An Improved Instrument for Determining Distances, especially applicable as a Station Keeper for Maintaining the Ordered Distances between Ships in a Fleet.
Year: 1893

page_text: Date of Application, 27th Feb., 1893
Complete Specification Left, 31st Aug., 1893-Accepted, 7th Oct., 1893
PROVISIONAL SPECIFICATION.
An Improved Instrument for Determining Distances, especially
applicable as a Station Keeper for Maintaining the Ordered
Distances between Ships in a Fleet.
LEWIS KNOX BELL Junior Army and Navy Club 10 St. James' Street
London S.W. Commander, Royal Navy do hereby declare the nature of this
invention to be as follows:-
This invention relates to an instrument primarily designed for assisting to
maintain the ordered distances between ships in a fleet, but which may be used in
conjunction with fixed marks, at known distances apart, to the ascertainment of
distances of objects so marked, or of known dimensions, from the observer. It is
based on the principle that if a transparent prism is interposed between the eye and
an object whose distance is considerable in comparison with the distance between
the eye and the prism, the angle subtended at the eye between the object as seen
directly outside the prism, and its image as seen through the prism, is practically
constant and equal to the angle of minimum deviation of the prism.
For this purpose I employ a prism of suitable angle fixed in a support which may
conveniently be held in the hand with its axis at right angles to the line of vision.
By imparting a slight motion of rotation round the axis the position of minimum
deviation is readily obtained, being the position in which the object as seen direct,
and its refracted image most nearly approach, and by rotating the prism so that its
axis moves in a plane which is at right angles or nearly so to the line of vision the
refracted image may be made to appear either above or below, or to right or left of,
or in any desired direction from the object as viewed direct.
In order to use the instrument as a station-keeper the observer must hold it as
above described so that he may see some prominent object on the ship on which he
maintains his distance (as for instance the mast head) directly and also through the
prism, using either one or both eyes, as suits his convenience. He must then note
the coincidence of the refracted image with some other point of the same ship
fixed relatively to the first, and preferably vertically below it. This being done
when the observer is at the ordered distance from the other ship, any change in his
distance will be indicated by a change of position of the refracted image, that is to
say, if it falls between the two selected points as seen directly, a decrease of distance
will be indicated, and if outside, an increase.
Dated this 27th day of February 1893.
LEWIS K. BELL,
Commander R.N.
COMPLETE SPECIFICATION.
An Improved Instrument for Determining Distances, especially
applicable as a Station Keeper for Maintaining the Ordered
Distances between Ships in a Fleet.
I, LEWIS KNOX BELL, care of Messrs. Woodhead & Co. of 44 Charing Cross
in the County of London, Commander Royal Navy, 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 relates to an instrument primarily designed for assisting to
maintain the ordered distances between ships in a fleet, but which may be used in
conjunction with marks which are fixed relatively to each other and at known
distances apart to ascertain the distances of objects so marked, or of objects of
...031

page_text: known dimensions, from the observer whether on sea or on land. It is based on the
principle that if a transparent prism be interposed in the position of minimum
deviation between the eye and an object whose distance is considerable in comparison
with the distance between the eye and the prism, the angle subtended at the eye
between the object as seen directly outside the prism, and its image as seen through
the prism, is practically constant and equal to the angle of minimum deviation of
the prism.
For this purpose I employ a prism a (as shewn in Figure 1) of suitable angle
fixed in a support which may conveniently be held in the hand with the axis
of the prism at right angles or nearly so to the line of vision as shewn in
Figure 2 which shews the instrument in vertical transverse section. By imparting
a slight motion of rotation to the prism round its axis the position of minimum
deviation is readily obtained being the position in which the object as viewed
directly outside the prism as shewn at z and its refracted image as seen through the
prism as at y most nearly approach and by rotating the prism so that its axis moves
in a plane which is at right angles or nearly so to the line of vision the refracted
image may be made to appear either above or below, or to the right or left of, or
in any desired direction from the object as viewed direct.
In order to use the instrument as a station keeper the observer must hold it as
above described and as shewn in Figure 2 so that he may see some prominent object
on the ship from which he is to maintain his distance (as for instance the main
truck) directly and also through the prism, using either one or both eyes, as suits
his convenience. He must then note the coincidence of the refracted image with
some other point of the same ship fixed relatively to the first, and preferably
vertically below it. This being done when the observer is at the ordered distance
from the other ship, any change in his distance will be indicated by a change of
position of the refracted image, that is to say, if it falls between the two selected
points as seen directly a decrease of distance will be indicated, and if outside an
increase.
For instance, if the angle of minimum deviation of the prism employed
be equal to the angle subtended by a perpendicular to the line of vision of 10 feet
at 100 yards, at that distance the refracted image of the main truck of a ship for
instance will coincide with a point on the mast 10 feet below the main truck as
viewed directly and at 200 yards 20 feet below, at 300 yards 30 feet below, and so
on; all within small and unimportant limits of error. To facilitate the ascertaining
of the distance by this means the observer may be provided with a drawing or
photograph of the ship or object viewed, the said drawing or photograph being
marked with a scale indicating various points at which the main truck or other
part selected will appear through the prism, and the distances of the object when
the image appears at these various points. In Figure 2A I have shewn such a
drawing marked with a scale.
Presume for instance that the instrument is made of two kinds for general use in
a fleet for instance the said prisms having respectively minimum angles of deviation
equal to the angles subtended by perpendiculars of 16 feet and 32 feet at one cable's
length (a distance of 200 yards is a cable's length). When using the 16 feet
instrument the refracted image will appear 16 feet below the real object at one cable
and 32 feet at two cables and so on within small limits of error.
For close quarters the 32 feet instrument should be used. In the diagram shown
in Figure 24 I have shown two scales one at each side one for the 16 feet instrument
and the other for the 32 feet instrument. I have also shown three spaces in one of
which is a drawing of part of a ship. In the other spaces (of which there may be
any number) drawings of other ships in the fleet may be made. It will be under-
stood of course that the objects drawn may be in any suitable number and they may
be objects of any suitable kind whose distances are to be measured the distances
between one point and another on the real object being known or approximately
known. Presuming that the object is as shewn a ship the horizontal lines are
drawn through points which are (taking the 16 feet and 32 feet instruments as the
ones in use) 16 feet apart on the real object. These are numbered in the example

