Improved Means or Apparatus for Correcting and Adjusting Telemeters or Range Finders.
Year: 1893

page_text: Date of Application, 22nd Sept., 1893-Accepted, 28th Oct., 1893
COMPLETE SPECIFICATION.
Improved Means or Apparatus for Correcting and Adjusting
Telemeters or Range Finders.
I, ERNST ABBE, Doctor of Philosophy, of Carl Zeiss Strasse at Jena in the
Grand-duchy of Saxe-Weimar, German Empire, do hereby declare the nature of
said invention, and in what manner the same is to be performed, to be particularly
described and ascertained in and by the following statement :-
In telemeters or range finders having a short base line the difference in the
direction or the convergence of the axes of sight of two telescopes situated at the
ends of the base line, these axes being simultaneously directed to a distant object,
is measured by means of a micrometer in order to derive from this difference-viz.
the parallactic angle of the object relative to the base line-the distance of the
object either arithmetically or by means of a mechanical translation. All apparatus
of this description are attended with the disadvantage that the zero point of the
micrometer scale corresponding with the truly parallel position of the axes of sight
of both telescopes cannot be permanently fixed or at any rate cannot be maintained
constant for any length of time, when the instrument is exposed to shocks and
shaking during transport or is subjected to considerable variations of temperature.
By means of this invention as hereinafter described this defect is practically
obviated by providing telemeters or range finders of the class hereinbefore referred to
with a supplementary apparatus which enables the observer to adjust the instrument
at any time and with the same precision as if he were enabled to avail himself for
the purpose of adjustment of a distant object whose distance is known beforehand.
Fig. 1 of the accompanying drawings represents in horizontal section a range
finder provided with an apparatus made according to this invention.
and
Fig. 2 is a diagram illustrating the effect of deviation as hereinafter explained,
Figs. 3 and 4 illustrate modified arrangements of the prisms employed in
apparatus constructed according to this invention.
In the following description of the apparatus it is assumed to be applied to a
telemeter of the most simple construction.
Two telescopes A' A² represented in Fig. 1 of the accompanying diagrams are
fixed parallel with one another at opposite extremities of a bar T the telescope A¹
being provided in the field of its eye piece with a fixed wire at a and the
telescope A2 being provided with an adjustable wire at a capable of being moved
to a measured distance in the field of the eye piece of this telescope by means of a
suitable micrometer adjustment M.
Two Prandl's double-reflecting prisms deflecting at an angle of 90° are
temporarily supported by means of any suitable fitting in front of the objectives of
the two telescopes so that their principal sections have an opposed position and are
parallel to one another and parallel with the plane of the axes of the telescopes.
Behind the eye piece o' of the telescope A¹ is placed a mirror S or reflecting prisin
in such a manner as to direct the light of the sky through the eye-piece in the
direction of the axis of this telescope. Now when the observer looks through the
eye-piece o² of the telescope A2 he sees upon the light back-ground and simul-
taneously with the adjustable wire a² in the field of this eye-piece the reflected
image of the wire a' in the other eye-piece, which is reflected four times by the
prisms and by means of the micrometer M he can now adjust the directly seen
wire a² until it coincides with the image of the other wire (a¹). When the angles
of deflection W and W2 of the two prisms according to the diagram Figure 2 are
exactly equal to 90° so that these prisms acting in combination turn a ray exactly
through an angle of 180 degrees from its direction at entering by reflecting it four

