Improvements in Telemeters.
Year: 1895

page_text: Date of Application, 26th Mar., 1895-Accepted, 29th June, 1895
Improvements in Telemeters.
I, FRANK LAWTON OLCOTT WADSWORTH, of The Ryerson Physical
Laboratory, University of Chicago, Chicago, in the County of Cook and State of
Illinois, United States of America, Physicist, 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 :-
My invention relates to the art of measuring distances, and the object of
my invention is to determine the distance between any two points in sight of
each other without the use of a chain with greater rapidity and accuracy than
heretofore.
My invention consists in a new and improved method of determining distances
and in convenient and novel apparatus adapted to ready use in the determination
of distance by this new method, the construction and operation of which is
hereinafter fully described.
To understand the novel features of the invention it will be well to briefly
review the present state of the art and refer to the different methods of measurement
of distance without the use of a chain or tape which are now used.
In brief they all depend on a measurement of one side and two angles of a
triangle, two of whose vertices are the points the distance between which is to be
determined.
In particular they may be divided into two general classes according to the
position of the base line or measured side of the triangle with reference to the
observer.
1st. When the base line passes through the position of the observer in which
case the determination of the distance involves the measurement of the two angles
between the base line and the lines from the extremity of the base to the distant
object. The measurement in this case may be reduced in complexity, though not
increased in accuracy, by placing at the two extremities of a known and fixed bas
two telescopes which are simultaneously directed at the distant object, the angle
between their optical axes viz. the angle subtended by the base line itself as
viewed from the distant object, being determined either by means of two circle
readings or by an automatic arrangement similar to that used in the Fiske range
finder.
Or the two telescopes may be rigidly connected with their optical axes parallel,
and the object brought on the cross wires of one by turning the whole system.
The same object is brought on the cross wires of the second by deflecting the rays
of light from the object before they reach the telescope either by reflection from a
mirror whose angle is variable or by passage through a prism.
In other cases one telescope is dispensed with by placing at one extremity of the
base a reflecting mirror which reflects the rays from the distant object to a second
mirror in front of the telescope at the other end of the base, which in turn reflects
them into this telescope together with those coming directly from the object. By
determining the divergence of these two sets of rays by the separation of the two
images in the field of the telescope we can measure the angle subtended by the
base line and therefore determine the distance.
2rd. To the second class belong those methods in which the base is at the

page_text: distant object. As it is to this class that the present invention relates we may with
advantage consider them somewhat in detail.
Devices of this class may be subdivided into two classes:
1st. Those in which the distance is determined by determining the length of the
base line subtended by a fixed angle, at the observing telescope.
To this class then belongs the stadia method of measurement, which consists in
placing in the focal field of the telescope two fixed cross wires which being at a
fixed distance apart subtend a known and fixed angle in the field of the telescope.
We then observe the length of rod intercepted between these two wires either by
moving two targets on the rod until they coincide with the two cross wires, in
which case the length of the rod is read off by the rodman; or else by reading
directly on the face of the rod the intercepted distance, the rod being suitably
graduated for that purpose.
This method involves then either a setting and reading by the rodman or person
in charge of the movable base, or 2nd, a double reading (one for each wire) of the
observer at the telescope.
To this class also belong those instruments in which two images of a rod are
formed in the field of the observing telescope by the use of a split objective or eye
piece, or by the interposition of a prism in front of the objective covering one-half
the field, as in Richard's invention, the angular separation of the two images
remaining constant and the distance of the rod being determined by their linear
separations, this being ascertained either by moving a target on the rod until its
two images are coincident or by letting the observer at the telescope read on the
rod itself the linear separation of the two images.
It will be noted that both these methods require either a setting and reading of
the target by an assistant or else a visual reading by the observer of the graduations
on the face of the rod.
To the second subdivision belong those methods in which the length of the rod
or base line is kept constant and the distance determined by a measurement of the
angular value of the rod at the observing telescope.
Heretofore the measurement of this angular value has in all devices employed
for this purpose been made by setting the cross wire of the observing telescope
first on one end or one target of the rod, and then on the other, and reading the
angle through which the telescope is turned between the two settings.
The objections to this method are:
1st. The doubling of the personal error of setting, introduced by the necessity
for two settings in the method of measurement heretofore used.
2nd. The time error caused by instrumental changes between the two settings.
3rd. The time required to make a measurement by this method which is much
greater than that required for example by the stadia method.
The method which I have invented to overcome these and other difficulties
makes the rod a base line of fixed length, as in the last class, and determines the
distance by the measurement either of the angle which it subtends at the telescope,
or of the lateral separation of two images of the rod.
The novelty of my invention, then, lies in the employment of a rod with two
marks or targets at a fixed distance apart as opposed to one in which the distance
is variable, and a means for measuring the angle subtended by this rod as viewed
from the observing telescope by a single setting of an attachment secured to the
telescope, as opposed to the measurement of this angle by two settings, between
which the axis of the telescope is moved through the angle to be measured.
The measurement by a single setting is accomplished by forming two images of
the fixed rod, one of these images being movable in the field of the telescope with
reference to the other, in consequence of which they may be separated by the exact
length of the rod, by bringing one image of the upper target into exact coincidence
with the other image of the lower target. Further, this movement of the images
is accomplished by the motion of one of the elements of the attachment to the
telescope, the motion being measured on a suitable scale, the graduations in which

