Improvements in Artificial Horizons for Sextants and like Nautical Instruments.
Year: 1896

page_text: Date of Application, 9th Apr., 1896
Complete Specification Left, 9th Jan., 1897-Accepted, 27th Feb.. 1897
PROVISIONAL SPECIFICATION.
Improvements in Artificial Horizons for Sextants and like Nautical
Instruments.
I, JAMES MORTON of Villiers Street South, Sunderland in the County of
Durham, Nautical Instrument Manufacturer, do hereby declare the nature of this
invention to be as follows:-
This invention relates to improvements in or relating to nautical instruments
and embraces an improved artificial horizon designed principally for use on board
ship in conjunction with the sextant quadrant or other reflecting instrument to
enable the navigator to measure altitudes of the heavenly bodies with accuracy
and facility of which the nature and construction may be described as follows:-
I make a close vessel which is constructed so as to be transparent by having
in each of two of its opposite sides a plane glass these two glasses being parallel
to each other with a space between them. Within this vessel from its upper side
and between the two glasses I hang a mirror or a frame having the mirror
attached to it which is suspended very delicately from one or more points (I prefer
two) by means of fine pivots or of threads wires links springs or such like; the
points of suspension being in a line parallel to the two glasses and also to the
plane of the mirror the mirror with its attached frame has freedom to swing
backward or forward through an angle of a few degrees between the two glasses
and come
to rest having its centre of gravity exactly perpendicular to its
suspension (except in cases to be mentioned) the mirror being adjusted so that
when at rest it hangs in a perpendicular plane and (when the vessel containing
it is upright) parallel to each of the two glasses. In this position on looking
into the mirror the image of the eye is reflected back to it in a horizontal line.
To fix the position of the eye I place an eye-piece having an aperture in it
at a convenient distance from the mirror and facing it.
On looking through the aperture its reflected image is seen its centre being
in a horizontal line. The surface of the eye-piece facing the mirror is for better
definition black and on the surface so formed is placed a white patch or plate
the under edge of which is straight and extends across the surface of the eye-piece
in a horizontal line just above the aperture and clear of it.
The reflected image of this line of division between the white and black which
for brevity I may call the index is designed to show the line of the horizon and
although I describe this index as placed as near to the aperture as practicable it is
not necessarily confined to that position for it might conveniently be placed at
any suitable angle to the line of vision above or below or at either side but at
whatever angle it is inclined from the line of sight the mirror would have to be
adjusted to hang at an inclination in the same direction of half that angle in
order that the image of the index may be reflected to the eye in a horizontal
direction.
The index being placed above the aperture in order to be clear of it the mirror
must be adjusted accordingly that is if I place the index say one degree relatively
to the surface of the mirror above the centre of the aperture the mirror will have
to be inclined in the same direction half a degree from the perpendicular and
then on looking through the aperture the reflected image of the index will be seen
in a horizontal direction as before.

page_text: To still oscillations or tremors of the mirror I fill the vessel containing it with
a suitable clear and transparent liquid.
Now on looking through the aperture the index may be seen in a truly
horizontal direction only in one particular part about the centre of the field of
view, when seen either above or below this there is an error arising from the
refractive power of the liquid so that any observation made above or below this
part of the field of view would not be correct. Now in practice it would be
difficult on board ship at sea and at times impossible to hold the sextant steady
enough while taking an observation to keep the images of the sun (or of a star)
and also of the index sufficiently near the one place in the field of view where
a correct observation could be made.
To obviate this I have devised the following methods of correcting the error
arising from refraction by the liquid :-
First by substituting for the plane glass between the observer's eye and the
mirror a spherically concave glass of such a curvature that the radius of the
curve is the distance between the concave that is the outer surface of the glass
and the aperture; it is then obvious that rays of light passing between the
aperture and the mirror will be perpendicular to the surface of the glass and
liquid and so not affected by refraction; in this case a convex glass as regards
the liquid of similar curvature would have to be fitted in the opposite side in
order that distant objects might be seen distinctly through the transparent vessel
and its contained liquid past the side of the mirror.
