then to indicate the distance of the staff.
Our Invention further relates to means for minimising or eliminating any liability
to error in the observation of apparent coincidence owing to want of accurate focussing.
This may be effected by placing the 'deflecting prism' with its thin edge perpendicular
to its diametral edge. On removing theeye-piece from the telescope and viewing the
objective partly through the deflecting prism and partly through the plate glass or
second prism, one seinicircular portion of the objective will appear to be shifted with
respect to the remaining semi-circular portion, indicating thereby, that the two beams
of light have not the same mean direction. By covering up certain portions of the
objective, when the deflecting prism is in its mean position, we may make the
uncovered portions of the objective to have no apparent shift with reference to one
another. In this case and for this position of the deflecting prism' (and approxi-
mately so for other positions), the beams will have the same inean direction, and want
of accurate focussing will not cause an apparent relative displacement of the two
images which we wish to bring into aligninent.
We may if thought desirable, gear the eye-piece or the objective with the mechanism
which moves the deflecting prism in such a way, that for any motion of the deflecting
prism we get a proportional though inuch smaller notion of the eye-piece or objective,
and of the telescope. By adjusting either the distance between the two marks, or the
angle of the deflecting prism or the ratio of gearing, but preferably for a final adjust-
ment the first; it will be possible to arrange that if the eye-piece be focussed upon
the image for one distance the focussing will be accurate for any other distance as soon
as the images of the two marks are brought into correct alignment.
The scale attached to the prism may be made of ivory or other transparent or
translucent material and may be read by means of a lens placed at an opening in the
tube opposite to the index, and a window may be provided on the opposite side of the

page_text: tube behind the scale for illuminating the latter, or if preferred a mirror or reflecting
prism may be used in addition to a lens for reflecting an image of the scale and index
to a second eye-piece, placed near and parallel to the first, preferably at a distance of
about 2 inches from the first, so that one eye can be used for adjusting the marks in
coincidence, while the other can be used simultaneously or nearly so for reading the
scale. Or the second eye-piece may be dispensed with, by so arranging the position
of the mirror or reflecting prison and lens, that an image of the scale may be reflected
to the eye-piece of the telescope.
When the distance to be measured is so short that it is not indicated on the scale
of the instrument, we may provide two other marks at say one tenth the distance
between the first, obtain alignment with them, and divide the readings by ten, and for
very great distances a base line ten times as great can be taken, in which case the
readings must be multiplied by ten.
The instrument may be mounted in a similar manner to a theodolite with graduated
circles to indicate the altitude and azimuth of the object observed, and with cross wires
for defining a line of collimation, or it may be mounted with a bubble tube in the same
manner as a levelling instrument, or in connection with a magnetic compass, or in
other suitable manner.
The instrument may be used for observing the distances of objects at sea, by
bringing the water line of the object into apparent coincidence with the horizon, in
which case the height of the station, from which the observation is made, above the
sea level forms the equivalent of the base.
In order to adapt the readings of the instrument to variable bases, we may provide
means for rotating the deflecting prism or prisms about an axis in their own plane
and perpendicular to their diametral edge, or the objective may be made compounded
of two achromatic objectives with provision for altering the distance between them, so
as to have the power within certain limits of varying the equivalent focal length of the
compound telescopic objective.
In either case a suitable scale may be provided in connection with the mechanism
so as to indicate the equivalent alteration in base.
Dated this 8th dose of Tul.. 1000
J. CLARK JEFFERSON, A.R.S.M., Wh. Sc.,
2, East Parade, Leeds, Agent for Applicants.
COMPLETE
SPECIFICATION.
Improvements in Telemeters or Instruments for Ascertaining the
Distances of Objects.
We ARCHIBALD BARR, Regius Professor of Engineering in the University of
Glasgow and formerly of the Yorkshire College of the Victoria University Leeds in
the County of York and WILLIAM STROUD, Professor of Physics of the Yorkshire
College of the Victoria University Leeds in the County of York 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:-
The object of our Invention is to provide an instrument whereby distances may be
readily determined without direct measurement, as by the use of a surveyor's chain'or
rods..
;
An instrument constructed in accordance with our invention consists of a telescope
having a prism of small angle, (preferably achromatic) moveable along the tube, and:
covering half its section, and hereinafter referred to as the deflecting prisın;' the.
remaining half being left vacant or preferably filled by a piece of plate glass of:
thickness optically equivalent to that of the prism, or about equal to the thickness of
the prism, or by a second prism of small angle with its edge oppositely placed with
reference to the first.