page_text: are either arbitrary, in which case the distance of the rod is determined by
reference to a table or represent directly the distance in feet or yards of the rod
from the instrument.
This method of measuring the angle enables me to determine the distance
between rod and telescope with greater accuracy and at the same time greater
rapidity than heretofore, and to dispense if necessary with both cross wires and tripod
or other support for the observing telescope. Greater accuracy is secured, 1st: by
reason of the fact, well known to those who have used a telescope, that two objects
may be brought into coincidence more accurately than the cross wires can be set
on a single object: 2nd: by reason of the fact that one setting is dispensed with:
3rd by reason of the fact that in consequence of this there is no error due to
instrumental changes in the interval between two settings and the observation may
be taken even when the rod or instrument is vibrating or swaying with the wind
without affecting the accuracy of the result.
:
Greater rapidity is secured, 1st, by measurement of the angle and hence
determination of the distance at a single setting, by direct reading on a scale if
desired.
It will be understood that I know that the method of measuring angles between
two objects by bringing them into coincidence in the field of a telescope by the aid
of two mirrors one of which covers half the field of the telescope is not new, it
having been used in the sextant for nearly two hundred years, but the combination
of this means of measurement of an angle, whose advantages have already been
briefly alluded to, with a portable telemeter rod of fixed length for the direct
purpose of rapidly and accurately determining distances is the distinguishing
novelty of my invention and it is to this general principle of measurement of
distance that I wish especially hereinafter to lay claim as new.
To carrying out my method I have devised several forms of telemeters, all
adapted to be readily attached to or detached from an ordinary surveyor's
telescope, and consisting of two or more refracting prisms, or mirrors or total
reflection prisms so disposed in the line of sight that when the telescope is directed
at the rod two images of the latter are seen, one by a series of refractions or
reflections through a train of prisms or mirrors, one of which is movable with
respect to the others, and the other image, either directly, or by a series of
refractions or reflections through a train of refracting prisms or reflecting mirrors
or total reflection prisins all of whose surfaces are fixed with reference to each
other.
By the relative movement of the surfaces of the first train the corresponding
image may be moved with reference to the other, so as to separate these two
This relative
images by the exact length of the rod as heretofore described.
movement is accomplished by a rotation of one, or in some cases two, of the prisms
or mirrors about an axis or axes which is parallel to their plane and at right
angles to the line of sight.
In the accompanying drawings,
Sheets 1, 2, 3, and 4 illustrate the use of movable prisms in carrying out my
method. Sheets 5, 6 and 7 show movable mirrors for the same purpose.
Sheet 8 shows a fixed pair of reflectors or a reflecting prism.
Referring to the several figures on these sheets,
Figure 1 shows a telescope provided with one form of my telemeter attachment,
directed towards a rod provided with two fixed targets.
Figure 2 is a side elevation of the attachment. Figure 3 is a front view of the
same. Figure 34 (Sheet 2) shows a similar view of an attachment having only
one prism instead of two.
Figure 4 is a diagram.
Figure 5 is a side elevation of an attachment having two refracting prisms
rotatable in opposite directions on the same axis. Figure 6 is a front view of the
same.

page_text: Figure 7 is a horizontal section on line 7-7 in Figure 6.
Figures 8, 9 and 10 are diagrams.
Figure 11 is a side elevation, partly in section, of an attachment in which the
refracting prisms are revoluble about an axis perpendicular to their refracting
edges. Figure 12 is a front elevation of the same.
Figure 13 is a side elevation of a form in which the refracting prisms are
inserted between the object glass and the eye piece of the telescope. Figure 14 is
a cross section of the same:
Figures 15 and 16 are diagrams.
Figure 17 is a side elevation of one form of attachment in which mirrors are
used. Figure 18 is a front elevation thereof.
Figure 19 is a diagram:
Figures 20 and 21 show different views of the rod as it appears when looked at
through the telescope.
Figure 22 is a side elevation and Figure 23 an end view of an attachment in
which total reflection prisms are used. Figures 24 and 25 are respectively similar
views of a modification. Figures 26 and 27 are similar views of a further
modification.
Figure 28 is a perspective view of a double total reflection prism.
Figure 29 shows a further modification in which two fixed mirrors, or a double
total reflection prism, are used. Figure 30 is an end view of the same.
Figure 31 is a diagram, and Figure 32 is a side view of the prism.
In describing my invention, I will first take up the forms shown on Sheets 1, 2, 3
and 4. The rotation of a refracting prism about any axis parallel or perpendicular
to its refracting edge will, as is well known, change the deviation of the rays
passing through it. If then we place a refracting prism over one-half the field
of the telescope with its refracting edge horizontal and leave the other half of the
field uncovered we shall see in looking into the field of the telescope two images
of the object at which the telescope is directed, one displaced vertically with
reference to the other by an angular amount equal to the deviation of the rays
which have passed through the prism.
By turning the prism about an axis either parallel or perpendicular to the faces
and the refracting edge the vertical separation of the two images may be altered
by reason of the principle just cited and in a certain position the two images will
be separated by an amount equal to the length of the rod, viz.: the lower target
of one image will be in exact coincidence with the upper target of the other
image. Then from the physical law governing the relations between the angular
position of the prism and the resulting deviation of the rays which pass through
it at that angle, the angular separation of the two images (viz.: the angles subtended
by the rod at the telescope) may be determined, and from this, knowing the length
of the rod, the distance of the rod from the telescope. In practice the angular
position of the prism with reference to the axis of the telescope is indicated by a
scale which may be graduated into distances in feet or yards or metres directly if
desired.
The telescope A may be of any usual construction, and may be mounted or not
upon a tripod, as desired. The rod B is provided with two targets or other
marks B B2, rigidly fixed at a carefully measured distance apart, say ten or
twelve feet.
The several modifications of my telemeter will be described in the order in
which they are illustrated.
The attachment shown in Figures 2 and 3 is composed of two refracting
prisms C C each covering one-half the field of the telescope, placed side by side,
the thick end of one corresponding with the thin edge of the other, and their
meeting edges being on the vertical diameter of the object glass of the telescope.
The object of using two prisms cach covering one-half the field instead of the
single prism covering only one-half the field is two-fold: 1st. To secure equality