Secondly another method of making the correction is to substitute for the plane
mirror a convex one of suitable curve.
A third method is while using the plane glasses as in the second ense and the
plane mirror to have the suspension of the mirror not perfectly flexible but
controllable or controlled by the action of one or more springs or wires of such
strength that when the instrument is swayed or tilted from the horizontal position
the springs deflect the mirror from its perpendicular position of rest in the
proportion of the difference between the angles of incidence and refraction of the
liquid used. The proportion between the sines of the angles of incidence and
refraction is constant but not so the angles themselves: I cannot therefore by the
last methods make the correction absolutely perfect but in the small angle
contained in the telescope or field of view the error remaining is so small as to be
inappreciable and practically I can make observation in any part of the field of view
without error.
By attaching the two parts of the artificial horizon together that is the
suspended mirror with its casing and the eye-piece is assumes the form of a small
telescope and may be attached to the sextant or equivalent instrument in the same
manner as are the other tubes or telescopes belonging to it; the line of sight is
then parallel to the plane cf the sextant and leads into its horizon glass; the
image of the index or horizon line and that of the sun or other body observed can
then be seen together in the field of view when the two images can be brought into
contact in the usual way and the angle registered by the sextant read off.
Instead of uniting the mirror and eye-piece the mirror arranged as above
described may be attached to any convenient part of the sextant frame between
the eye-piere and the horizon glass or beyond the horizon glass through or past
which it may be seen or the mirror may be altogether detached from the sextant
and used in the same way as the ordinary artificial horizon by observing it through
the eye-piece in the sextant.
For the purpose of observing faint objects as stars or planets propose instead
of making the instrument with plane glasses as described to substitute leuses for
these thus converting it into a telescope or otherwise to use a telescope such as is
in ordinary use with the sextant and to combine with it in a binocular form my
artificial horizon as herein foreshadowed but having suitable lens or mirror
arrangement according to the maguitying power required. I have thus an instru-
ment resembling a binocular telescope one barrel containing the telescope and the

page_text: other the artificial horizon; with one eye I look through the telescope into the
horizon glass and with the other eye into the artificial horizon.
In some of the above referred to arrangements it is not always necessary to have
two transparent glasses in the vessel one being sufficient when it is required only
to look into the mirror and not past it.
For night observation it is necessary to illuminate the index line in order that
its image may be visible in the mirror.
Dated this 8th day of April 1896.
A. CRAWHALL CHAPMAN,
3 St. Nicholas Buildings, Newcastle-on-Tyne, Patent Agent.
Improvements in Artificial Horizons for Sextants and like Nautical
Instruments.
I, JAMES MORTON of Villiers Street South, Sunderland in the County of
Durham, Nautical Instrument Manufacturer, 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:-
This invention has reference to improvements in or relating to nautical instru-
ments and embraces an improved artificial horizon designed principally for use on
board ship in conjunction with the sextant, quadrant or other reflecting instrument
to enable the navigator to measure altitudes of the heavenly bodies with accuracy
and facility, the nature and construction of which I will describe with reference to
the annexed drawings in which the same reference letters indicate the same or
analogous parts.
A simple form of my invention is shown in Figures 1 to 4; Figure I is a plan,
Figure 2 a longitudinal section, Figure 3 a front elevation of the eye-piece and
Figure 4 a cross-section on line X Y of Figure 2. In this arrangement, I construct
a closed vessel of brass or other suitable metal of a form such as is shown at A,
and in order to render its side or sides transparent where required, I secure in each
of two of its opposite sides a plane glass a a, these two glasses being parallel to
each other with a space between them. Within this vessel between the two glasses,
I arrange a mirror B attached to a frame b which may be of talc, mica, sheet brass
or the like and which is suspended very delicately from one or more points, on for
example the cover-plate a¹, by means of fine pivots, or of threads, wires, links,
springs, or such like. In Figure 2, the suspension is effected by a spring strip of
brass or other suitable material a² connected at one end to the cover-piece a¹ and
at the other to the mirror frame b; or as shown in Figure 7 where the spring
strip a² is replaced by the small spiral springs a³ a³. In another way illustrated
by Figure 8, the frame is suspended by the arms b¹ b' from eyes a¹ a connected
to the cover-piece a¹. The points of suspension being in a line parallel to the two.
