Improvements in Optical Instruments.
Year: 1897

page_text: Improvements in Optical Instruments.
I, HENRI LOUIS HUET, of 114, Rue du Temple, Paris, France, Optician, do
hereby declare the nature of this invention to be as follows:-
This invention relates to an optical prism or block applicable to optical instru-
ments generally. The effect of the prism, whose faces are so disposed as to produce
three successive reflections of the light rays, is to reverse the image with or with-
out lateral displacement of the latter according as the sum of the three angles..
at which the ray is reflected is equal to 180° or according as the difference
between the sum of the first and last of these angles on the one hand and the
intermediate angle on the other hand is equal to two right angles. The property
of reversing the image permits in particular of replacing, in terrestrial telescopes.
producing erect images, the terrestrial eye-piece, which has the objection of
absorbing the light and greatly increasing the length of the telescope, by an
astronomical eye-piece which is highly luminous and very short but cannot be
used alone for terrestrial observations owing to the reversal of the image. Owing
to the flat form of the prism the instrument is not cumbersome and may be carrier
in the pocket.
By the optical prism of this invention the images may be reversed either in one
direction so as to be symmetrical with regard to a right line parallel to their
plane, or in two directions perpendicular to each other so as to be symmetrical
with regard to a right line perpendicular to their plane, the reversal of the image
being in both cases equivalent to a rotation of 180° about the symmetrical
axis.
I
My invention is illustrated in the accompanying drawings in which
Figure 1 represents a vertical section of a prism for reversing the
image in one direction without lateral displacement of the latter. Figure 2 is
a modification with different values of angles of reflection. Figure 3 is a per-
spective view of another form of prism for producing a reversal of the image in
two directions perpendicular to each other. Figure 4 is a perspective view and
Figure 5 a plan of a prism for reversing the images in one direction with lateral
displacement. Figure 6 is a perspective view of the same prism modified so as
to produce a reversal of the image in two directions perpendicular to each other
in each case with lateral displacement. Figures 7 and 8 represent a perspective
view and plan respectively of a prism by which the image is reversed in one
direction with displacement. Figure 9 shews a modified form of Figures 7 and 8
for effecting the reversal of the image in both directions. Figure 10 is a plan of
another modification. Figure 11 is a modification shewing the prism divided
into two parts for increasing the lateral displacement of the image. Figure 12
is a plan of a special means applied to the optical prism for preventing any
reflecting surface around the pencil of rays utilized. Figures 13 and 14 shew
diagrammatically the arrangement of one of the two barrels of a binocular glass
for direct vision, utilizing one or other form of the prism.

page_text: The reversal of the image in one direction by means of the prism is produced
by three successive reflections from faces so inclined as to impart to a ray perpen-
dicular to the face of the prism at which it enters, three deviations. When the
difference between the sum of the first and of the last of these angles of reflection
and the intermediate angle is equal to two right angles the image is reversed
without any lateral displacement. In this case the prism is symmetrical and
the intermediate reflecting face is at a suitable distance from the intersection
of the other reflecting faces, a distance which is easily calculated in each case
knowing the aperture of the luminous film which is to enter and leave the prism
and the trigonometrical lines of the angles chosen.
In the case of Figure 1 the angles of reflection are respectively equal to 120°,
60°, 120°. The intermediate angle being less than 90° the face BC should be
silvered otherwise there would not be total reflection. On the other hand, in the
case of Figure 2, where the angles of reflection are respectively equal to 135°,
90°, 135°, the intermediate angle being equal to 90° it is not necessary to silver
the face B C. In order to produce the reversal of the image in a second direction
perpendicular to the first, it is only necessary to replace the plane BC by a
dihedron of 90° whose arris M N is perpendicular to the intersection of the
faces A E, E D, as in Figure 2. When the sum of the three angles of reflection.
whereby the ray is deflected, is equal to 180° the image is reversed and displaced
in the lateral direction; in this case the prism is particularly intended to be
applied to stereoscopic instruments such as photographic or direct vision binoculars
in which the displacement of the image may be utilized for increasing the distance
apart of the objectives relatively to that of the eye-pieces and thereby to produce
in accordance with known principles a stereoscopic effect.
The deviations may be obtained under any angles but preferably the faces of the
prism will be so arranged as to obtain two deviations of 45° and one of 90°.
As with these angles of deviation two combinations may be made, according to
-the order in which they are placed, it follows that the prism may be arranged in two
ways. Figures 4 and 5 shew in perspective and horizontal section the form of
the prism corresponding to the following order of the deviations, i.e. 45°, 90°, 45°.
The prism is terminated by the ingress and egress faces S¹ T and P Q, PI Q¹
perpendicular to the direction of the light and presenting three reflecting
faces M N, M¹ N¹ M T, M¹ T¹ and R S, R¹ S¹; the inclination of the first and
last of these reflecting faces to the direction of the light is 112°30' whilst the
intermediate face is parallel to it. Figure 5 shews the course of the luminous
rays and demonstrates that the reversal of the image is produced, the position
of the rays A a a a A¹ and Bbbb B¹ being the reverse at the egress to what it
was at the ingress relatively to the median ray Oooo 0¹. The faces M N, M¹ N¹
and R S, R¹ S¹ which correspond to a deviation of 45° should be silvered or tinned
otherwise total reflection would not be produced; this precaution is not required
for the faces M T, M T if the field is not to exceed about 10°.
Figures 7 and 8 shew a prism in which the order of the deviation of the ray
is as follows:-90°, 45°, and 45°, or inversely. The ingress faces G L, G¹´L'
and the egress faces H I, H¹ I' are perpendicular to the direction of the light;
the first (or the last) reflecting face G H, G¹ H¹ are inclined at 45° to the direction
of the light and the two other faces K L, K' L' and I J, I¹ J¹ are inclined
at 112°30'.
Figure 8 shews as in the preceding case the mode of producing a reversal of
the image. For the reasons previously given, the faces I J, I J¹ and K L, K¹ L'
should be silvered or tinned.
In the two forms shewn in Figures 4 5, and 7 8, the images are displaced
laterally to a certain extent. With the second form this displacement may be
increased at will by dividing the prism into two on the plane H L, H' L' and
separating the two portions as shewn in Figure 11. With one or other of these
arrangements it is only necessary in order to produce the reversal in the second

