Improvements in Sextant Attachments.
Year: 1894

page_text: Date of Application, 11th Dec., 1894
Complete Specification Left, 17th May, 1895-Accepted, 6th July, 1895
PROVISIONAL SPECIFICATION.
Improvements in Sextant Attachments.
"
I, ALEXANDER MELVILLE CLARK, of the Firm of A. M. and Wm. Clark, of
53, Chancery Lane, in the County of Middlesex, Fellow of the Chartered Institute
of Patent Agents, do hereby declare the nature of this invention (a communication
to me by Thomas Tapley Helenus Ferguson, of Hankow, China, Inventor,) to be
as follows:-
This invention relates to a sextant attachment designed to convert the ordinary
sextant into a measuring instrument for measuring large angles, say from
120 degrees to 240 degrees, as well as angles from 0 degrees to 20 degrees.
The invention consists principally of a full-silvered glass or mirror and a half-
silvered glass, of which the latter is substituted for a horizon glass in the plate of
the instrument, and the mirror is attached to a vernier indicating a graduated
arc.
The invention also consists of certain parts and details and combinations of the
same, as will be hereinafter described.
Reference is had to the accompanying drawings, forming part of this Specification,
in which
Figure 1 is a plan view of the improvement.
Figure 2 is an enlarged face view of the half-silvered glass. Figure 3 is an end
view of the same.
Figures 4 and 5 are diagrammatic views illustrating the use of the instrument for
measuring greater angles than can be measured by the ordinary sextant; and
Figure 6 is a sectional plan view of a modified form of the improvement.
Similar letters of reference indicate corresponding parts in all the figures.
The sextant, of any approved construction, is provided with the usual plate A
carrying a pivot B for the vernier C indicating on the graduated arc D forming
part of the plate A. On one end of the latter, opposite the pivot B, is mounted,
movable vertically up and down but not round its own axis, a ring E for supporting
the telescope, and between the said ring, which is essentially the same as the other
telescope ring K, and on the pivot G. is mounted to turn on the plate A the half-
silvered glass F, shewn in detail in Figures 2 and 3.
The half-silvered glass F is provided with a suitably constructed frame G sup-
porting a half-silvered glass G, as indicated in Figure 2, the said glass being beld
in place in the frame by set screws G' which relatively take in the same position as
The frame G is
the set screws on the horizon glass of an ordinary sextant.
provided with a base G³, preferably made circular and provided at its under side
and in the centre with a pivot G extending downward to fit into a corresponding
aperture formed in the plate A. On the base G³, on opposite sides of the pivot G4,
are held clamping screws G and G extending through segmental slots A¹ and A'
respectively, formed in the plate A of the sextant. The segmental slots A1 and A2
permit of turning the half-silvered glass F, 120 degrees, the glass being fastened in
place at either end of the slots by the set screws G and G in such a way that when
the glass F is turned (against clock hands) in its ultimate position it will take in
the same position relative to the under glass, as the horizon glass of an ordinary
sextant (ie. parallel). The free end of the index arm is provided with vernier,
tangent screws, etc., exactly like that on other sextants.

page_text: The half-silvered glass is placed in the position on the plate A ordinarily occupied
by the horizon glass of the sextant, which horizon glass is removed so that the half-
silvered glass stands in alignment with the index glass I over the pivot B and the
ring E carrying the telescope. Between the half-silvered glass F and the index
glass I are arranged the usual four shades, while the other shades are removed so
that when two extremely bright objects are to be observed, such as the sun and the
reflected sun; the shading is to be done by dark glasses which fit to the eye-piece
of the telescope, and of which three shades should be provided. Upon the vernier C
is provided at C¹ the usual, or any suitable, support for a microscope.
