Improvements in Telemeters.
Year: 1890

page_text: Date of Application, 9th Aug., 1890
Complete Specification Left, 9th May, 1891-Accepted, 8th Aug., 1891
PROVISIONAL SPECIFICATION.
Improvements in Telemeters.
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 to be as follows:-
The object of our invention is to provide means for readily and accurately
determining distances without direct measurement.
We employ a deflecting prism of small angle as an attachment to a theodolite,
level, or other telescopic instrument in such a manner that two points subtending a
constant angle at the instrument are indicated upon a graduated staff held at the
point whose distance is required. The interval between those points divided by
the angle of deviation of the prism in radian measure will give the distance of the
staff. The prism may be placed within the telescope or preferably in front of the
objective and it may be used in any one of three ways.
(1) A telescope provided with cross-wire may be used, in which case a reading
of the staff may be taken in the ordinary way, and then the prism introduced into
the path of the rays with its thin edge perpendicular or nearly so to the length
of the staff and a second reading taken, the difference in the two readings furnishing
the interval referred to above.
(2) The prism may be introduced so as to intercept about half of the rays
traversing the objective from the staff (or two prisms each covering a portion of the
objective may be used with the same object) in which case two images of the staff'
will be seen superimposed the one upon the other or side by side in which case the
difference of the readings of any two points of the staff which appear in juxta-
position will furnish the required interval. To facilitate the reading of the staff in
the latter case we may provide a special mark to be placed at zero or other convenient
position upon the staff and observe the position of this mark in the one image upon
the graduated scale in the other image.
(3) The prism may be placed with its thin edge at right angles to the staff and
a reading taken and then the prism turned through 180° in its own plane (or
reversed so as to accomplish the same result). In this case the double of the angle
of deviation of the prism would give the angle subtended by the interval between
the two readings.
In these cases the distance determined will be the distance of the staff from the
prisin. When the telescope is moveable about a vertical axis as in the case of a
theodolite or levelling instrument and it is desired to read the distance of the staff
directly from this axis the mark above referred to may be placed away from its
nominal position by an amount equal to the distance on the staff which corresponds
to the distance of the prism in front of the axis.
On sloping ground the instrument may be arranged to give horizontal instead of
actual distances by arranging that the staff shall be held perpendicularly to the
line of vision of the telescope or nearly so and also perpendicularly to the cross
wire used for taking the readings, and providing means for causing the prism to
rotate about an axis parallel to the optical axis of the telescope and in such a
manner that when the line of collimation of the telescope is horizontal the thin edge
of the prism shall be parallel to the cross wire and shall rotate from this position
through angles equal to those through which the telescope is turned about a
horizontal axis at right angles to the line of collimation. If the telescope is mounted
in Y's like an ordinary Y theodolite the readings may be taken at the intersection
of the cross wires and the telescope together with the prism may be caused to rotate
in the Y's by means of bevil gearing or other equivalent mechanism through angles

page_text: equal to those through which the instrument is turned about its horizontal axis. Or
to accomplish the same result the prism may be arranged so as to be capable of
rotation relatively to the telescope and provided with graduated circle and index
and be rotated by hand through an angle equal to the angle of altitude at which
the line of collimation is directed as indicated by the vertical circle of the instrument
or otherwise. Or the telescope may be pivoted upon a horizontal axis making an
angle of 45° with the line of collimation. Or a mirror may be attached to the
telescope in front of the objective having its plane at an angle of 22° with the line
of collimation to accomplish the same result.
We may provide suitable sights attached to the levelling staff to enable the staff-
man to hold it perpendicularly to the line of sight of the telescope when desired,
and also a circular or other bubble glass for the vertical adjustment of the staff.
We may support the prism in such a manner that it can be at will placed in
front of the whole of the objective, or so as to cover only half, or to be entirely
removed from the path of the rays for which purpose we have a special arrangement
for supporting the prism by means of pivotted arms from a ring which fits the
telescope in place of its ordinary cap.
A prism may be used in a similar manner to that above described without the
use of a telescope by placing it in front of the eye so that the distant staff may be
seen partly through and partly passed it, or two prisms may be so used the eye
being placed so as to look partly through the one and partly through the other.
Dated this 8th day of August 1890.
BOTTOMLEY & LIDDLE,
154, St. Vincent Street, Glasgow, Applicants' Agents.
COMPLETE SPECIFICATION.
Improvements in Telemeters.
We, ARCHIBALD BARK, 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 :-
The object of our invention is to provide means for readily and accurately
determining distances without direct measurement.
We employ an achromatic deflecting prism of small angle as an attachment to a
theodolite, level, or other telescopic instrument in such a manner that two points
subtending a constant angle at the instrument are indicated upon a graduated staff
held at the point whose distance is required. The prism is placed in front of the
objective. The interval between the points observed divided by the angle of
deviation of the prism in radian measure will give the distance of the staff; the
staff being held approximately normal to the lines of sight. The prism may be
used in any of the following ways:—
(1.) A. telescope provided with cross wires may be used, in which case a reading
of the staff may be taken in the ordinary way, and then the prism introduced into
the path of the rays with its thin edge perpendicular or nearly so to the length of
the staff, and a second reading taken, the difference between the two readings
furnishing the interval referred to above.
(2.) The prism may be introduced so as to intercept about half of the rays
traversing the objective from the staff, (or two prisms each covering a portion of
the objective may be used with the same object) in which case two images of the staff
will be seen superimposed the one upon the other or side by side, and the difference
of the readings of any two points of the staff which appear in juxta-position will
furnish the required interval. To facilitate the reading of the staff in this case we
may provide a special mark to be placed at zero or other convenient position upon
L