Connected with the prism or prisins is a scale for indicating the distance of the
object observed. A staff is also provided with two prominent marks upon it at a

page_text: known distance apart, and observations of the distance of this staff are readily effected
by the observer directing the telescope so as to see the marks on the staff, the line
joining the marks being perpendicular or nearly so to the line of vision of the
telescope. To enable the staff-man to hold the staff correctly, sights may be attached
to it at right angles to its face. The staff should also be held perpendicular to the
edge of the deflecting prisin, or the telescope may be rotated about its line of vision
to accomplish this requirement. By moving the deflecting prism or prisms to and
fro along the tube by some suitable and convenient mechanism the observer adjusts
the image of one mark, formed by rays traversing the aforesaid deflecting prisin, into
apparent coincidence or alignment with the image of the other mark, formed by rays
which have traversed the remaining half section of the tube or second prism. The
scale attached to the prism may be so graduated as then to indicate the distance of
the staff from the instrument.
'
Our Invention further relates to means for minimising or eliminating any liability
to error in the observation of apparent coincidence owing to want of accurate
focussing. This inay be effected by placing the deflecting prism with its thin edge
perpendicular to its diametral edge. On removing the eye-piece from the telescope
and viewing the objective partly through the plate glass or second prism, one
semicircular portion of the objective will appear to be shifted with respect to the
rem ining semi-circular portion, indicating thereby that the two beams of light have
not the same mean direction. By covering up certain portions of the objective when
the deflecting prism is in its mean position, we may make the uncovered portions of
the objective to have no apparent shift with reference to one another. This covering
may be effected at the objective itself, or by means of a suitable diaphragm attached
to the telescope tube, or to the prism carrier. In this case and for this position of
the deflecting prisin, and approximately so for other positions, the beams will have the
same mean direction, and want of accurate focussing will not cause an apparent relative
displacement of the two images we wish to bring into alignment.
We may if thought-desirable gear the eye-piece carrier (or the objective) with the
mechanism which moves the deflecting prism in such a way, that for any motion of
the deflecting prism we obtain a proportional though much sinaller inction of the eye-
piece or objective. By adjusting either the distance between the two marks, or the
angle of the deflecting prism, or the ratio of gearing, it will be possible to arrange
that if the eyepiece be focussed upon the image for one distance, the focussing will be
accurate for any other distance as soon as the images of the two marks are brought
into correct alignment.
The scale attached to the prismu inay be made on metal and viewed through an
aperture or window in the tube, or it may be made of ivory or other transparent or
translucent material, and may be read by means of a lens placed at an opening in the
tube opposite to the index, and a window inay be provided on the opposite side of the
tube behind the scale for illuminating the latter; or if preferred a mirror or reflecting
prism may be used in addition to a lens for reflecting an image of the scale and index
to a second eye-piece near the first. This distance between the eye-pieces may if
desired be about 2 ins., so that one eye can be used for adjusting the marks in
co-incidence, whilst the other can be used simultaneously or nearly so for reading the
scale. Or the second eye-piece may be dispensed with, by so arranging the position
of the mirror or reflecting prisu and lens,that an image of the scale may be reflected
to the eye-piece of the telescope. Or again the scale may be viewed by a lens or
microscope situated within or near to the horizontal axis of the instrument when
mounted as a theodolite.
When the distance to be measured is so short,that it is not indicated on the scale of
the instrument, we may provide two other marks say at a distance apart equal to about
one tenth of the distance between the first pair (the actual distances to be calculated
as described below),obtain aligninent between thein and divide the readings by ten;
and similarly for very great distances a base line about ten times as great may be
taken, in which case the readings must be multiplied by ten. In this case the scale is

page_text: graduated to give the distance of the object from a point at a distance in front of the
objective equal to its focal length.
In order to avoid the necessity of bringing the image of the staff always to one
exact position (longitudinally) in the telescope tube, it is advisible to fix the scale
index relatively to the objective. If the instrument be not provided with crosswires,
the focussing is most conveniently done by moving the eye piece. When cross-wires
are provided, it will be most convenient to move the end of the instrument carrying
them so as to bring them into the plane of the images, as is usually done in dubipy
levels, rather than to move the objective relatively to the tube as is common in
theodolites.
The different base-lengths for use upon shorter or longer distances than those for
which the instrument is adapted with the main base lengths may be calculated as
follows: Let n be the multiplier intended to be used, B the main base and b the
subsidiary base, the focal length of the objective and d the angle of deviation of the
deflecting nrism than
The instrument may be mounted in a similar manner to a theodolite with graduated
circles to indicate the altitude and azimuth of the object observed, and with cross wires
for defining a line of collimation; or it may be mounted with a bubble tube in the
same manner as a levelling instrument, or in connection with a magnetic compass, or
a plane table, or in other suitable manner.
When the instrument is to be used as a levelling instrument or theodolite, we may
provide means for readily removing the deflecting prism out of the way of a portion
of the rays, which would ordinarily traverse it, so that it may not interfere with the
use of the instrument for levelling or measuring angles of azimuth and altitude.
In this case a parallel plate of glass is most conveniently used in connection with the
deflecting prisin instead of a second prism, so that the rays traversing it may not be
deviated in direction.