The
graduation on the arc D remains the same, with the exception that immediately
above each numeral denoting degrees another is engraved which, added to the numeral
below, will give 240 degrees. Thus above 0 degrees will come 240 degrees, above
30 degrees is engraved 210 degrees, so as to aggregate 240 degrees. Above
60 degrees is 180 degrees, above 100 degrees is 140 degrees, etc. To convert the
sextant into a large-angle measuring instrument, the horizon or half-silvered glass F
is turned in its position, the telescope is taken from its usual ring and screwed or
otherwise fastened in the ring E, and the readings are taken from the upper added
numerals on the arc D.
Previous to being able to measure large angles the following adjustments have
to be made on the sextant
1. To set the index glass perpendicular to the plane of the sextant the same as
is done with ordinary sextants.
2. To adjust the line of collimation, to set the axis of the telescope at its proper
distance from the plane of the sextant, screw on the plain tube, unclamp the index
and move it to the right and even off the arc (for which purpose the first telescope
ring has to be removed from its square socket to allow the index arm to pass off the
arc) until on looking through the sight-hole over the edge of the silvered portion
of the horizon glass the reflection of the back of the same glass is visible in the
index glass. Raise the telescope ring till the reflected edge of the silvered half is
visible above the edge itself. Then slowly lower the ring until the reflected edge
just disappears. The line drawn from the sight-hole over the edge of the silvered
half will then be perpendicular to the axis of the index glass, which is the
adjustment required.
3. To set the axis of the telescope parallel to the plane of the sextant, same as
with ordinary sextants.
4. To set the horizon glass perpendicular to the plane of the sextant, turn the
arm of the index still farther to the right until the index glass becomes parallel to
the horizon glass, both facing to the right. Hold the instrument so that the images
of a star, or distant terrestrial object reflected in each of the mirrors, coincide.
If, on moving the index arm, it is found that one of the images passes above or
below the other, the horizon glass has to be adjusted by the screw at the top.
5. To set the plane of the horizon glass vertical to that of the index glass when
the index of the latter points at 180 degrees, select a good-sized well-made plane
speculum, such as a toilet mirror. Place it on a stand so that it faces some well
defined vertical line in the far distance-a flagstaff, or the edge of a building.
Place yourself in front of the mirror, some distance off, say 15 to 20 feet, and after
having temporarily obscured the index glass by raising a few shades look at the
reflection of the mirror in the horizon glass. In the mirror will appear an image
of the sextant itself and beyond that the landscape containing the selected vertical
line. Get an assistant to slightly move the mirror in vertical and horizontal
direction until the image of the vertical line (flagstaff) as seen in the mirror falls
behind the reflection of the horizon glass in the same mirror. Part of it will then
be visible through the upper part of the horizon glass. Adjust the mirror further
so that the image of the horizon glass is bisected by the image of the flagstaff
behind it. In practice this operation entails no difficulty. With regard to the
observer, or more accurately with regard to the middle of the horizon glass, the
flagstaff and its image in the mirror will make exactly an angle of 180 degrees.

page_text: If now the shades before the index glass are removed the image in that glass ought
to coincide with the image seen in the mirror. If not, the horizon glass is not
adjusted and has to be slightly moved by means of its lower screw till the contact
takes place. The position of the horizon glass when arrested by the clamping screws
in either position should be accurate enough to enable this adjustment to be
dispensed with. In each case, however, the index error has to be found. This can
be done for any large angle by measuring the angle and its difference from
360 degrees, either by objects on shore, or by stars at sea, or by the horizon on both
Another
sides of the observer, in which case the sextant is held vertical.
of
way
completing the adjustment, or finding the index error, is to select two sharp objects
which span an angle of about 120 degrees and first measure it with the sextant in
its ordinary position and afterwards when it is fitted as a large-angle instrument.
A great advantage of the large-angle sextant, is the possibility of measuring
every angle twice, once for its value below 180 degrees and once for its value above.
By taking several of such sets any error in construction of the sextant,
centralization, etc., will become apparent.
As shewn in Figure 4, the sextant is adjusted for measuring greater angles than
when adjusted for ordinary work. It will be seen that the telescope is removed
from its usual place at K and held in the rings E. Figure 5 shews the lines of
reflection from different objects coinciding in the field of the telescope.