page_text: of illumination of the two images; 2nd. To secure increased angular separation of
the two images without increasing the refracting angle of the prism.
The prisms are firmly held in a frame D, which is hinged preferably at its lower
end, on pivots E whose common axis is transverse to the line of vision and to the
line joining the targets. The pivots are held in ears fon a short tube F, which
is fitted to slip snugly over the large end of the telescope. Any other suitable
node of securing the attachment to the telescope may be used, care being taken
that the axis of the pivots E shall be not only tiansverse to the line of vision but
also perpendicular to the line joining the targets B¹ B².
To measure the angular movement of the frame D, a scale G and index H are
provided, one fastened rigidly to the frame D and the other to the tube F. In
the drawing, the scale is shown attached to the frame and the index to the tube;
but their relative positions may be changed if desired. When the prisons are
vertical or at right angles to the line of vision, the index stands at zero, as shown
in Figure 2.
It is assumed that the index of refraction of the prisms, and the angle between
the two faces of each, are known. Knowing these and also the minimum deviation
of the prism, the angle of divergence of the ray through each prism can be
calculated for each degree of inclination of the frame D as shown by the scale G.
The mathematical formula for this calculation is well known and need not be
given here. Having determined these angles, and knowing the fixed base B' B²,
the distances corresponding to the degrees on the scale G can be readily calculated,
and if desired they may be entered on the scale.
In order, therefore, to measure a distance from the telescope to the rod, it is
simply necessary to observe the rod through the telescope and then turn the
frame D on its pivots until one of the images of the upper target coincides with
the other image of the lower target, when the index H will indicate on the scale
the distance sought. Thus, in Figure +, with the rod at station 1, the target will
coincide with the prisms tilted to position 11. If the rod is moved nearer, as to
station 2, the frame D will have to be tilted still further to position 2¹.
more distant station, as 3, the frame will stand at 31.
For a
It is evident that the greatest distance which can be measured with a given set
of refracting prisms is that at which the rod subtends the angle of deviation
produced by the prisms in their 0 position. To increase this distance one of the
prisms as C may be made removable. The angular separation of the images will
now be half as great as before, and the maximum distance that can be measured
will be doubled. Thus with a prisin of 1° giving a minimum angular deviation of
about 30' and a rod 12 feet long we can measure a maximum distance of about
1400 feet.
With the attachment shown in Figures 2 and 3, the images must be observed
exactly in the centre of the field of the telescope, since any deviation of the line
of sight from the axis of the telescope is equivalent to a change in the position of
the prisms. In order to overcome this objection and render the action of the
prisms independent of the position of the axis of the telescope, I have devised the
modification shown in Figures 5, 6 and 7. The prism C2 is here held in a frame I
which is journalled on trunnions ii in arms K projecting in front of the object
glass of the telescope. The frame may have a rearwardly extending handle ¹ to
enable it to be turned more readily. A graduated sector scale 12 is attached to
the trame 1.
The other prism C is held in a frame L fitting inside the frame I and supported
on a short shaft / journaled concentrically in the hollow trunnion i of the frame 1.
The outer end of the shat carries an index L' moving over the scale I³.
On the trunnioni is keyed a grooved pulley M which is connected by a belt m
with a similar pulley M' on a short shaft m' journalled in one of the arms K and
carrying on its inner end a sector gear N, which meshes with a similar gear N¹
ou the inner frame L. From this, or any other arrangement of equivalent
mechanism to produce the same result, it follows that when the frame 1 and its