page_text: the staff and observe the position of this mark in the one image upon the graduated
scale in the other image.
(3.) The prism may be placed covering the objective with its thin edge at right
angles to the staff and a reading taken; then the prism turned through 180° in its
own plane (or reversed so as to accomplish the same result) and a second reading
taken. In this case the double of the angle of deviation of the prism would give
the angle subtended by the interval between the two readings.
In these cases the distance determined will be the distance of the staff from the
prism. When the telescope is moveable about a vertical axis as in the case of a
theodolite or levelling instrument and it is desired to read the distance of the staff
directly from this axis, the mark above referred to may be placed away from its
nominal position by an amount equal to the distance on the staff, which corresponds
to the distance of the prism in front of the axis. Thus if the prismn have an angle
of deviation of 1 in 100 and the staff be graduated in hundredths of a foot, each
division intercepted on the staff corresponds to a distance of 1 foot. Then if the
prism be placed of a foot in front of the vertical axis and the interval on the staff
be counted from the 1 foot graduation, the mark M, Figures 1 and 2, may be
placed of a division above the graduation when the reading of the staff which
appears in contact with the image of the mark, say 243, 2 minus 100 i.e. 143, 2
gives the distance of the staff in feet from the vertical axis of the instrument.
On sloping ground the instrument may be arranged to give horizontal instead of
actual distances by arranging that the staff shall be held perpendicularly to the line
of vision of the telescope or nearly so and also perpendicularly to the cross wire
used for taking the readings, and providing means for causing the prism to rotate
about an axis parallel to the optical axis of the telescope and in such a manner that
when the line of collimation of the telescope is horizontal the thin edge of the prism
shall be parallel to the cross wire and shall rotate from this position through angles
equal to those through which the telescope is turned about a horizontal axis at
right angles to the line of collimation. If the telescope is mounted in Y's like an
ordinary Y theodolite, the readings may be taken at the intersection of the cross
wires and the telescope together with the prism may be caused to rotate in the Y's
by means of bevel gearing or other equivalent mechanism through angles equal to
those through which the instrument is turned about its horizontal axis.
Or to
accomplish the same result the prism may be arranged so as to be capable of
rotation relatively to the telescope and provided with graduated circle and index
and be rotated by hand through an angle equal to the angle of altitude or depression
at which the line of collimation is directed as indicated by the vertical circle of the
instrument or otherwise.
Or usually the prism would be used as on level ground, and the observations
reduced so as to give the horizontal distance in the manner commonly adopted with
other tacheometers or telemeters.
We may provide suitable sights attached to the levelling staff to enable the staff-
man to hold it perpendicularly to the line of sight of the telescope when desired,
and also a circular or other bubble glass for the vertical adjustment of the staff.
"
Instead of the staff being read directly from the instrument, a movable "target'
may be provided to be moved up or down in accordance with directions from the
observer, and its position read after its adjustment.
The principle of operation of the instrument is indicated in Figure 1, where
Pis the prism placed in front of a telescope T. M is a mark upon the staff S, shown
also in Figure 2. Readings corresponding to those above assumed are indicated in
the figure. Convenient arrangements for supporting the prism are shown in
Figures 3 to 10. In Figures 3 to 7, F is a frame carrying the prism P; C is a
cap fitting on to the end of the telescope T. The frame F is connected to the cap C
by means of two connecting rods R R. The arrangement is such that the prism
may be placed completely in front of the objective as shown in Figures 3 and 4, or
so as to cover only a portion of the objective as shown in Figures 6 and 7, or
entirely removed as shown by dotted lines at Fin Figure 6. Figures 8, 9, 10

page_text: show another arrangement where the frame F carrying the prism P pivoted by
incans of an axis A to the cap C, which fits on to the telescope T, of which L is
the objective. The prism may be placed so as to cover the whole of the field as
shown in Figures 8 and 9, or entirely removed as shown at F in Figure 9, or
placed so as to cover a portion of the objective as shown at Figure 10.
We do not however limit ourselves to the exact details of mounting shown.
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 :-
1. The combination of a telescope with cross wires or their equivalent and an
achromatic prism capable of being interposed at will in the path of the rays outside
the objective for the purpose of indicating distances by two successive readings on
a graduated staff.
2. The use of an achromatic prism placed in the path of a portion of the rays by
which a graduated staff is seen so as to give two images of the staff superimposed
or side by side whereby the distance of the staff may be determined by observing
where a graduation (or special mark) of the one image falls on the graduations of
the other.
Dated this 8th day of May 1891.
BOTTOMLEY & LIDDLE,
154, St. Vincent Street, Glasgow, Applicants' Agents.