Manipulation of the large-angle sextant is slightly more difficult than that of the
ordinary sextant, but with a little practice presents no noteworthy difficulties.
After contact is made in the middle of the field of the telescope, this contact will
no longer be when both objects are brought above or below the middle line. They will
then separate or overlap. In case the collimation of the telescope is perfect, the
However,
true angle will be when contact is made in the exact centre of the field.
it is always possible to get the exact angle by gently rocking the instrument so that
both objects move from the upper part of the field to the lower, and vice-versa. The
curves described by each object should just touch without intersecting. The small
column which holds the arm to which the microscope is attached, is placed slightly
higher up on the index arm, as shewn in the drawings. It should not interfere
with the line of vision of the sextant in either position.
This instrument permits latitude to be determined on shore by meridian altitudes
of the sun in an artificial horizon when the altitude is from 60 degrees upwards.
Travellers in low latitudes from about 35 degrees North to 35 degrees South, have
to relinquish this means of determining the latitude for a considerable portion of
the year, or even throughout the year, and limit themselves to star observations by
night, unless they possess elaborate surveying instruments, such as a transit
instrument, or succeed in finding a natural horizon. For the same reason, it adds
considerably to the choice of stars for latitude observations in enabling travellers
by land to take such stars as should have a meridian altitude exceeding
60 degrees.
Every angle taken by the attachment can be remeasured in such a way as to
eliminate instrumental errors. For instance, an angle of 150 degrees can be
remeasured by taking the other angle of 210 degrees which is also formed by the
same lines. The mean of these two observations should always be 180 degrees.
If not, the difference between this mean and 180 degrees is the index error which
may be additive or subtractive. The large-angle sextant might be used in surveying
for traversing, which operation, through the frequent occurrence of large angles,
forbids the use of a sextant. Especially in setting out railway curves it might
For use in sounding
be used with much advantage, even to replace the theodolite.
in a river between two beacons, by clamping the index at 180 degrees and looking
into the telescope one would be enabled to tell the exact moment when the boat is
between the two beacons on the line which unites them. The attachment may be
used in many more cases in which it is imperative to measure large angles quickly,
under conditions which preclude the use of instruments which have stands, such as
theodolites, etc.

page_text: The arrangements of the parts are such that any ordinary sextant can be readily
converted into the combination instrument by an instrument maker of ordinary
skill; without the least detriment to either accuracy or ease of working of the
sextant itself. The instrument thus produced is, therefore, still a sextant, and as
none of the usual sextant fittings have been removed, beyond a few shades, loses
none of its efficiency as a reflecting instrument for measuring angles up to
130 degrees. The attachments described above will only add to its efficiency
with no other sacrifice but that of adding slightly to the weight of the instrument.
Instead of mounting the horizon glass on a circular base, as before specified, an
arrangement like the one represented (in ground plan) in Figure 6 might be
employed with the advantage of doing away with the turning of the horizon glass
each time the sextant is to be converted into its large-angle position. The
adjustments can then be made once for all and the mère shifting of the telescope
from the first to the second ring is all that is required to bring about the necessary
conversion. Two horizon glasses N and N¹, each in every respect similar in way
of mounting, mode of adjustment, etc., to the horizon glass. on an ordinary sextant,
are mounted perpendicularly on a flat base O in the positions indicated in Figure 6,
the horizon glass N' however, being broader than the horizon glass N. They form
together exactly an angle of 60 degrees and are to be both adjusted perpendicular
to the plane of the sextant. Both are half-silvered on the sides facing each other;
with the silvered portions below and reaching to exactly the same distance off the
base. The line of vision of the first telescope will partly be reflected into the
index glass above mentioned and partly pass through the unsilvered portion of N.