page_text: prism are turned in one direction, the frame L and its prism are turnel in the
opposite direction, the angle of divergence between then being indicated by the
relative positions of the scale 12 and index L. Furthermore, since the coincidence
of the images of the targets depends with this attachment, upon the angle between
the two prisms, the line of sight may be shifted somewhat with reference to the
attachment without affecting the apparent superposition of the images.
The range of both the attachments described above may be greatly increase l
by the interposition of a second refracting prism or set of prisms between the
movable prism and the object glass of the telescope.
In Figure 7 these prisms P P are shown in section, held in a frame Q, which
slides in grooves in the frame F just in front of the telescope. They are so
arranged that their thick ends may be made to stand opposite the thin ends of
prisms C2 C3, or vice versa, as shown in Figures 8 and 10, the frame Q being
removable and reversible, end for end. In the first position, the divergence of the
rays produced by passing through C2 and C³ is reduced again by P and P', so
that the movable prisms must be turned through a larger angle when the targets
are at a considerable distance than they would if the prisms P P were not used.
There is, therefore, less liability of error in getting a reading at long distance, and
a possibility of securing coincidence at any distance up to 0. For intermediate
distances. the telemeter may be used without the removable prisms, while for
shorter distances they may be inserted in a reversed position, as shown in Figure 10
so as to increase the angular divergence.
This feature of a removable and reversible prism is applicable to the attachment
shown in Figures 2 and 3, and also to a telemeter having but a single rotatable
prism.
In Figures 11 and 12 is shown a modification in which the refracting prisms
rotate about an axis perpendicular to the refracting edge instead of parallel to it.
As in this case the prisms cannot well be arranged to cover half the field they are
made to cover the whole field. A double total reflection prism C is then arranged
to cover one-half the field as shown, the object of this prism being to laterally
divert the rays which pass through it (as shown by the dotted lines) and therefore
prevent the shifting of this image by the motion of the prisins, the image formed
by the rays which pass through the prisms being alone affected. The construction
of the particular mechanism shown is as follows:
At the forward ends of two arms K is secured a tubular support R in which are
rotatably sleeved two annular frames S S', one in front of the other and preferably
concentric with the axis of the telescope. The adjacent edges of those frames are
cut into level gear teeth to mesh with a similar pinion T journalled in a suitable
bearing R on the bolder R. One of the frames carries an annular scale, preferably
aims attached to thin spokes s¹. The other frame carries an index s² co-operating
with the scale. When one frame is rotated in one direction the other rotates
in the opposite direction, and the relative angular movement is shown by the index
and scale.
In each frame is mounted a refracting prism which may be clamped against a
shoulder in the frame by a threaded ring s3 screwed into the frame.
When these prisms C4 C5 stand as shown in Figure 11, that is, with their
refracting edges turned in the same direction the ray of light passing through them
is deviated from a straight path by an amount depending on the angle of the prisms,
this angular deviation being the maximum which is desired, viz. the angular value
of the red at the least distance. Upon rotating the prisms on their common axis
it will be readily seen that the angle of the combination and therefore the deviation
of the ray is continually diminished in a vertical direction while remaining constant
and equal to 0 in a horizontal direction until, when each of the prisms has been
moved through 90°, (viz.: when their relative angular movement as indicated by
the scale and pointer is 180°), their refracting edges are turned in opposite directions
and, if the two prisms have the same refracting angle, the effective angle of the
combination is 0, i.e. there is no deviation of the ray which passes through them.

page_text: The relation between the angular motion of the prisms and the deviation of the
refracted ray can be readily calculated as before and from this relation together
with the known length of the rod the distance of the latter can readily be deter-
nined, as herein before explained, a scale of feet or yards being engraved directly
on the scale s if desired.
A still further inodification of my invention is shown in Figures 13 and 14, where
the prisms are interposed between the object glass and the eye piece of the telescope.
In the tube of the telescope is incorporated a short section A', which may be made
removable if desired, comprising a casing A², preferably cylindrical. In each side
of the casing is a bearing a, said bearing receiving short shafts U U', the axes of
which are in line and lie transverse to the line of sight and to the line joining the
targets on the rod. On the inner end of each shaft is secured a thick refracting
prism, said prisms being lettered respectively C7 C, and each covering one-half
the field of the telescope and rotating on a line near their front edges, as shown in
Figure 13. On the outer end of each shaft is a gear or segment gear, meshing
with an intermediate gear journalled on a stud a projecting from the casing.
Attached to the gear u' is a disc V having a graduated rim; an index w is secured
to the casing or to tube A' adjacent to the edge of the disc. By turning the disc
the prisms are rotated in opposite directions, through ninety degrees or less, from
the zero position shown in Figure 16 in which the two images of the rod are in
exact coincidence and form a single image, to that of maximum divergence of the
images shown in Figure 15. The distance of the rod when the images coincide,
can be read on the graduated disc as before. By a proper relation between the
thickness and the refracting angle of the prisms we may considerably vary the
angular position required for exact superposition of the two images.
It will be understood that the essential point of this part of my invention is the
employment in connection with a portable telemeter rod or base of a device
attachable to any ordinary telescope consisting of two or more refracting prisms
which by their rotation on an axis either parallel or perpendicular to their
refracting edges and transverse to the line joining the two targets, may be made to
measure at one setting the angular value of the rod or base at the observing
instrument and thus afford us at once the distance when we know the length of the
rod. I do not confine myself to the special form of construction nor to the special
arrangement of prisms which I have shown and described as many other forms
have been devised by me for accomplishing the object aimed at and only typical
forms of the telemeter have been shown in the drawings. For short distances the
targets need not be placed so far apart as suggested above; or in order that one
rod may be made to answer for all distances an extra target may be placed between
the other two, as shown dotted in Figure 1.
The modifications of my telemeter attachment in which mirrors are used will
now be described in succession. That shown in Figures 17 and 18 is composed of
two plane mirrors X X¹ the former being rigidly fastened in a frame D' having a
short tube d adapted to slip easily upon the object glass end of the telescope, where
it can be fastened in any desired manner, so as not to interfere with or be affected
by the focusing of the telescope, which in a surveyor's telescope is usually
accomplished by moving the object glass. The mirror X may be mounted in the
frame in any suitable way, but I prefer to press it firmly by means of set screws cc
against yielding abutments, such as the spring E' and the piece of cork F¹, which
permit a slight adjustment with reference to the axis of the telescope, for the
purpose hereafter to be described. This mirror covers one-half of the field of the
telescope, on one side of a vertical diameter as shown in Figure 18. It is adjusted
approximately to an angle of 45 degrees with the line of vision.
Rising from the tube d is a standard d' having a rearwardly extending arm d² at
the extremity of which is a lateral boss in which is journalled a wrist pin supporting
a bar G which carries at its front end the mirror X', located above the mirror X
and parallel therewith in its lowest position. A spring H attached to the arm ď³