glasses and also to the plane of the mirror, the mirror B with its attached frame b
has freedom to swing backward or forward through an angle of a few degrees
between the two glasses and come to rest having its centre of gravity exactly
perpendicular to its suspension (except in cases to be mentioned), the mirror being
adjusted so that when at rest it hangs in a perpendicular plane and (when the
vessel A containing it is upright) parallel to each of the two glasses a a.
position on looking into the mirror the image of the eye is reflected back to it in
a horizontal line.
To fix the position of the eye, I place an eye-piece C having an aperture c in it

page_text: at a convenient distance from the mirror and facing it. This is simply and
conveniently effected by the use of the frame tube D fixed to the vessel A and
eye-piece and of a form such as is showa with light hole d and cleansing
hole d',
On looking through the aperture e, the reflected image is seen its centre being in
a horizontal line.
The surface of the eye-piece C facing the mirror B is for better definition black
as at e and on the surface so formed is placed a white patch or plate e the under
edge of which is straight and extends across the surface of the eye-piece C in
a horizontal line just above the aperture and clear of it as at 1-1 in Figure 4.
The reflected image of this line of division between the white c and black e',
which for brevity I may call the index, is designed to show the line of the horizon
and although I describe this index as placed as near to the aperture e as practicable,
it is not necessarily confined to that position, for it might conveniently be placed
at any suitable angle to the line of vision above or below, or at either side, but at
whatever angle it is inclined from the line of sight, the mirror B would have to be
adjusted to hang at an inclination in the same direction of half that angle in order,
that the image of the index may he reflected to the eye in a horizontal direction,
as will be readily understood by opticians.
The above mentioned index being placed above the aperture e in order to be
clear of it, the mirror B must be adjusted accordingly, that is, if I place the
index 1-1 say one degree relatively to the surface of the mirror B above the
centre of the aperture c, the mirror will have to be inclined in the same direction
half a degree from the perpendicular and then on looking through the sight
aperture, the reflected image of the index will be seen in a horizontal direction as
before.
To still oscillations or tremors of the mirror, I fill the vessel A, containing it,
with a suitable clear and transparent liquid, such as water or alcohol or a mixture
of these, packing rings to secure tight joints being arranged where required as, for
example, at p p Figure 2.
Now on looking through the aperture c, the index 1-1 may be seen in a truly
'horizontal direction only in one particular part about the centre of the field of
view; when seen either above or below this there is an error arising from the
refractive power of the liquid used in the vessel A so that any observation made
above or below this part of the field of view would not be correct. Now in practice
it would be difficult on board ship at sea and at times impossible to hold the sextant
steady enough while taking an observation to keep the images of the sun (or of
a star) and also of the index sufficiently near the one place in the field of view
where a correct observation could be made.
To obviate this, I have devised the following methods of correcting the error
arising from refraction by the liquid :-
First, as shown in section in Figure 5, I substitute for the plane glass a between
the observer's eye at c and the mirror B a concave glass a** of such a curvature
that the radius of the curve is practically equal to the distance between its circum-
ference and the aperture e; it is then obvious that rays of light passing between
the aperture e and the mirror B will be perpendicular to the surface of the glass a**
and liquid and so not affected by refraction; in this case a convex glass a, as
regards the liquid, of similar curvature would have to be fitted in the opposite
side of the vessel A in order that distant objects might be seen distinctly through
the transparent vessel and its contained liquid past the side of the mirror B.
Secondly, another method of making the correction is shown in section in
Figure 6. Here I substitute for the plane mirror B a convex one B' of suitable
curve, the glasses a a remaining plane as in Figure 2.