Now, the horizon glass N1 is so adjusted that the whole field of vision which passes
through the upper unsilvered half of the horizon' glass N will be clear of the
horizon glass N₁, or any of its solid parts. The horizon glass N takes up the
same position relatively to the index glass and the first telescope ring, as
the horizon glass does on an ordinary sextant. The second horizon glass N¹ is
placed to intercept the line of vision from the second telescope. Its lower
silvered half will intercept and reflect half of the field of this second telescope
and the upper portion being clear glass will let through the line of vision
which passing over the silvered half will also pass over the silvered half of
the horizon glass N and ultimately strike the index glass where it is reflected.
The horizon glass N' is centred on the axial line of the second telescope which
passes through the centre of the horizon glass N and strikes the centre of the
index glass. The plate O can be attached to the framework of the sextant, and as
both the frames which hold the glasses N and N' are soldered on the plate O Do
other adjustments need be made than those well-known adjustments which can be
made with the screws P and P¹ (above) and Q' and Q (below). As these frames
are well known sextant parts they need not be drawn in elevation. Finally,
a screen R made of thin blackened brass is fastened by the screw 'S into the plate O
round which screw, as pivot, the screen is movable at will, so that it may take in
either the position S R or S R. In the former position it serves to screen off
various reflections from the back of the glass N. In the latter it is thrown back
so as not to interfere with the line of sight from the first telescope. The screen is
of a square or erect angular shape. Instead of the screen R a set of two or three
shaded glasses might be used, which, in that case, would have to be placed so that
if down they are perpendicular to the line of sight from the first telescope.
Dated this 11th day of December 1894.
A. M. & WM. CLARK,
53, Chancery Lane, London, Chartered Patent Agents.

page_text: I ALEXANDER MELVILLE CLARK, of the Firm of A. M. & Wm. Clark of
53 Chancery Lane in the County of Middlesex, Fellow of the Chartered Institute
of Patent Agents, do hereby declare the nature of this invention (a communication
to me by Thomas Tapley Helenus Ferguson, Inventor, of Hankow, China) and in
what manner the same is to be performed, to be particularly described and
ascertained in and by the following statement :-
This invention relates to a sextant attachment designed to convert the ordinary
sextant into a measuring instrument for measuring large angles say from 120°
to 240°, as well as angles from 0° to 120°.
The invention consists principally of a full-silvered glass or mirror and a half-
silvered glass, of which the latter is substituted for a horizon glass in the plate
of the instrument, and the mirror is attached to a vernier indicating on it
graduated arc.
The invention also consists of certain parts and details and combinations of the
same, as will be hereinafter described and then pointed out in the claims.
Reference is to be had to the drawings accompanying my Provisional
Specification, in which similar letters of reference indicate corresponding parts in
all the figures.
Figure 1 is a plan view of the improvement;
Figure 2 is an enlarged face view of the half-silvered glass;
Figure 3 is an end view of the same;
Figures 4 and 5 are diagrammatic views illustrating the use of the instrument for
measuring greater angles than can be measured by the ordinary sextant ; and
Figure 6 is a sectional plan view of a modified form of the improvement.
The sextant of any usual or approved construction, is provided with the plate A
carrying a pivot B for the arm carrying at its free end a vernier which indicates
on the graduated are D forming part of the plate A. On one end of the latter
opposite the pivot B, is mounted movable vertically up and down, but not round its
own axis a ring E, for supporting the telescope, and between the said ring, which is
essentially the same as the other telescope ring K, and on the pivot G is mounted to
turn, on the plate A, the half-silvered glass F, shown in detail in Figures 2 and 3.
The half-silvered glass F is provided with a suitably constructed frame G
supporting a half-silvered glass G¹ as indicated in Figure 2, the said glass being
held in place in the frame by set screws G2, which relatively take the same position
as the set screws on the horizon glass of an ordinary sextant. The frame G is
provided with a base G³, preferably made circular and provided at its under side
and in the centre with a pivot G4 extending downward to fit into a corresponding
aperture formed in the plate A. On the base G³, on opposite sides of the pivot G³,
are held clamping screws G³ and G6, extending through segmental slots A¹ and A'
respectively, formed in the plate A of the sextant. The segmental slots A' and A²
permit of turning the half-silvered glass F 120°, the glass being fastened in place at
either end of the slots by the set screws G and G6, in such a way, that when the
glass F is turned (against clock hands) in its ultimate position it will take the same
position relative to the under glass, as the horizon glass of an ordinary sextant,
(i.e. parallel). The free end of the index arm is provided with vernier, tangent
screws, etc., exactly like that on other sextants.