page_text: That is, the distance of the rod is reversely proportional to the movement of the
screw required to produce coincidence. It must be noted that this distance is
measured not from the center of the instrument but from the line X X¹.
The object of the marks 6' 63 on the rod is to enable the mirrors X X¹ to be set
to exact parallelism for the zero position of the screw and furnish a means of
readily testing that parallelism at any time when the rod is at any distance from
the instrument. To do this it is only necessary that the marks should be just the
distance X X' from the center of the targets. Then when the screw is turned
to 0 and the telescope directed at the rod the marks on one image should coincide
exactly with the targets in the other image. If they do not so coincide they are
made to do so by a minute adjustment of the lower mirror X by means of the
screw c¹. In the operation of measuring distances it is only necessary under
ordinary circumstances to verify the correctness of this zero for every forty or
fifty consecutive measurements, and not even then unless great accuracy is
demanded..

page_text: The mirror X is also provided with slight angular adjustment at right angles to
that just described by means of screw c", the object being if desired to obtain a
light lateral separation of the two images.
In the modifications shown on Sheet 6 of the drawings, the mirrors are replaced
by total reflection prisms with the object of avoiding the silvering necessary when
mirrors are used.
Figure 22 shows the front end of the telescope with its object glass tube movable
by means of the milled head A³, as usual. Slipped upon the end of the tube is
the frame D carrying a fixed prism X2. Hinged upon screws L² carried in ears
on the top of the frame D², and transverse to the axis of the telescope and to the
line joining the targets is an arm G2 to the front end of which is secured the
movable prism X³, placed with its reflecting side (the hypothenuse of the triangle)
normally parallel with the similar side of the prism X2. A ray of light striking
the prism X will therefore be reflected into the telescope, as shown by the dotted
line.
The end of the arm G2 has a screw 1* tapped through it, the point of the screw
resting on a plane surface a parallel with the axis of the telescope, so that axial
movement of the frame D2 will not vary the angle of the arm G2 to the axis of
the telescope. The screw carries a graduated disc * co-operating with a scale K
mounted on the arm G².
The prism X covers one-half the field of the telescope, as shown in Figure 23,
the other half being either left uncovered; or provided with a double total
reflection prism X, in order that the two images of the rod may be seen under the
same optical conditions and therefore appear equally illuminated in the field of the
telescope. When the double prism is omitted, the image seen directly is more
brightly illuminated, and it is not so easy to determine when the two images
exactly coincide.
The double total reflection prism also avoids the necessity for correcting the
length of the rod by the quantity B2 z, since the two images are reflected from
two surfaces placed at the same level, to wit, the rear surface of the prism X³ and
the rear surface of the upper part of the double prism X'.
In this attachment, as in the preceding one, the system of reflectors is moved
with the object glass in focusing the instrument, and hence if we wish to measure
from the center of the instrument we must add to the distance x a distance equal
to the distance of the line X2 X3 from the center of the instrument.
The modification shown in Figures 24 and 25 avoids the necessity for this
correction. The fixed total reflection prism X is mounted on a hanger M² secured
The movable total
to the telescope at its horizontal transverse axis of movement.
reflection prism X is located below the prism X', and the ray coming to it and
reversed in its direction by the two prisms is again reversed and thrown into the
telescope by a double total reflection prism X' having its reflecting surfaces
perpendicular to each other, and secured to a ring D³ mounted on the front end of
the telescope. The path of the ray between the prisms X5 X' is parallel with the
axis of the telescope, so that the axial movement of the prism X' when the object
glass is adjusted does not affect the operation of the instrument.
The movable prism X is attached to a radius arm N2 pivoted on the cross axis
of the telescope, and adjusted by the usual tangent screw O bearing against the
lug n on the arm. The angular movement of the arm and hence of the reflecting
prism may be measured by any suitable means of the requisite delicacy, such as a
long vernier m² n³ read by a high power eye piece P' attached to a radius rod P³
so as to swing over the graduated arcs m² n¹. In place of this arrangement the
arm N2 may carry a reading microscope R² (shown in dotted lines) by means of
which the graduations on the arc m² can be read.
A second total reflection piece X8 with parallel reflecting surface may be placed
in front of the other half of the objective, as indicated in dotted lines in Figure 24,
the object being as in the second modification to avoid the correction for X X' to
the length of the rod and to secure equal illumination of the two images.
B

