AN IMPROVED APPARATUS FOR INDICATING THE DIRECTION OF OBJECTS AND MEASURING ANGLES
Year: 1861

page_text: LETTERS PATENT to Edward Sang, of No. 2, George Street, in the City
of Edinburgh, North Britain, Actuary and Civil Engineer, for the
Invention of "AN IMPROVED APPARATUS FOR INDICATING THE DIRECTION OF
OBJECTS AND MEASURING ANGLES."
Sealed the 10th October 1861, and dated the 26th July 1861.
PROVISIONAL SPECIFICATION left by the said Edward Sang at the
Office of the Commissioners of Patents, with his Petition, on the 26th
July 1861.
I, EDWARD SANG, of No. 2, George Street, in the City of Edinburgh,
North Britain, Actuary and Civil Engineer, do hereby declare the nature of
the said Invention for "AN IMPROVED APPARATUS FOR INDICATING THE DIRECTION
OF OBJECTS AND MEASURING ANGLES," to be as follows:-
This Invention consists in the use of a piece of glass or other transparent
substance, so constructed that when a straight line fixed in regard to the
piece of glass is directed to any object and the eye brought near to its edge
so as to look partly at the object and partly into the glass, an image of the
object will be seen as well as the object itself, the coincidence of the image
with the object being a proof that the said line is pointed accurately to that
object.

page_text: SPECIFICATION in pursuance of the conditions of the Letters Patent, filed
by the said Edward Sang in the. Great Seal Patent Office on the
27th November 1861.
TO ALL TO WHOM THESE PRESENTS SHALL COME, I, EDWARD
SANG, of No. 2, George Street, in the City of Edinburgh, North Britain,
Actuary and Civil Engineer send greeting.
WHEREAS Her most Excellent Majesty Queen Victoria, by Her Letters
Patent, bearing date the Twenty-sixth day of July, in the year of our Lord
One thousand eight hundred and sixty-one, in the twenty-fourth year of Her
reign, did, for Herself, Her heirs and successors, give and grant unto
me, the said Edward Sang, Her special license that I, the said Edward
Sang, my executors, administrators, and assigns, or such others as I, the
said Edward Sang, my executors, administrators, or assigns, should at
any time agree with, and no others, from time to time and at all times
thereafter-during-the term therein expressed, should and lawfully might
make, use, exercise, and vend, within the United Kingdom of Great
Britain and Ireland, the Channel Islands, and Isle of Man, an Invention
for AN IMPROVED APPARATUS FOR INDICATING THE DIRECTION OF OBJECTS AND
MEASURING ANGLES," upon the condition (amongst others) that I, the said
Edward Sang, my executors or administrators, by an instrument in writing
under my hand and seal, or under the band and seal of one of them, should
particularly describe and ascertain the nature of the said Invention, and in
what manner the same was to be performed, and cause the same to be
filed in the Great Seal Patent Office within six calendar months next and
immediately after the date of the said Letters Patent.
NOW KNOW YE, that I, the said Edward Sang, do hereby declare
the nature of my said Invention, and in what manner the same is to be
performed, to be particularly described and ascertained in and by the
following statement, reference being had to the accompanying Drawings,
and to the letters and figures marked thereon, that is to say:―
My said Invention consists in the use of a piece of glass or other trans-
parent substance so constructed as that when a certain straight line, fixed in
regard to the piece of glass, is directed to any object, and the eye brought near
to its edge so as to look partly at the object, and partly into the glass, an image
of the object may be seen as well as the object itself, the coincidence of the
image with the object being a proof that the said straight line is accurately
pointed to that object.
And in order that my said Invention may be fully understood I shall now
proceed more particularly to describe the same, and for that purpose I shall

page_text: refer to the several Figures on the Sheet of Drawings hereunto annexed the
same letters of reference indicating corresponding parts throughout all the
Figures.
The piece of glass above referred to is herein-after designated as the crystal
pointer, and the above-mentioned straight line, as the line of direction. The
crystal pointer is to have four surfaces ground and polished flat in such a way
that their planes if produced to meet would bound a four-faced solid or
tetrahedron as represented in Figures 1 and 3 of the Drawings, or it may'
have five surfaces so ground and polished as that their planes when produced
may bound a five-faced solid or pentahedron as shewn in Figure 2. The
general action of the instrument is as follows:-The light which comes from
an object in front of the surface B C D, Figure 1 is refracted there and
entering the glass falls partly on the surface A D B partly on A C B.
That portion of the light which falls on A, D, B, is reflected internally so as
to meet the surface A C B, and is there again reflected to meet the fourth
surface A D C, at which surface it is refracted a second time and emerges
from the glass. It is requisite that the light should emerge from the glass
free from angular dispersion, and that the image which it produces be not
distorted. For this end the reflecting surfaces A C B, A D B, may have
any convenient angle, but the relative positions of the two refracting surfaces,
for this part of the light, must be such that A C D is parallel to the image of
B C D as formed by two reflections, one in A D B the second in A C B.
In the crystal pointer represented by Figure 1 the inclination of the reflecting
surfaces AC B, A D B is 80° and the face B C D is equally inclined
to these, and the proportionate dimensions suitable for optical flint glass
of considerable refractive power, are A B=4-400; C D=1.923; A C=
2.348; B D 2.298; A D=2.693; B C=3.096. That portion of the
light which falls on A C B is thence reflected to A D B and experiences there
a second reflection. The fourth surface, in order to suit this part of the
light must be parallel to the image of B C D as reflected first in A, C, B,
and then in A, D, B, this position is indicated by the dotted lines A, c, d.
When B C D, is equally inclined to the reflecting surfaces the plane
A cd, is placed symmetrically to A D C, B c being equal to B D, and B d
to B C. The glass may be cut along either of the planes A C D, or A c. d,
or it may be cut along both, and so take the pentahedral form A B, ce D
shown in Figure 2. On looking into the surface A C D or A c d of the
glass thus shaped the images of objects in front of B. C D are seen, aud by
turning the piece of glass about, the image of any object may be brought into
the direction of the object itself, so that by bringing the eye to look partly