page_text: direction perpendicular to the first, to replace any one of the three planes of
reflection by a dihedron of 90° whose arris is parallel to the direction of that
plane. In this case it is not necessary to silver or tin the faces of the dihedron,
at least unless a very considerable field is required. Figure 6 shews this form
applied to the plane R¹ S¹ of the first combination and Figure 9 shews the
same form applied to the plane K L, K' L' of the second combination. These
figures also shew that a ray C c c c contained in a horizontal plane at the upper
part of the prism is thrown back by the dihedron in the direction of C¹ in a
horizontal plane of the lower part and consequently that the reversal is produced
in the vertical direction as well as in the horizontal direction.
The values of the angles of reflection which have been specified are merely
given as examples because they appear to be the most suitable as regards the
form and construction of the prism and because only one face requires to be
silvered. But they may be modified according to circumstances; for instance
the reversal of the image may be obtained by three angles of reflection which
are equal to 60°, as shewn in Figure 10, but in this case all the faces require to
be silvered.
In order to avoid the formation around the pencil of rays utilized, of reflecting
surfaces which veil the images by false lights and even produce other images,
which are superposed upon the first, all the faces or portions of faces which are
not used for the transmission of the pencil of useful rays are incised with saw-
cuts of suitable depth and the grooves thus formed are filled up with black
varnish or other substance capable of completely arresting the light, as shewn.
in Figure 12.
The invention thus enables the reversal of the image to be obtained in optical
instruments generally whenever required, and at the same time the displacement
of the optical axis to a greater or less extent. In particular this system of optical
prism is applicable to stereoscopic instruments, enabling in conformity with well.
known principles a considerable relief to be given to the images by the separation
of the objectives a distance apart greater than that of the eye-pieces (photographic
and direct vision binoculars).
Figures 13 and 14 shew diagrammatically one of the two body portions of a
binocular in which one or other of the two combinations above described is
employed.
After having traversed the objective and the prism combination which may
be placed in any order, the luminous rays from the object produce an erect image-
since the objective on the one hand and the prism on the other each produces a
reversal. If this image be viewed with an astronomical eye-piece 01 01 the-
objects will be seen placed as when viewed with the naked eye. As regards the..
difference of distance apart of the objectives and of the eye-pieces, it is evident.
that it may be much greater by employing the second combination, as shewn in
Figure 14, than when using the first, as shewn in Figure 13, and so with the relief
effect. On the other hand the first arrangement has the advantage over the second
of greatly diminishing the dimensions of the telescope for a given focal value of
the objective.
It will be evident that in each case for prisms with plane ingress and egress
faces, lens prisms may be substituted either by giving these faces suitable
curvatures or by mounting thereon ordinary lenses so as to form prism objectives.
It will be understood that this system of optical prism may be made of any
required dimensions and may be mounted in any suitable way and be applied
to any optical instrument in which displacement of the image may be utilized
to increase the distance apart of the objectives relatively to that of the eye-pieces,
Doted this Oth day of June 1897
A. M. & WM. CLARK,
Chartered Patent Agents, 53, Chancery Lane, London.