page_text: In Figures 26 and 27 is shown a further modification in which only two mirrors
are used, but both rotate about axes parallel to each other and transverse to the
line of sight and the axis of the instrument. The object of this modification is to
secure a small relative angular motion between the surfaces by means of
comparatively large angular motion of the mirror system as a whole, thus enabling
the angular motion to be read off with sufficient accuracy on an open scale without
the aid of any screw, vernier or microscope. To secure this object the upper
mirror X is secured to the upper end of a pair of rock arms Y journalled on a
horizontal axis on the ends of arms K³ projecting from the ring K³ on the front
end of the telescope. Passing eccentrically through these journals are the pivots.rl
of the lower mirror X. To one of these pivots a pointer T is secured, and the
adjacent rock arm Y has a horizontal arm or elbow Y parallel with the pointer
and provided with a transverse screw y¹ bearing against the pointer and held
yieldingly in place by a spring t¹. The pointer moves over a graduated scale Z
fixed to the telescope, and the screw y¹ has a finger y² whose tip lies adjacent to a
scale y on the rock arm Y.
'
Upon reference to the diagram, Figure 27, it is plain that a given are on the
scale Z subtends a greater angle at the axis of rotation of the rock arm than at
the axis of the pivots, the latter being farther from the scale than the former.
Hence when the arm: Y' and pointer T¹ are simultaneously, moved, and the position
of the system of mirrors X X 10 is changed, their relative positions are also
changed by an amourt equal to the difference between the angles 1, 2, 3 and 1, 4, 3
in Figure 27, that is, the angular divergence of the mirrors is equal, to the
angle 2, 1, 4.
[+
The parts may be so proportioned that a movement of the arm:Y' and pointer T¹
over the entire scale Z (about thirty degrees) will cause a divergence of the
mirrors of but one-half a degree. But since it is necessary to obtain a divergence
of about 2 degrees for a range of distance from 0 up to 4,000 feet, the screw y'
has been provided. The pitch of the screw is such that one turn. (or, any, given
number of turus) will separate the arm Y¹ and pointer T by an amount sufficient
to affect the relative positions of the mirrors to an extent equal to a traversing of
the entire scale 'Z'by the combined arm and pointer. The amount of divergence
thus obtained may be increased by minute increments by moving the pointer and
arm'over the scale as before. A further divergence is obtainable by giving the
screw y¹ another turn (or given number of turns), the finger y indicating the
number on the scale y³.
In conclusion it will be observed that the arrangement of mirror surfaces in all
of these modifications is such that a motion of the attachment as a whole or of the
mirror systems as a whole is without effect on the accuracy of the result. Hence
no care need be taken to adjust the attachment with reference to the axis of the
telescope, but the same may be readily taken off and replaced whenever it is
desired to use it, without interfering with the ordinary use of the instrument.
The details of construction shown and described are not deemed absolutely
essential. For instance the arm G¹ in Figure 17 may be of any convenient length
and the pivot about which it turns may be placed at any point so long as the axis
of rotation is perpendicular to the line of sight and the line joining the two
targets.
Again, the joint movement of both mirrors is shown in the fourth modification,
or the double total reflection prism, or its equivalent combination of mirrors, may
be used in any one of the four forms shown for the purpose heretofore described.
Again, in the first or any of the other forms, we may, instead of a mirror
covering one-half the field, use a mirror covering the whole field, one-half only
being silvered, or the whole being covered with a semi-transparent coat of silver
through which one image may be seen by direct transmission of the light. Finally,
any other well known method of accurately measuring small angles of rotation
other than those shown in this connection may be employed, one accurate method
of doing this being to provide a second small telescope or microscope placed above

page_text: the main observing telescope and directed to a scale attached to some part of
the instrument and seen by reflection from a small mirror rigidly attached to
and moving with the mirror X', the deflection of the zero point of this scale as
measured in the field of the microscope or telescope determining the angular
motion of the mirror.
Instead of placing the system of fixed and movable mirrors in front of an
ordinary telescope we may in special cases place them between the objective and
the eye piece of a specially constructed telescope.
Various other modifications in the construction of the mirrors and the means
for measuring the angles, such as will occur to persons skilled in the art, may
be adopted.
The third part of my invention relates to the method which in my previous
synopsis of distance measuring methods I have designated as Class 2-1st, and is
in brief the determination of the linear distance subtended by a fixed angle whose
vertex is at the observer's station. This fixed angle has been determined in
previous forms either by the use of fixed cross wires in the focal field of the
observing telescope (stadia wire method) or by placing in front of the telescope
and covering one-half the field of the latter, a refracting prism which bends the
rays passing through it by a certain amount depending on the angle of the prism,
as in a certain well known patented instrument.
The objection to the first of these forms is that the telescope requires to be
firmly mounted on a tripod or otherwise, while the reading is being made: and
that it is difficult to read accurately the points of intersection of wires on the rod
at great distance.
The objection to the second form is, that while it dispenses with the use of a
tripod or support for the observing telescope, and likewise increases the accuracy
by making the measurement depend upon the observance of a single coincidence of
two images, it requires the introduction of a refracting prism into the optical path
of the rays which form one of the images in the observing telescope.
It is well known that when an object is viewed through such a prism, the image
of the object is surrounded by a fringe of colored light on account of the difference
in refractive power of the prism for different kinds of light; and the sharpness of
definition is thereby lost, rendering the accurate setting of any part of the
refracted images into coincidence either with the cross wire or with the direct
image (which is formed by the rays which pass through the uncovered half of the
objective) very difficult; and, if the prism has a large refracting angle, impossible.
The accuracy of the measurement would, other things being equal, increase with
an increase of the fixed angle of measurement; but, in order to increase this angle,
we must with the aforesaid patented form of instrument increase correspondingly
the refracting angle of the prism; and this as already pointed out increases the
indistinctness of the image, and more than negatives the increase of accuracy
which would otherwise result.
The only possible method of obviating this indistinctness in the case of a
refracting prism is to make it achromatic, eiz.: to make a compound prism of
two kinds of glass similar to the compound achromatic telescope objective and
this very considerably increases its cost.
To avoid all this and other difficulties as well as to obtain certain advantages not
possessed by the patented form of instrument, I use instead of a prism covering
one-half the field, a combination of two reflecting surfaces which are fixed at a
certain small and predetermined angle to one another. This combination is placed
in front of the telescope in such a way as to cover one-half the field of the latter;
and so that in addition to the image of the object, at which the telescope may be
directed, as seen directly through the uncovered half, there is another image of
the same object seen after a double reflection from the two reflecting surfaces,
displaced with reference to the first by an angular amount which is double the
angle between the aforesaid two reflecting surfaces.