The half-silvered glass is placed in the position on the plate A ordinarily
occupied by the horizon glass of the sextant, which horizon glass is removed so
that the half-silvered glass stands in alignment with the index glass I over the
pivot B, and the ring E carrying the telescope.
Between the half-silvered glass F and the index glass I, are arranged the usual

page_text: four shades, while the other shades are removed so that when two extremely bright
objects are to be observed, such as the sun and the reflected sun, the shading is to
be done by dark glasses which fit to the eye piece of the telescope, and of which
three shades should be provided. Upon the index arm C is provided at C¹ the
usual or any suitable support for a microscope.
The graduation on the arc D remains the same, with the exception that
immediately above each numeral denoting degrees another is engraved, which,
added to the numeral below, will give 240°.
Thus above 0° will come 240°,
above 30° is engraved 210° so as to aggregate 240°.
100° is 140°, etc.
Above 60° is 180°, above
To convert the sextant into a large angle measuring instrument, the horizon or
half-silvered glass F is turned in its position, the telescope is taken from its usual
ring and screwed or otherwise fastened in the ring E, and the readings are taken
from the upper added numerals, on the arc D.
Previous to being able to measure large angles the following adjustments have
to be made on the sextant ;
1. To set the index glass perpendicular to the plane of the sextant the same as
is done with ordinary sextants.
2. To adjust the line of collimation, to set the axis of the telescope at its proper
distance from the plane of the sextant, screw on the plain tube, unclamp the index
and move it to the right and even off the arc, (for which purpose the first telescope
ring has to be removed from its square socket to allow the index arm to pass off the
arc), until on looking through the sight hole over the edge of the silvered portion
of the horizon glass, the reflection of the back of the same glass is visible in the
index glass. Raise the telescope ring till the reflected edge of the silvered half is
visible above the edge itself. Then slowly lower the ring until the reflected edge
just disappears. The line drawn from the sight-hole over the edge of the silvered
half will then be perpendicular to the axis of the index glass, which is the adjust-
ment required.
3. To set the axis of the telescope parallel to the plane of the sextant, same as
with ordinary sextants.
4. To set the horizon glass perpendicular to the plane of the sextant, turn the
arm of the index still farther to the right until the index glass becomes parallel to
the horizon glass, both facing to the right. Hold the instrument so that the images
of a star, or distant terrestrial object reflected in each of the mirrors coincide. If,
on moving the index-arm it is found that one of the images passes above or below
the other, the horizon glass has to be adjusted by the screw at the top.
5. To set the plane of the horizon glass vertical to that of the index glass when
the index of the latter points at 180°, select a good sized well-made plane speculum
such as a toilet mirror. Place it on a stand so that it faces some well defined
vertical line in the far distance-a flagstaff, or the edge of a building; place
yourself in front of the mirror, some distance off say 15 to 20 feet, and after having
temporarily obscured the index glass by raising a few shades, look at the reflection
of the mirror in the horizon glass. In the mirror will appear an image of the sextant
itself and beyond that the landscape containing the selected vertical line. Get an
assistant to slightly move the mirror in vertical and horizontal direction until the
image of the vertical line (flagstaff) as seen in the mirror falls behind the reflection
of the horizon glass in the same mirror. Part of it will then be visible through the
upper part of the horizon glass. Adjust the mirror further so that the image of the
horizon glass is bi-sected by the image of the flagstaff behind it. In practice this
operation entails no difficulty. With regard to the observer or more accurately,
with regard to the middle of the horizon glass, the flagstaff and its image in the
mirror will make exactly an angle of 180°. If now the shades before the index
glass are removed, the image in that glass ought to coincide with the image seen in
the mirror. If not, the horizon glass is not adjusted and has to be slightly moved'
by means of its lower screw till the contact takes place. The position of the horizon.
