IMPROVEMENTS IN TELESCOPES USED IN INSTRUMENTS FOR MEASURING ANGLES
Year: 1876

page_text: LETTERS PATENT to Adolph Moser, of Aix la Chapelle, in the Kingdom of
Prussia, but at present of Southampton Street, Strand, in the County of
Middlesex, Engineer, for the Invention of "IMPROVEMENTS IN TELESCOPES
USED IN INSTRUMENTS FOR MEASURING ANGLES."
ealed the 13th April 1877, and dated the 6th November 1876.
PROVISIONAL SPECIFICATION left by the said Adolph Moser at the Office
of the Commissioners of Patents on the 6th November 1876.
ADOLPH MOSER, of Aix la Chapelle, in the Kingdom of Prussia, but at present of
"IMPROVE-
Southampton Street, Strand, in the County of Middlesex, Engineer.
MENTS IN TELESCOPES USED IN INSTRUMENTS FOR MEASURING ANGLES."
My Invention relates to telescopes used in instruments for measuring angles,
such as theodolites, telemeters, and the like, and consists in the application of
refraction for the measurement of small angles. For this purpose I provide the
telescope with two prisms of glass or other suitable transparent substance of a
small but determined angle of refraction set face to face, either in front of or behind
the objectiv, one of these prisms being kept in a fixed position, while the other
can be turned round its axis by means of a tangent screw or other suitable
mechanism; their relative position can be.modified so that the rays of light reflected
from a distant object will either pass through without refraction or under any
angle within twice the aforesaid determined angle, and which may vary according
to the purpose for which the instrument is to be used. For instance, assuming that
an angle of refraction of half a degree. be adopted for each prism, then the rays of
light can be made to deviate from the direction of the axis of the telescope at any
angle between zero and one degree, and which angle is indicated by suitable
scales for minute reading of the arc through which the aforesaid prism has been
turned.
The zero position will correspond to the case when no refraction takes place,.
but when the prism is turned round half a revolution then the refraction is exactly
one degree, because in the first case the two prisms are in such a relative position
that the rays of light are caused to deviate by one prism on one side, and by the
other on the other side of the axis of the telescope under the same angle, and the
action of the one will therefore be compensated by that of the other. In the second

page_text: case the deviation caused by each prism is equal to its angle of refraction, and as
both angles are on the same side of the axis of the telescope the whole deviation
will then be twice this angle. But between these two extreme positions the prism
being turned round to a greater or less extent a greater or less deviation of the rays
will be produced in proportion to the versed sines of the arc described by the
prism.
For operating with this instrument for the measurement of a small angle sub-
tended by two distant objects, I first with the prism index at zero direct the
telescope towards one of the objects, centering it by the cross threads. I then turn
the prism until the other object is by refraction brought also to the cross threads,
and knowing by the index scale the extent to which the prism has been turned for
this purpose I can readily compute the angle of refraction caused by the combined
action of the two prisms, and thence ascertain the angle which is subtended by the
two objects.
A table of angles corresponding to such divisions of the prism scale may he com-
puted, and the angles subtended by the two objects observed can then be ascertained
by simple reference to the table.
SPECIFICATION in pursuance of the conditions of the Letters Patent filed by
the said Adolph Moser in the Great Seal Patent Office on the 1st May
1877.
ADOLPH MOSER, of Aix la Chapelle, in the Kingdom of Prussia, but at present
of Southampton Street, Strand, in the County of Middlesex, Engineer. "IMPROVE-
MENTS IN TELESCOPES USED IN INSTRUMENTS FOR MEASURING ANGLES."
My Invention, relates to telescopes used in instruments for measuring angles,
.such as theodilites, telemeters, and the like, and consists in the application of
refraction for the measurement of small angles by means of two prisms of glass or
other suitable material of a small but. determined angle of refraction set face to face,
either in front of or behind the objective, one of these prisms being kept in a fixed
position, while the other can be turned round its axis by means of a tangent screw
or other suitable mechanism; their relative position can be modified so that the
rays of light will either pass through without refraction or under any angle within
twice the aforesaid determined angle. These prisms or circular prismatic discs are
made to refract the rays of light reflected from a distant object in passing through
at precisely the same determined angle, but which angle may vary according to the
purpose for which the instrument is to be used and the accuracy of measurement
required. For geodetical instruments I should mostly adopt an angle of thirty
minutes; for astronomical, a smaller one; but whatever angle may be adopted it
must be as far as possible perfectly exact; this being however attended with con-
siderable difficulty if the prism is made of a single plate I compose each prism of
two plates with even surfaces by joining them together with Canada balsam, the
combined angle of refraction of both being somewhat more than thirty minutes, or
whatever other angle may be adopted; the prism can be adjusted to that angle
with the greatest precision by gradually turning the plates upon each other in
opposite directions. The said two prisms being thus prepared they are applied to
the telescope as follows.

