Improvements in. Reflecting Instruments for Measuring
Year: 1893

page_text: Date of Application; 11th May, 1893.
Complete Specification Left, 10th Feb., 1894 Accepted, 21st Apr., 1894
PROVISIONAL SPECIFICATION.
Improvements in. Reflecting Instruments for Measuring Angles
JOHN HOLMES BLAKESLEY, 39 Victoria Street Westminster S. W...Civil
Engineer JAMES HENRY STEWARD 406 Strand London WC. Opticiant do
hereby declare the nature of this invention to be as followså se
1st. The application to reflecting instruments of the sextant type of an improved
index mirror, divided and so mounted and arranged that one half is fixed & the
other moveable. The reflection of both lines of vision through one or more
mirrors, instead of one being direct, as in ordinary sextants.
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2nd. The application of a moveable mount to carry the horizon glass (or mirror)
and the telescope (or sight hole) so that the said mount can work through any
convenient arc for keeping the line of vision clear through this and the two index
mirrors (or halves of index mirror).
These improved mechanical and optical arrangements enable us to read angles
on the divided arc of the instrument without parallactic error of the instrument,
while the movements and adjustments hereby described permit of any angle
between 0 degrees & 180 degrees being observed and measured by the instrument
Dated this Tenth day of May 1893.
COMPLETE SPECIFICATION...
Improvements in Reflecting Instruments for Measuring: Angles
JOHN HOLMES BLAKESLEY 39 Victoria Street Westminster, S.Wa Civil
Engineer, JAMES HENRY STEWARD 406 Strand, London, WC Optician 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:-
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The want of a sextant which will readily & accurately read angles up to
180 degrees has long been felt by surveyors, and the iniproved instrument herein-
after figured and described attains this object. The essential part of the improve-
ment consists of the application to reflecting instruments of the sextant type of
an improved index mirror divided, or two mirrors, so mounted & arranged that one
half, or one, is fixed & the other half mirror or mirror is moveable. The reflection
of both lines of vision through one or more mirrors instead of one being direct as
in ordinary sextants. To accomplish this we mount the two (or divided) mirrors 'A. A'
on a common axis B as shewn in the accompanying Figures 1. 2. 3, and by any
suitable mechanical motion, preferably the racked segment, C. as shewn in
Figs. 2 & 3 and the pinion D in Fig. 3, cause a movement to be made that enables
the mirrors A. A' to be placed at any angle between 0 & 90° with one another, and
at the same time move the vernier K round the divided arc L, and therefore rays
coming from points whose distance subtends any angle from zero to 180° can be
reflected in parallel directions to the eye and read off on the instrument. A horizon
glass E is fitted onto mount or arm F and has a twofold use. First, if the eye
looked direct into the two index mirrors, the head of the observer would often be
in the way of rays in measuring smaller angles whereas the horizon mirror occupies