glass when arrested by the clamping screws in either position; should be accurate

page_text: enough to enable this adjustment to be dispensed with. In each case however, the
index error has to be found. This can be done for any large angle by measuring
the angle and its differénce from 360°, either by objects on shore, or by stars at sea,.
or by the horizon on both sides of the observer, in which case the sextant is held..
vertical. Another way of completing the adjustment or finding the index error, is-
to select two sharp objects which span an angle of about 120° and first measure it.
with the sextant in its ordinary position and afterwards when it is fitted as a large-
angle instrument. A great advantage of the large angle sextant, is the possibility
of measuring every angle twice, once for its value below 180° and once for its value
above. By taking several of such sets any error in construction of the sextant,.
centralization, etc., will become apparent. As shown in Figure 4 the sextant is.
adjusted for measuring greater angles than when adjusted for ordinary work. It.
will be seen that the telescope is removed from its usual place at K and held in the
rings E. Figure 5 shows the lines of reflection from different objects coinciding in,
the field cf the telescope.
Manipulation of the large-angle sextant is slightly more difficult than that of the
ordinary sextant, but with a little practice presents no noteworthy difficulties.
After contact is made in the middle of the field of the telescope, this contact will
no longer be when both objects are brought above or below the middle line; they
will then separate or overlap. In case the collimation of the telescope is perfect
the true angle will be when contact is made in the exact centre of the field.
However, it is always possible to get the exact angle by gently rocking the instru-
ment so that both objects move from the upper part of the field to the lower, and
vice versa.
The curves described by each object should just touch without inter-
secting. The small column which holds the arm to which the microscope is attached,.
is placed slightly higher up on the index arm, as shown in the drawings. It should.
not interfere with the line of vision of the sextant in either position.
This instrument permits latitude to be determined on shore by meridian altitudes.
of the sun in an artificial horizon when the altitude is from 60° upwards. Travellers
in low latitudes from about 35° north, to 35° south have to relinquish this means of¨*
determining the latitude for a considerable portion of the year or even throughout
the year, and limit themselves to star observations by night, unless they possess
elaborate surveying instruments, such as a transit instrument or succeed in finding
a natural horizon. For the same reason it adds considerably to the choice of stars.
for latitude observations in enabling travellers by land to take such stars as should
have a meridian altitude exceeding 60°.
Every angle taken by the attachment can be re-measured in such a way as to-
eliminate instrumental errors. For instance an angle of 150° can be re-measured by
taking the other angle of 210° which is also formed by the same lines.
The mean
of these two observations should always be 180°. If not, the difference between this
mean and 180° is the index error which may be additive or subtractive.
The large-angle sextant might be used in surveying for traversing, which
operation, through the frequent occurrence of large angles forbids the use of a.
sextant. Especially in setting out railway curves it might be used with much.
advantage, even to replace the theodolite.
For use in sounding in a river between two beacons, by clamping the index at
180° and looking into the telescope, one would be enabled to tell the exact moment.
when the boat is between the two beacons on the line which unites them.
The attachment may be used in many more cases in which it is imperative to
measure large angles quickly, under conditions which preclude the use of instruments.
which have stands, such as theodolites, etc.
The arrangements of the parts are such, that any ordinary sextant can be readily
converted into the combination instrument by an instrument maker of ordinary skill,
without the least detriment to either accuracy or ease of working of the sextant
itself. The instrument thus produced is therefore, still a sextant, and as none of
the usual sextant fittings have been removed beyond a few shades, loses none of
its efficiency as a reflecting instrument for measuring angles up to 130°.
The

page_text: attachments described above will only add to its efficiency with no other sacrifice
but that of adding slightly to the weight of the instrument.