page_text: direction of the optical axis; and Fig. 5 shews the prisms in a position by which
the refraction is exactly twice that of each single prism.
The short tube a containing the objective, being part of the telescope tube, to
which it is fastened by screws, requires no explanation; the piece b is fitted over
this, and having upon its circumference two slits c, c, .it can be turned round. 90°,
and fixed by means of the set screws d, d; it contains the prism e, the worm
wheel i with the prism f, and has the mechanism for turning the prism f attached
to it. The prism e, which always remains stationary during the act of measuring,
is so fixed in its place that the angle of refraction is horizontal when the arm g is
vertical, whereas for vertical measurements that angle must be vertical, and the
said arm will then be in a horizontal position. This arm carries a double bearing
for the tangent screw or worm h, which works in the teeth of the worm wheel i.
A short cylindrical projection in the centre of this wheel fits inside the piece b, and
holds the prism f; the angle of refraction of this prism, will therefore be varied in
a horizontal and vertical sense by the movement of the worm wheel, the circum-
ference of which being provided with 360 teeth each turn of the screw will move
it round its centre with the prism f, which it contains, exactly one degree. The
number of degrees moved are indicated by the division on its inner surface and
the index ; they extend to half a circle, and are therefore 180 in number, as in
Fig. 2. One end of the worm h carries a small disc 7, the circumference of which
being divided into 60 equal parts, as in Fig. 1, they will indicate the angular
minutes which the prism f has been turned; smaller angles down to 10 seconds are
indicated by the vernier upon the index m, which division is sufficiently minute
for geodetical measurements with instruments of ordinary size, as a difference of
10 seconds in the angular movement of the worm wheel will not cause a variation
of one-tenth of a second in the angle of refraction, and it is hardly necessary to
observe that while the former is 180° the latter is only 1°.
It is advantageous to fasten the prisms in rings, as shown in the Drawing, because
they can then easily be brought into the exact position required. The prism f
must be fixed in such a position that its refracting angle be exactly opposite to
that of the prisme (as in Fig. 4) when the index stands upon zero, because the
rays of light reflected from a remote object will then be refracted under the same
angle, but on the other side of the optical axis, therefore there will be mutual com-
pensation and no deviation at all, and this will be the zero point. But when the
worm wheel has been turned round 180° and the prisms e and f brought in the
relative position shewn in Fig. 5 then the rays of light will deviate in the plane of
the refracting angle exactly 1° from the direction of the optical axis, when the
angle of refraction of each prism is thirty minutes. Between these limits, from
zero to one degree, we can imagine any number of equal parts, and assuming it to
be 36,000, each part will represent one-tenth of a second, any number of which
can be determined with the greatest accuracy by calculation, and the only limit to
the observation of so minute an angle will depend upon the size and perfection of
the telescope, the light reflected from the object aimed at, and the distinguishing
power of the human eye.
circle
=
It will be easily understood that however minute these angles may be they are
exactly in proportion to the versed sines of the angles described by the radius of a
1; for instance, supposing the angle described by this radius is 60° the
versed sine will be half the radius, and the angle of refraction 900 seconds or
15 minutes. Therefore in order to find the latter it is only necessary to ascertain
the corresponding versed sine of the former. But a table may be calculated con-
taining all the angles of refraction corresponding to those described by the said
radius for the most accurate angular measurements which the eye may be able to
distinguish. A great number of trials with a theodilite provided with such a
telescope have proved the accuracy of this system when everything is executed
with the necessary care but the prisms should be made of the best homogeneous
transparent material. I have found best hard crown glass to answer very well,
and if the joint angle of refraction of both prisms does not exceed one degree the

page_text: chromatic aberration is but very slight, and will be almost or entirely imperceptible
when the measurement by refraction is confined to an angle of 30 minutes or
less, but the divided circles of the instrument must then be provided with smaller
divisions.
To measure the angle between two distinct objects I first bring the prism fupon
zero and direct the hair cross of the telescope upon one of these objects, keeping it
fixed in this position; I then turn the prism by means of the mechanism before
described until the hair cross covers the other object. From the angle thus
described I find the corresponding versed sine which represents the angle sought
for, in seconds or parts of a second, either by calculation or by referring to the
table. When the angle to be measured is greater than the limited angle of
refraction of both prisms it is necessary to measure first with the divided circle and
then the fractional part by refraction. When vertical angles are to be measured
the piece b must be turned round 90°, and it will then be in the position shewn in
Fig. 2 by dotted lines, in which it must be kept during the act of measuring. The
prisms may also be placed between the eye piece and the objective, they can
then be less in diameter in proportion to the section of the cone of light within the
telescope.
I have already stated that when the refracting angles of the prisms are either
exactly opposite to each other, as in Fig. 4, or both in the same direction, as in
Fig. 5, there will in the first case be no deviation of the rays of light, and just the
same as if there were no prisms in the telescope; in the second case the deviation
will be exactly one degree (that of each single prism being 30 minutes). But when
the prism fis brought into any other position the object will not only appear to
move in one direction, but also at right angles to it, and it is therefore necessary
to move the telescope vertically when the angle to be measured by the prisms is
horizontal, and horizontally when the same is vertical. This does not however in
the least affect the accuracy of the measurement provided the mechanisms for both
movements are sufficiently correct, as is the case in all good instruments of that
kind.
Having thus described the nature of my Invention and the manner of performing
the same, I hereby declare that I do not claim any part of the mechanism nor the
especial combination of those parts as before described, but what I do claim as my
Invention is, the application of two prisms in dioptric telescopes for the purpose of
measuring small angles by means of refraction on the principle before described, of
which the following is a brief repetition :-I make each of the aforesaid prisms to
refract the rays of light at precisely the same suitably adopted angle, say 30 minutes,
dividing this angle, say into 1,800 seconds, and the radius of a circle = 1 into the
same number of equal parts, the arc between the radius and any one of the sines
erected from these parts will represent the angle described by the worm wheel,
while the number of parts upon the corresponding versed sine represent the angle
between the two objects in seconds, but I do not confine my claim to any particular
angle of refraction, nor to any special subdivision of the adopted angle; I set these
prisms face to face, either in front of or behind the objective in the telescope, and
while one of them is kept in a fixed position during the act of measuring the other
can be turned round its axis by suitable mechanism, thereby modifying their
relative position, and consequently also the angle of refraction, which is indicated
by the said mechanism as before described and calculated with the assistance of the
above principle.
In witness whereof, I, the said Adolph Moser have hereunto set my hand
and seal, this Twenty-first day of April, in the year of our Lord One
thousand eight hundred and seventy-seven.