page_text: The use of this system has four distinct advantages over the use of a refracting
prism: First, the first cost is less than that of an achromatic refracting prism
which is necessary if good definition is to be obtained. Second, as the rays of light
which pass the combination have suffered no refraction, but only reflection, the
resulting image is not colored in the least, no matter how great the angle between
the two reflecting surfaces may be; consequently any fixed angle of measurement
may be used without prejudicing the accuracy of the result by causing indistinctness
in the displaced image. Third, the angle between the reflecting surfaces may be
adjusted so as to make the same combination useful for either very great or for very
small distances, while with the prism there is no method by which the refracting
angle and therefore the angular displacement of the refracted image may be
changed, and we must therefore, if the best results are to be obtained, have a set
of prisms of different angles, one for short, another for medium, and a third for
long distances, which still further increases the expense. Fourth, by a suitable
disposition of the second reflecting surface, I can make the point from which the
distance is measured, the center of the instrument instead of some point in front of
the same, as in the case of the stadia wire method, or some point in the prism itself
as in the patented form.
This last point is of particular importance when the attachment is used in
connection with transit instruments, in which the point of measurement is the
center of the instrument.
In order to still more thoroughly explain my invention and emphasize the
particular points of advantage above mentioned, I will now describe a particular
form of attachment constructed in accordance therewith and shown in Sheet 8 of
the accompanying drawings.
C and C are the two reflectors (Figure 29) which are secured in a frame D*,
which slips over the end of the telescope A in place of the usual cap. These
reflectors are arranged to cover one-half the field of the telescope on one side of a
diameter as shown in the drawing, and are inclined at a small angle to one another
so that the ray indicated by the dotted lines, which enters the telescope after
double reflection is slightly inclined to the optical axis of the latter, this inclination
determining the small fixed angle of measurement which is one of the factors in the
determination of the distance. A screw E secured by a lock-out F2 bears against
the back surface of one of the mirrors C³, the mirror being kept against the point
of this screw by means of a spring or block of elastic material H2. By turning
this screw in one direction or the other the inclination of the reflected ray to the
optical axis can be varied for the purpose herein before described.
The method of using the instrument is as follows: The instrument is first
adjusted or calibrated by placing it in position and measuring off a distance X2 B2,
Figure 31,-say 1,000 feet. Then a rod B with two marks or targets at B¹
and B2 (say 10 feet apart) is placed at B2, the telescope directed upon it, and
the caps carrying the mirrors turned on the telescope until the edge of the
mirror system is parallel to the length of the rod. Two images of the rod
will be seen in the eye piece of the telescope, which will be in line with
each other, but displaced laterally with reference to each other by an amount
depending on the angle of inclination of the two surfaces C° C. This angle
is then varied by means of the screw E² until the upper mark or target of
one image is in exact coincidence with the lower mark or target of the other
image. Then the lock-nut F2 is screwed down, locking the screw, and thus fixing
the two mirrors with reference to each other. We then know the angle between
the incident ray before reflection and the axis of the telescope, which in this case,
is the angle B 2 B2 whose tangent is 108. Then any other distance is very
readily determined by placing the rod at the point whose distance from the
instrument is desired, and measuring the lateral separation of the two images in the
telescope in any of the customary ways of determining such measurements, as for
instance, by providing the rod with a scale graduation like a stadia rod, and noting