Instead of mounting the horizon glass on a circular base, as before specified, an
arrangement like the one represented (in ground plan) in Figure 6, might be
employed with the advantage of doing away with the turning of the horizon glass
each time the sextant is to be converted into its large-angle position. The adjust-
ments can then be made once for all, and the mere shifting of the telescope from the
first to the second ring is all that is required to bring about the necessary conversion.
Two horizon glasses N and N₁, each in every respect similar in way of mounting,
mode of adjustment, etc., to the horizon glass on an ordinary sextant, are mounted
perpendicularly on a flat base O in the positions indicated in Figure 6, the horizon
glass N', however, being broader than the horizon glass N. They form together
exactly an angle of 60° and are to be both adjusted perpendicular to the plane of
the sextant. Both are half-silvered on the sides facing each other, with the silvered
portions below and reaching to exactly the same distance off the base. The line of
vision of the first telescope will partly be reflected into the index glass above named,
and partly pass through the unsilvered portion of N. Now, the horizon glass Ni
is so adjusted that the whole field of vision which passes through the upper,
unsilvered half of the horizon glass N will be clear of the horizon glass N' or any
of its solid parts. The horizon glass N takes up the same position relatively to the
index glass and the first telescope ring, as the horizon glass does on an ordinary
sextant. The second horizon glass N' is placed to intercept the line of vision from
the second telescope. Its lower, silvered half will intercept and reflect half of the
field of this second telescope and the upper portion, being clear glass, will let
through the line of vision which, passing over the silvered half, will also pass over
the silvered half of the horizon glass N, and ultimately strike the index glass where
it is reflected. The horizon glass N¹ is centered on the axial line of the second
telescope, which passes through the centre of the horizon glass N and strikes the
centre of the index glass.
The plate O can be attached to the framework of the sextant, and as both the
frames which hold the glasses N and N¹ are soldered on the plate O, no other
adjustments need be made than those well known adjustments which can be made
with the screws P and P¹ (above) and Q and Q1 (below). As these frames are well
known sextant parts they need not be drawn in elevation.
Finally, a screen R made of thin blackened brass is fastened by the screw S into
the plate O, round which screw as pivot the screen is movable at will, so that it
may take in either the position S R or S R¹. In the former position it serves to
screen off various reflections from the back of the glass N. In the latter it is
thrown back so as not to interfere with the line of sight from the first telescope.
The screen is of a square cr erect angular shape. Instead of the screen R a set of
two or three shaded glasses might be used, which, in that case, would have to be
placed so that if down they are perpendicular to the line of sight from the first
telescope.
The above described attachments can also be made to the instrument known as
the box sextant which is essentially a sextant and would as such readily lend itself
to the present improvement and acquire thereby the same increased utility."
A small part of the side of the enclosing case would only have to be cut away to
allow the line of vision of the 2nd telescope as reflected from the 2nd horizon glass
to pass out of the box.
Having thus 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 combination with the frame, graduated arc, pivoted mirror, and other
ordinary attachments of a sextant, of a half silvered glass which is movable
vertically, and adapted to turn horizontally, and occupies the place of the horizon
glass, as shown and described.

page_text: 2. The combination with the frame, graduated arc, pivoted mirror, and other
ordinary attachments of a sextant, be it of the shape hereabove described, or
partly enclosed in a metal case and known as a box sextant, of a half silvered glass.
which is adjustable horizontally and vertically and occupies a place with regard to
the index and horizon glasses, substantially as shown and described.
3. The combination with the frame, graduated arc, pivoted mirror, and other
ordinary attachments of a sextant, be it of the shape hereabove described, or partly
enclosed in a metal case and known as a box sextant, of a half silvered glass in
alignment with the index and the horizon glasses, a ring adapted to hold the-
telescope in alignment with the said three glasses and a set of numerals engraved
on the metal arc above the usual figures denoting degrees, substantially as shown.
and described.
Dated this 17th day of May 1895
A. M. & WM. CLARK,
53 Chancery Lane, London, Chartered Patent Agents.