page_text: a small space and by being put on the moveable arm or mount F can always be
brought to such a position that the rays to be observed do not cut it, but pass either
on the same or different sides of it and secondly, since the deviation of a ray
reflected from two mirrors depends on the relative inclination of the two mirrors
and not upon their absolute position.a slight tremor of the hand which would make
the image oscillate of an object viewed by reflection in one mirror only leaves the
image steady when viewed by reflection in two mirrors.
The moveable arm F on which the horizon mirror E is placed turns on an
axis H Fig. 3 concentric with that of the moveable index mirror A as indicated
in drawing Fig. 2 at El & F¹ which arm also carries the telescope Fig:-7 or eye
diaphragm G Figs. 1. 2. 3., and consequently the horizon mirror.being once, placed
to reflect the centre or axis of the index.mirror to the eye diaphragm always does
so however this arm may be turned. The illustration in Figure 5 shews this for
ray O coming from one distant object is first reflected from the moveable index
mirror A on to the horizon mirror B. & then to the eye aperture G.
Fig. 6 shews the effect of the reflection of a small-angle & Fig. 4 the reflection
of the two rays when the mirrors are parallel and therefore shewing zero (0) on the
divided arc L, providing the instrument is in proper adjustment.
The position, of the arm F is varied at will see FI Fig. 2 and if placed as far
towards the zero end of scale as it will go the position will be found convenient for 2
observing all angles up to about 70°, the rays from both objects passing to one
side of it as in Fig. 6 and for greater angles, if it is turned from this position
through an angle. about 15° or 20° less than the position of vernier estimated for
the new angle this will be found a convenient one for avoiding the paths of the
observed rays which now pass on different sides of it as shewn in Fig. 5. This 2
arrangement of course implies a different position of the observer relatively to the
instrument for different angles but this apparent disadvantage vanishes with a very
little practice in using the instrument.
The arrangement of the mirrors as described above reduces the parallax of the
instrument practically to zero. This is an important point as great errors can
creep in to the ordinary box sextant on this account when observing an object at a
small angular distance with another, at a considerable distance. The parallactic
error of the instrument can obviously never be greater than half the breadth of the
index mirror divided by the distance of the object observed and if this is observed
in the centre of the index, mirror the parallax is zero.. An adjustment is arranged
.at. V. for placing the mirrors A. A' in true position for reading exactly zero as
performed in ordinary sextants and the regular accessories of reading microscope R
& detachable telescope Fig. 7 are also provided together with dark glasses for
solar observation.
Figure 3 shews a sectional drawing of the instrument across X. X. with the various
parts alluded to in description and now recapitulated alphabetically.
A. is the moveable index mirror.
A the fixed index mirror.
B. the centre or common axis on which the index-mirror A moves.
C. the racked segment to which is fixed the vernier arm.
1

page_text: An important use for this instrument which as explained is free from parallactic
error is to take vertical heights by measuring the angular distance first between the
base of the object observed and one on the ground plumb from the instrument, and
second the top of the distant object on to the same ground object-the difference of
readings on arc being the angle. An artificial horizon may be used instead of a
plumb line.
By setting the vernier to 90° and using the instrument with a plumb line, rough
levelling may be done with a considerable amount of accuracy. With vertical
angles, given a still day fairly accurate altitudes & depressions can be obtained by
this instrument by taking the angle between the object & the plumb line &
subtracting 90° from the observed angle, the result being positive for an altitude
& negative for a depression. Fig. 8 shews this method of use. For setting out
curves with this wide angle sextant proceed as follows. On coming to the end B
of a straight A B (see Fig. 9) if the total deflection 8 of the curve is known set the
sextant to the angle 180°- & set off the angle A B C equal to this value, and
lay out B C on the chord of the curve large enough to give the radius of curvature
required. (If R is the minimum radius allowed & C is the chord must be at least
2
8
8
2
C
equal to R sin.) At C set off the same angle B. C. D., equal to A. B. C., & C D is
the new straight. To lay out the curve keep the sextant at the angle & walk round
such an are that the images of the ranging rods at B & C are exactly superposed
in the instrument. This arc (dotted B. C.) is the curve required.
Having now particularly described and ascertained the nature of our said inven-
tion and in what manner the same is to be performed we declare that what we
claim is :-
1. The application to sextants and instruments of this class of a divided or
double index mirror so mounted and arranged that one half is fixed when in
adjustment and the other moveable, so that the reflection of both lines of vision are
through one or more mirrors instead of one being direct as in ordinary sextants,
substantially as figured and described.
2. The application of a moveable mount to carry the horizon glass in sextants
and instruments of this class to work through a convenient arc for keeping the line
of vision clear through this and the two index mirrors (or halves of index mirror)
for the purpose set forth and substantially as figured and described.
3. An instrument for measuring any angle from 0 to 180 degrees without
parallactic error substantially as figured and described.
Dated this Ninth day of February 1894.
J. H. STEWARD.
JOHN H. BLAKESLEY.