page_text: times the correct zero position of the range finder in which the line of sight of the
telescope A is exactly parallel with that of the telescope A' is immediately obtained
by adjusting the two wires a' and a² so as to coincide as above described. On the
other hand when the angles W and W differ the issuing ray is inclined to the
direction of the entering ray at an angle represented by = W'W' and
the described adjustment of the moveable wire a leads to an inclination of the two
axes of sight relatively to one another which is equal to the angle and this
position of the range finder corresponds with its correct adjustment upon an object
the parallactic angle of which relative to the base line of the telemeter is we and
the distance of which from the observer is e (when A represents the length of
A
20
the said base line or distance between the objectives of the two telescopes).
It follows that in this case the instrument, that is to say the micrometer M, is
known to be correctly adjusted when it indicates accurately the angle w or the
distance e corresponding thereto, whilst the two wires a¹ and a² coincide.
The angle of deflection of a prism of the class referred to and in consequence the
parallactic augle w = WW2 characteristic to a given pair of such prisms can be
determined once for all with perfect accuracy and the value w remains permanent
and unchanged inasmuch as only the principal sections of the two prisms are main-
tained approximately parallel, the angles W, W not being influenced, if the
prisms should be partially turned in the plane of their principal sections.
A pair of prisms of this description with the difference of known value con-
sequently constitutes, for any telemeter, an unchangeable standard or known
distance which the observer is able to carry about and use when required as herein-
before described in order to adjust the instrument afresh just in the same way as if
a distant object of known distance had been provided for the purpose.
Various mechanical arrangements may be employed for the purpose of easily
and quickly placing the prisms in front of the objectives of the telemeter.
For example the prisms may be fixed separately in plain cases adjustably
supported upon hinges or pivots connected with the ends of the telescopes so that
they can be moved in and out of position for use without loss of time or they may
be arranged to slide on guides in front of the objectives. The two prisms may
also be connected together by a light bar or a frame hinged or pivoted to the
body of the telemeter in such a manner as to enable both prisms at once to be
readily and accurately placed in the required positions.
The supplementary adjusting apparatus hereinbefore described is applicable to
telemeters of all descriptions in which two telescopes are employed with separate
or partially separate fields of the eye-pieces. It is perfectly immaterial whether
the axial lines of these telescopes are straight as in Figure 1 or broken, for
example partially directed parallel to the base line. The adjusting apparatus is
also independent of the construction and action of the measuring apparatus by
which the parallactic angle of the distant object and the distance corresponding
with such angle is measured as with every method of observation and with every
measuring apparatus a perfect adjustment is always possible by directing the
telemeter towards an object situated at a known distance from the observer,
a method of adjustment to which the subject of this invention is wholly equivalent.
The construction of the apparatus may be simplified whilst retaining the same
principle of working when the telemeter is constructed with a sufficiently short
base line. In this case the pair of double-reflecting prisms having a deflection of
90 degrees as herein before described are replaced by one double-reflecting prism
with a deflection of 180 degrees, the principal section of which is formed by a
single right angle isosceles triangle whose reflecting catheti are separated to the
distance separating the objectives of the two telescopes as represented in Figure 3.
According as the angle between the reflecting surfaces deviates more or less from
90 degrees a piece of glass of this description constitutes a standard for a greater
or smaller (but always capable of being accurately determined) angle w which as
herein before explained can be utilized as an equivalent of a given distance when

page_text: the said piece of glass is placed in front of the objectives of a telemeter with the
principal section of the prism approximately parallel with the plane of the axes of
the two telescopes.
In place of this single reflecting prism in the form of a solid bar of glass two
prisms the principal section of each forming a right angled isosceles triangle may
be cemented to the ends of a cylindrical glass tube as represented in Figure 4.
With both of these arrangements the absolute permanency of the angle w is to
be relied on only within such limits of dimensions of the base line as render .any
deformation caused by pressure or variations of temperature either impossible or
practically unprejudicial.
Having now particularly described and ascertained the nature of the said inven-
tion and in what manner the same is to be performed I declare that what 1
claim is :
1. In telemeters or range finders having two telescopes an arrangement for
obtaining a given or predetermined difference in the direction of the lines of sight
of the said telescopes consisting of two double-reflecting prisms of known deviation
(90 degrees or thereabouts) placed in front of the objectives of the telescopes in a
position in which the wire or other mark in the field of the eye-piece of one of the
telescopes being illuminated from the back becomes visable in the field of the
eye piece of the other telescope and enables the instrument to be corrected and
adjusted in the same way as it could be corrected and adjusted by directing it on
to a distant object situated at a known or measured distance from the observer
all substantially as hereinbefore described with reference to the accompanying
diagrams.
2. In apparatus for correcting and adjusting telemeters as herein before described
and claimed the arrangement consisting of two reflecting prisms permanently and
rigidly connected together and causing a deviation of known extent being equal to
180 degrees or thereabouts substantially as hereinbefore described with reference
to Figures 3 and 4 of the accompanying diagrams.
3. The arrangements and combinations of parts constituting the improvements
in means or apparatus for correcting and adjusting telemeters or range-finders
substantially as hereinbefore described with reference to the accompanying
diagrams.
Dated the 22nd day of September 1893.
J. H. JOHNSON & Co.,
Agents.