page_text: in a scale of cables from the top line downwards-0, 1, 2, 3, &c. in the left hand scale
and 0, 1, 1, 1 &c. in the right hand scale. By observing with the instrument the
point on the mast on which the refracted image of the truck appears to fall and
referring to the corresponding point on the diagram the distance may be at once
read off on the scale of cables using in the example shown, the left hand scale for
the 16 feet instrument and the right hand scale for the 32 feet instrument.
Similarly the scale may be marked from the water line upwards in which case the
position of the water line of one image on the other image would indicate the
distance.
For long distances the instrument may be used in combination with a telescope.
For this purpose the prism may be secured in a tube as shown in sections at right
angles to each other in Figures 3 and 4 the prism being marked a and the tube b.
This tube can be readily slipped, on the end of the telescope as shown in Figure 5.
The prism may be placed inside the telescope between the eye-piece and the object
glass instead of outside if desired.
The prism need not be secured in the holder so that the object is seen over the
top of the prism. It can for instance be fixed as shewn in Figures 6 and 7 as
regards the hand instrument and in Figures 8 and 9 as regards the fitting for a
telescope so that the object is viewed directly from one side of the prism or in any
position so that it does not include the whole field and in the case of binocular
telescopes it may include the whole field of one tube.
The prism may be made adjustable in its holder so that the minimum angle of
deviation is not always employed the prism being rotated (as indicated in Figure 10)
on an axis transverse to the line of vision until the angle of deviation is increased
to the desired extent.
From the foregoing description it will be evident that the invention can be used
generally not only for taking and maintaining the ordered distances between ships
and for determining the distance of objects at sea but also for determining the
distance of objects on land.
For the purpose of obtaining small angles of deviation with greater facility than
can be done by grinding one prism accurately two prisms may be used in combination
for instance Figure 11 shows two prisms of equal angle which can be easily produced
by cutting one prism (Figure 12) in two parts along the dotted line.
If the prisms be placed together as in Figure 11 there is no deviation of a ray in
passing through but by inclining them one to the other as in Figure 13 a sinall and
adjustable angle of deviation of the ray is obtained depending on the inclination of
the prisms to each other.
This adjustability can be readily effected by mounting the prisms in the holder in
carriers so that they can be rotated and then by a pinching screw or otherwise be
retained in position.
This adjustable combination can be used in the same manner as the single
prism.
For field range finding a prism may for example be used refracting say 1 foot at
100 yards and the prism may be either a single one or be made as described and be
adjusted until the required amount of refraction is obtained and it is then fixed.
With this instrument the distance of a man from the observer is readily obtained
with approximate accuracy by means of a reference diagram as shown in Figure 2B.
Assuming a man to be 5 feet high the refracted image of the top of his head will
be seen at the various points traversed by the horizontal lines and reference to the
scale will give the distance of the man observed from the observer. Distances of
other objects such as horses or stags for instance can be similarly ascertained.
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. For the purposes described the use of a prism or combination of prisms by
which the distance of objects can be observed by the relative distances apart of the

page_text: object as seen directly and a refracted image of the object as seen through the prism
or prisms substantially as herein before described.
2. A prism or prisms for observations as claimed by the preceding claiming clause
mounted in a holder for convenience in use substantially as described and shown in
Figures 1, 2, 6 and 7 of the accompanying drawings.
3. A prism or prisms as claimed by the preceding first claiming clause combined
with a telescope as described or with an attachment for a telescope as described in
Figures 3, 4, 5, 8 and 9 of the accompanying drawings.
4. For use in combination with a prism or combination of prisms as claimed by
the preceding first claiming clause a diagram divided and provided with a scale
made to correspond with the degree of refraction of the prism or prisms or
combination or combinations of prisms substantially as hereinbefore described and
illustrated by Figures 2 and 2 of the accompanying drawings.
Dated thin 20th of A
1000
J. H. JOHNSON & Co.,
Agents.