According to a third method, while using the said plane glasses a a as in
Figures 2 and 6, and the plane mirror B, I arrange the suspension of the mirror B
not perfectly flexible or free as shown in Figure 8 but controllable or controlled by
the action of one or more springs or wires such as a² and a3 a³ Figures 2 and 7

page_text: respectively of such strength that when the instrument is swayed or tilted from the
horizontal position the springs or equivalent suspensions serve to deflect the mirror
from its perpendicular position of rest in the proportion of the difference between-
the angles of incidence and refraction of the liquid used, as represented by the
dotted lines in Figure 2.
The proportion between the sines of the angles of incidence and refraction is
constant, but not so the angles themselves; therefore, by the last methods the
correction cannot be made absolutely perfect, but in the small angle contained
in the telescope or field of view the error remaining is so small as to be inappreciable,
and practically observation can be made in any part of the field of view without
error.
By thus attaching the two parts of the artificial horizon together, that is
the suspended mirror B and its casing A, and the eye-piece C, by the tube D, the
instrument assumes a small and rigid form and it may be attached as a whole to the
sextant or equivalent instrument G, Figures 1 and 2, by an extended part E inserted
through the lug or rising piece F thereof and secured by the nut H practically in the
same manner as are the other tubes or telescopes belonging to it; the line of
sight is then parallel to the plane of the sextant G and leads into its horizon
glass; the image of the index or horizon line 1-1 and that of the sun or other
body observed can then be seen together in the field of view when the two images
can be brought into contaes in the usual way and the angle registered by the
sextant read off.
Instead of uniting the mirror B and the eye-piece C as described, the mirror B,.
arranged as above described, may itself be attached to any convenient part of the
sextant frame between the eye-piece and the horizon glass or beyond the horizon
glass through or past which it may be seen, or the mirror may be altogether
detached from the sextant and used in the same way as the ordinary artificial
horizon by observing it through the eye-piece in the sextant..
For the purpose of observing faint objects as stars or planets, instead of making
the instrument with plane glasses a a as described, I substitute lenses for these
thus converting it into a telescope, or I may use a telescope such as is in ordinary
use with the sextant and thus I combine with it in a binocular form my artificial
horizon, but in such case a suitable lens or mirror arrangement is necessitated
according to the magnifying power required as will be well understood. I have
illustrated this arrangement in plan and end elevation respectively in Figures 9.
and 10 wherein K is my artificial horizon as abeve described connected by a
plate L to a small telescope J ard the whole attachment connected to the
sextant G by the part or couple F; with this arrangement the observer can
with great ease and steadiness with one eye look through the telescope into the
horizon glass and with the other eye into the artificial horizon.
In some of the above referred to arrangements it is not always necessary to have
two transparent glasses in the vessel one being sufficient when it is required only
to look into the mirror and not past it.
For night observation it is necessary to illuminate the index line in order that
its image may be visible in the mirror.
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. An artificial horizon adapted for attachment for sextants, quadrants, and
like nautical instruments substantially as herein described and illustrated in the
drawings.
2. The use in an artificial horizon for the purpose above referred to of a closed
and partly transparent vessel such as A containing a clear transparent liquid and
mirror B suspended therein the error arising from refraction by the liquid
being corrected by either of the methods described in combination with an eye-
piece such as C or a small telescope substantially as herein described.

page_text: 3. The arrangement of my improved artificial horizon as described in reference
to and shown in Figures 1 to 4 of the drawings in which a rigid tube is used to
connect the closed mirror containing vessel A with the eye-piece C.
4. The several arrangements of the side glasses of the closed vessel A and of
the mirror B substantially as described-respectively in reference to Figures 1, 2
and 5 and 6 in combination with an eye-piece or a small telescope for the purposes
herein specified.
5. The combination of parts forming the binocular arrangement of artificial
horizon adapted for attachment to sextauts and the like as described in reference
to and shown in Figures 9 and 10.
Dated this 8th day of January 1897.
For the Applicant,
J. C. CHAPMAN,
Chartered Patent Agent.
70 Chancery Lane, London, W.C., and
3 St. Nicholas' Buildings, Newcastle-on-Tyne.