page_text: I, HENRI LOUIS HUET, of 114, Rue du Temple, Paris, France, 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:-
This invention relates to an optical prism or block applicable to optical instru-
ments generally. The effect of the prism whose faces are so disposed as to produce
three successive reflections of the light rays, is to reverse the image with or with-
out lateral displacement of the latter according as the sum of the three angles
at which the ray is reflected is equal to 180° or according as the difference
between the sum of the first and last of these angles on the one hand and the
intermediate angle on the other hand is equal to two right angles. The property
of reversing the image permits in particular of replacing in terrestrial telescopes
producing erect images, the terrestrial eye-piece which has the objection of
absorbing the light and greatly increasing the length of the telescope, by an
astronomical eye-piece which is highly luminous and very short but cannot be
used alone for terrestrial observations owing to the reversal of the image. Owing
to the flat form of the prism the instrument is not cumbersome and may be carried
in the pocket.
By the optical prism of this invention the images may be reversed either in one
direction so as to be symmetrical with regard to a right line parallel to their
plane, or in two directions perpendicular to each other so as to be symmetrical
with regard to a right line perpendicular to their plane, the reversal of the image
being in both cases equivalent to a rotation of 180° about the symmetrical
axis.
My invention is illustrated in the drawings accompanying my Provisional
Specification in which Figure 1 represents a vertical section of a prism for reversing
the image in one direction without lateral displacement of the latter. Figure 2 is
a modification with different values of angles of reflection. Figure 3 is a per-
spective view of another form of prism for producing a reversal of the image in
two directions perpendicular to each other. Figure 4 is a perspective view, and
Figure 5 a plan, of a prism for reversing the image in one direction with lateral
displacement. Figure 6 is a perspective view of the same prism modified so as
to produce a reversal of the image in two directions perpendicular to each other
in each case with lateral displacement. Figures 7 and 8 represent a perspective
view and plan respectively of a prism by which the image is reversed in one
direction with displacement. Figure 9 shews a modified form of Figures 7 and 8
for effecting the reversal of the image in both directions. Figure 10 is a plan of
another modification. Figure 11 is a modification shewing the prism divided
into two parts for increasing the lateral displacement of the image. Figure 12
is a plan of a special means applied to the optical prism for preventing any
reflecting surface around the pencil of rays utilized. Figures 13 and 14 shew
diagrammatically the arrangement of one of the two barrels of a binocular glass
for direct vision, utilizing one or other form of the prism.
The reversal of the image in one direction by means of the prism is produced
by three successive reflections from faces so inclined as to impart to a ray perpen-
dicular to the face of the prism at which it enters, three deviations. When the
difference between the sum of the first and of the last of these angles of reflection
and the intermediate angle is equal to two right angles the image is reversed
without any lateral displacement. In this case the prism is symmetrical and

page_text: the intermediate reflecting face is at a suitable distance from the intersection
of the other reflecting faces, a distance which is easily calculated in each case
knowing the aperture of the luminous film which is to enter and leave the prism
and the trigonometrical lines of the angles chosen.
In the case of Figure 1 the angles of reflection are respectively equal to 120°,
60°, 120°. The intermediate angle being less than 90° the face B C should be
silvered otherwise there would not be total reflection. On the other hand, in the
case of Figure 2, where the angles of reflection are respectively equal to 135°,
90°, 135°, the intermediate angle being equal to 90° it is not necessary to silver
the face B C. In order to produce the reversal of the image in a second direction
perpendicular to the first, it is only necessary to replace the plane B C by a
dihedron of 90° whose arris MN is perpendicular to the intersection of the
faces AE, E D, as in Figure 3. When the sum of the three angles of reflection
whereby the ray is deflected, is equal to 180° the image is reversed and displaced
in the lateral direction; in this case the prism is particularly intended to be
applied to stereoscopic instruments, such as photographic or direct vision binoculars,
in which the displacement of the image may be utilized for increasing the distance
apart of the objectives relatively to that of the eye-pieces and thereby to produce
in accordance with known principles a stereoscopic effect. The deviations
may be obtained under any angles but preferably the faces of the prism will be
so arranged as to obtain two deviations of 45° and one of 90°. As with these
three angles of deviation two combinations may be made, according to the order
in which they are placed, it follows that the prism may be arranged in two ways.
Figures 4 and 5 shew in perspective and horizontal section the form of the prism
corresponding to the following order of the deviations, i.e. 45°, 90°, 45°.
The prism is terminated by the ingress and egress faces S T, S¹ T¹ and PQ,
P Q perpendicular to the direction of the light and presenting three reflecting
faces M N, M¹ N¹, M T, M¹ T and R S, R¹ S¹; the inclination of the first and
last of these reflecting faces to the direction of the light is 112°30' whilst the
intermediate face is parallel to it. Figure 5 shews the course of the luminous
rays and demonstrates that the reversal of the image is produced, the position
of the rays A a a a A and B bbb B¹ being the reverse at the egress to what it
was at the ingress relatively to the median ray Oooo O¹. The faces M N, M! N'
and R S, R¹ S¹ which correspond to a deviation of 45° should be silvered or tinned
otherwise total reflection would not be produced; this precaution is not required
for the faces M T, M¹ T¹ if the field is not to exceed 10°.
Figures 7 and 8 shew a prism in which the order of the deviation of the ray
is as follows:-90°, 45° and 45°, or inversely. The ingress faces G L, G¹ L₁
and the egress faces H I, H¹ I are perpendicular to the direction of the light;
the first (or the last) reflecting face GH, G₁ H₁ is inclined at 45° to the direction
of the light and the two other faces K L, K¹ L' and I J, I J¹ are inclined
at 112°30'.
Figure 8 shews as in the preceding case the mode of producing a reversal of
the image. For the reasons previously given, the faces I J, I' J and K L, K¹ L'
should be silvered or tinned.
In the two forms shewn in Figures 45, and 78, the images are displaced
laterally to a certain extent. With the second form this displacement may be
increased at will by dividing the prism into two on the plane H L, H¹ L¹ and
separating the two portions as shewn in Figure 11. With one or other of these
arrangements it is only necessary, in order to produce the reversal in the second
direction perpendicular to the first, to replace any one of the three planes of
reflection by a dihedron of 90° whose arris is parallel to the direction of that
plane. In this case it is not necessary to silver or tin the faces of the dihedron,
at least unless a very considerable field is required. Figure 6 shews this form
applied to the plane RS, R¹ S₁ of the first combination and Figure 9 shews the
same form applied to the plane KL, K' L' of the second combination. These