page_text: past the edge of the glass the object may be seen at the same time with its
image. The image then appears as if the object were turned round through
an angle which is double the angle of the two reflecting surfaces; that image
which belongs to the surface A c d being turned to the left, that which
belongs to A c d being turned to the right. If the double surface A Ce,
A De be cut as in Figure 2, and if the eye be brought over the ridge A e
both images may be seen at once and either of these may be collated with the
object. This form of the crystal pointer has thus two distinct lines of direction
and, on this account is only applicable to special purposes.
The dimensions of the form shown in Figure 2, are inclination of A c B
to A D B 80°; A B=4,400; A c=A D=2-693; B c=B D=2-298
A =2-368; ce=De=0.819 and Be=2.492; which proportions are
suitable for optical flint glass. The most advantageous arrangement is to have
the two reflecting surfaces A C B, A D B exactly perpendicular to each other
in which case the surfaces A cd, A C D, coincide in position, and the images
appear as inverted or turned round through 180°; they are thus coincident
with each other and there appears to be only one image. This form of the
crystal pointer is shown in Figure 3, in which the reflecting surfaces AC B,
A D B, are perpendicular to cach other. The relative positions of the re-
fracting surfaces are, in this case, easily obtained, it being sufficient to make
the line A C equal to B C, and AD to B D. It is most convenient, however,
to make all the four lines A C, C B, BD, DA of one length. The inclination
of the two refracting surfaces AC D, BC D, should be determined so that
the sine of its half be represented by the formula:-
sin
1+(1+82)
49.
in which 2 is the index of refraction of the substance used; and I recommend
that, in working the surfaces, the angle should exceed rather than fall short
of the value of 4 as determined by the above formula. For flint glass the
angle may be made about 126°, and the proportionate dimensions suitable for
this material as in Figure 3, are A B=4400; C D=2-000; A C=C B=
B D D A 2-615. In this form of the crystal pointer the line of direction
is a parallel to A B, drawn through the centre of gravity of the complete tetra-
hedon A B, C D. For the convenience of use a small part is bevilled off the
tetrahedron along the edge C D the surface so exposed being left dull, and the
eye is brought to look past the middle of this part when making observations.
Small portions may also be truncated from the ends at A and B and from the
thin parts near C and D, for the purpose of rendering them less fragile ; in
this
way the form shown in Figure 4, is obtained, and this is the form which

page_text: I prefer for ordinary use. So far as the principle is concerned, the size of
the crystal pointer is of no moment. It may be made as small as the difficulty
of manipulation will allow, and the chief advantage gained by making it
large is the superior precision with which the angles can be cut. For this
reason no scale of measurement has been attached to the Drawings. I
have found that when the line CD of the complete tetrahedron is from
one inch to one inch and a half, the size is convenient both in the making
and in the using, and I have also found that optical flint glass answers well
as a material of which to construct the pointer. The crystal pointer may
be set in any appropriate box or receptacle which does not permanently cover
the surfaces AC D, C B D, and the bevilled part at C D, and it is proper
to place a darkened surface at a little distance so as to intercept any extraneous
light which might be reflected into the eye by the surface -A C D, or by
BC D. The manner of applying the crystal pointer to the indication of
direction may be easily understood from a single example. If the pointer
be secured to the outside of the tube of a powerful telescope, which is turned
to some well defined distant object, and if it be then so adjusted as that the
image of that object agrees with the object itself, the line of direction of the
pointer is then parallel to the optical axis of the telescope and the pointer
may be used as a finder for bringing any other object into the field of view.
Its use in the measurement of angles may be examplified in its application
to the altitude and azimuth circle or theodolite. If the crystal pointer be
attached to the horizontal axis of a theodolite and be furnished with proper
adjusting screws in the manner well known to instrument makers, so that
its line of direction may be adjusted to be perpendicular to that axis, it will
then serve for the measurement of angles in altitude and azimuth. It affords
greater facilities than the telescope for testing the accuracy of its collimation,
because looking from the end A we can adjust its line of direction to an
object and then turning the axis half round in its place, 'we can examine the
coincidence looking from the end B, when both coincidences are exact the
line of direction is accurately perpendicular to the axis. Its compactness
and the ease with which it can be used give to the crystal pointer a great
advantage over the telescope whenever a greater degree of precision than to
half a minute of angle is not required.
I may here observe that I do not confine or restrict myself to any particular
method of mounting the said piece of glass or to any particular application for
effecting the indication of direction, or to the measurement of angles farther
than is implied in causing the image of an object as seen through the glass
to agree with the object itself.

page_text: Having now described and particularly ascertained the nature of my said
Invention, and the manner in which the same is or may be used or carried
into effect, I would observe, in conclusion, that what I consider to be novel
and original, and therefore claim as the Invention secured to me by the herein-
before in part recited Letters Patent, is, I claim as my Invention the method
of shaping a piece of glass or other transparent substance, so that light entering
it in a certain direction may undergo two internal reflections, and may emerge
from it in the same direction without angular dispersion, by which arrange-
ment the image of an object as seen through the glass may be made to agree
with the object itself.
In witness whereof, I, the said Edward Sang, have hereunto set my hand
and seal, this Twenty-first day of November, One thousand eight
hundred and sixty-one.
EDWARD SANG. (L.S.)