"
The instrument may be arranged to read horizontal instead of actual distances
for moderate angles of elevation or depression, by rotating the telescope tube about
its longitudinal axis through an angle equal to the angular altitude or depression
through which the telemeter is turned from the horizontal: since the effect of
displacements in a vertical plane caused by rotating the telescope through an angle
is the same as if the prism angle of the prism were smaller in the proportion cos 0,
which is also the ratio of the horizontal to the actual distance. This may be accom-
plished by supporting the telemeter in a similar manner to that adopted in Y theodolites,
and provided with any suitable mechanism such as bevel wheels or levers which will
cause the telemeter to rotate within the Y's as it is inclined upwards or downwards.
In this case, the edge of the deflecting prism should be horizontal when the line of
vision of the telescope is horizontal, and the staff should be held in a vertical plane,
through the instrument. In order to enable the staff man readily to accomplish the
latter requirement, an ordinary bubble tube or circular bubble cell may be provided
on the staff.
The instrument may be used for observing the distances of objects at sea, by
bringing the water line of the object into apparent coincidence with the horizon, in
which case the height of the station from which the observation is made above the
sea level forms the equivalent of the base, i.e. if the height above sea level be 50 ft.,
the instrument would be graduated with a scale of reciprocals, as though for use upon
a staff with two marks 50 ft. apart.
And in order that our Invention and the manner in which the same may be
performed may be more readily and clearly understood, reference is given to the
accompanying sheet of drawings and to the letters and marks thereon.

page_text: DESCRIPTION OF DRAWINGS.
Fig. 1 is a longitudinal vertical section. Fig. 2 a horizontal section at right angles
to Fig. 1. Fig. 3 is a cross section on line A A of Figs. 1 and 2; and Figs. 4 and 5
shew two forms of the staff, with marks thereon whose distance apart serves as the
base line for the observations.
a, a¹, are the tubes of the telescope or instrument, the tube a¹ being capable of
longitudinal motion within the tube a, for the purpose of bringing the image of the
object on the cross wires xx. d is the deflecting prism arranged so as to cover half
the circular section of the beam of light, as shewn in the cross section Fig. 3, and d¹
is a piece of plate glass of a thickness approximately equal to that of the prism.
The thin edge of the prism d is at right angles to the diametral edge.
The deflecting prism d and the plate are mounted in a frame piece e, which
is capable of being moved axially or longitudinally within the tube or tubes a a¹.
Connected with this frame piece e is a scale of transparent or translucent material,
and marked with a scale of reciprocals for indicating the distance of the object
observed. The scale is guided in the frame g, attached to the tube a opposite a hole
or window in the tube a, and the frame g is provided with an index or pointer i for
indicating the mark or marks to be read off from the scale. The frame piece cand
consequently also the deflecting prism d and the scale fare moved axially or longitu-
dinally within the tube a by means of the screw j, which passes through a tapped
hole or nut formed in or on the frame piece e, the screw j being capable of rotary but
not of longitudinal notion in the fixed bearings. The screw j can be thus rotated
by means of the milled head l.
A window and lens m are provided in the side of the tube a opposite to the index or
pointer i. is a rod upon which the lower part of the frame e is guided. This
rod is bent in an oblique manner at the end nearest the eye-piece as shown at s
Fig. 2, so as to effect the removal of the deflecting prism from the path of the rays,
when the prism carrier is brought to the extremity of its path, the scale being flexible
enough or the frame g so formed as to allow of this motion.
A diaphragm e¹ is formed on the prison carrier e of sufficiently small dimensions,
with the object as hereinbefore described of minimising or eliminating any liability to
error in the observation of apparent coincidence owing to want of accurate focussing.
n is the focussing head attached to the tube a and actuating a pinion which gears into
a rack attached to the tube a³, which supports the cross wires and eye-piece.
It will be readily understood that we do not limit ourselves to the exact form and
details of the instrument as herein illustrated and described, as these may be varied
without altering or departing from the nature of our said Invention, but
Having now particularly described and ascertained the nature of our said Invention,
and in what manner the same is to be performed, we declare that what we claim is :-
1º. A telemeter or range finder consisting of a telescope provided with a deflecting
prism (or prisms) placed in and moveable along a portion of the rays from the
objective to the eye piece, for the purpose of bringing into apparent coincidence the
images of two marks at a known distance apart upon a staff or other object, and of
indicating by the position of such prisın (or pristns) the distance of the staff from
the instrument.
2º A telemeter or range finder consisting of a telescope provided with a deflecting
prism (or prisms) placed in and moveable along a portion of the rays froin the
objective to the eye piece for the purpose of bringing into apparent coincidence
the images of the water line of an object at sea and the visible horizon, and of
indicating by the position of such prism (or prisms) the distance of the object.
Dated this 7th day of Mov 1890