page_text: such coincidence, the distance of the rod from the point 2 will be :
in this case, 100 y.
distance
tangent angle B₁ B
It is evident that by making the vertical distance between the surface of the two
mirrors such that this distance is to the distance from the center of the telescope
mounting as B2 B₁: B²x², the point will coincide with said center of instrument
rendering any correction to the measured distance unnecessary.
It is evident that we may make the angle B¹² B2 anything we please by turning
the screw in or out. For long distances say 1000 to 3000 feet, it will be convenient
to use a smaller angle than 1 to 100 (for example B³ 2 B2) in order to avoid the
necessity for a rod of very great length; while for short distances (1 to 500 feet)
the accuracy will be considerably increased by making the angle larger, say 1 to 50,
or 1 to 25 even.
If great constancy of angle is desired under severe conditions of use we may
secure it by making the two reflecting surfaces the two sides of a solid block of
glass, Figure 32. This has also the advantage of not requiring any silvering,
since the surfaces e p, gh, are both faces of total reflection for rays entering
through the faces e h, g p. We may avoid the necessity of using silvered mirrors >
in the form shown in Figure 29 by using instead of mirrors equivalent total
reflecting prisms as indicated by the dotted lines.
Having now particularly described and ascertained the nature of my said inven-
tion and in what manner the same is to be performed, I declare that what I
claim is :-
1. The means for determining distances herein shewn and described, consisting
of a portable rod having two targets fixed a known distance apart, and a device
rotatable on an axis for deflecting the image of one of said targets until it coincides.
with that of the other, substantially as described.
2. The means for determining distances herein shewn and described, consisting
of a portable rod having two targets fixed a known distance apart, and a device
movable transversely to the line of sight for deflecting the image of one of said
targets until it coincides with that of the other, substantially as described.
3. The combination with a telescope, of a frame adapted to slip over the
objective, and a refracting prism pivoted in said frame on an axis perpendicular to
the vertical diameter of the field, substantially as described.
4. The combination with a telescope of a frame adapted to slip over the
objective, and a refracting prism covering half the field to one side of a vertical
diameter thereof, and rotatable on an axis transverse to said diameter, substantially
as described.
5. The combination with a telescope and with a portable telemeter rod or base
of known length, of a refracting prism rotatable on an axis perpendicular to the
rod, substantially as described.
6. The combination with a telescope, of a refracting prism covering half the
field and rotatable on an axis transverse to the line of sight, which passes through
the prism, substantially as described.
7. The combination with a telescope, of two prisms each covering half the field,
the thick end of one lying adjacent to the thin end of the other, and rotatable on
an axis transverse to the line of sight, substantially as described.
8. The combination with a telescope, of two prisms, each covering half the field
and rotatable on an axis transverse to the line of sight, and a scale and index for
indicating the angular movement of said prisms, substantially as described.
9. A telemeter apparatus consisting of a rod provided with two fixed targets, a
telescope and a tube adapted to slip over the end of the telescope, a frame pivotally

page_text: mounted therein on an axis perpendicular to the line joining the targets, and a
prism mounted in said frame, substantially as dscribed.
10. The combination with a telescope, of two prisms, independently rotatable on
a common axis transverse to the line of sight, substantially as described.
11. In a telemeter, the combination with two prisms independently rotatable on
concentric axes, transverse to the line of sight, of means for simultaneously rotating
them in opposite directions, substantially as described.
12. In a telemeter, the combination with a suitable support, of a frame journalled
on an axis transverse to the line of sight, a second frame journalled concentrically
therewith, gearing connecting the two frames to cause them to move in opposite
directions and a prism mounted in each frame, substantially as described.
13. In a telemeter, the combination with a prism rotatable on an axis transverse
to the line of sight, of a second prism adapted to be inserted and removed from a
position in line with the first prism, substantially as described.
14. In a telemeter, the combination with a prism, rotatable on an axis transverse
to the line of sight, of a second prism, adapted to he placed in line with the first,
and reversible in that position, substantially as described.
15. In a télemeter, the combination with a prism rotatable on an axis transverse
to the line of sight, of a second prism adapted to be inserted in line with the first,
and with either of its faces perpendicular to the line of sight, substantially as
described.
16. An apparatus for determining distances consisting of a rod having two
targets at a predetermined distance apart, and a telescope provided with two or
more closely adjacent reflecting surfaces one of which is movable with respect to
the others, whereby the images of the two targets can be brought into coincidence,
substantially as described.
17. An apparatus for determining distances consisting of a rod having two fixed
targets, separated a known distance apart, and a telescope provided with two
closely adjacent reflectors covering one-half the field, and relatively movable from
a position parallel with each other, substantially as described.
18. An apparatus for determining distances, consisting of a rod provided with
two fixed targets, and a telescope provided with two closely adjacent reflectors
relatively movable on an axis transverse to the line of sight and to the line joining
the targets, substantially as described.
19. An apparatus for determining distances, consisting of a rod provided with
two fixed targets, and a telescope provided with a frame, a reflector mounted in
the frame and covering half the field of the telescope, an arm hinged to said frame
on an axis transverse to the line of sight, a graduated scale for determining the
movement of said arm, and a second reflector attached to said arm in line with the
first reflector and closely adjacent thereto, substantially as described.
20. The combination with a telescope having a system of adjustable reflectors
intercepting the rays entering one-half the field, of a double total reflection prism
covering the other half of the field, substantially as described.
21. The combination with a telescope having an axially adjustable objective, of
a system of reflectors supported adjacent to the cross axis of the telescope, and a
double total reflection prism mounted on the tube holding the objective, substantially
as described.
22. The combination with a telescope having an axially inovable objective, of a
hanger M2 near the cross axis of the telescope, a total reflection prism secured to
said hanger, a similar prism secured to a radius arm N2, and a double total reflection
prism mounted on the objective tube, substantially as described.
23. The combination with a telescope, of a frame adapted to slip over the
objective end of the telescope, and capable of rotation thereon, a reflector rigidly
mounted on one side of said frame and inclined forward in front of the telescope, a
second reflector mounted on the other side of the frame, and also extending forward
of the telescope, and means for adjusting the angle between the reflecting surface,

page_text: said reflectors covering only one-half the field of the telescope, substantially as
described.
24. The combination with a telescope of two reflectors mounted thereon and
facing each other in planes which intersect at a small angle, said reflectors covering
one-half the field of the telescope and being separated by a vertical distance
bearing the same ratio to their distance from the center of the telescope as the
normal base line bears to its distance from said center, substantially as described.
Dated this 25th day of March 1895.
W. E. HEYS & SON,
Agents for the Applicant.