page_text: figures also shew that a ray C c c c contained in a horizontal plane at the upper
part of the prism is thrown back by the dihedron in the direction of c C in a
horizontal plane of the lower part and consequently that the reversal is produced
in the vertical direction as well as in the horizontal direction.
The values of the angles of reflection which have been specified are merely
given as, examples because they appear to be the most suitable as regards the
form and construction of the prism and because only one face requires to be
silvered, but they may be modified according to circumstances; for instance
the reversal of the image may be obtained by three angles of reflection which
are equal to 60°, as shewn in Figure 10, but in this case all the faces require to
be silvered..
In order to avoid the formation around the pencil of rays utilized, of reflecting
surfaces which veil the images by false lights and even produce other images
which are superposed upon the first, all the faces or portions of faces which are
not used for. the transmission of the pencil of useful rays are incised with saw-
cuts of suitable depth and the grooves thus formed are filled up with black
varnish. or other substance capable of completely arresting the light, as shewn
in Figure 12.
The invention thus enables the reversal of the image to be obtained in optical
instruments generally whenever required and at the same time the displacement
of the optical axis to a greater or less extent. In particular this system of optical
prism is applicable to stereoscopic instruments, enabling in conformity with well
known principles a considerable relief to be given to the images by the separation
of the objectives a distance apart greater than that of the eye-pieces (photographic
and direct vision binoculars).
Figures 13 and 14 shew diagrammatically one of the two body portions of a
binocular. in which one or other of the two combinations above described is
employed.
After having traversed the objective and the prism combination which may
be placed in any order, the luminous rays from the object produce an erect image
since the objective on the one hand and the prism on the other each produces a
reversal. If this image be viewed with an astronomical eye-piece OO" the
objects will be seen placed as when viewed with the naked eye. As regards the
difference of distance apart of the objectives and of the eye-pieces, it is evident
that it may be much greater by employing the second combination, as shewn in
Figure 14, than when using the first, as shewn in Figure 13, and so with the relief
effect. On the other hand the first arrangement has the advantage over the second
of greatly diminishing the dimensions of the telescope for a given focal value of
the objective.
It will be evident that in each case for prisms with plane ingress and egress
faces, lens prisms may be substituted either by giving these faces suitable
curvatures or by mounting thereon ordinary lenses so as to form prism objectives.
It will be understood that this system of optical prism may be made of any
required dimensions and may be mounted in any suitable way and be applied
to any optical instrument in which displacement of the image may be utilized
to increase the distance apart of the objectives relatively to that of the eye-pieces.
Having now particularly described and ascertained the nature of the said inven-
tion and in what manner the same is to be performed I declare that what I
claim is:
1. An optical prism or block for producing the reversal of the image, having its
faces so disposed in relation to each other as to cause three successive reflections
of the light rays as specified.
2. An optical prism or block of the kind specified, whereby the reversal of the
image is effected without lateral displacement thereof and in which the difference

page_text: between the sum of the first and last of the angles of reflection and the inter-
mediate angle is equal to two right angles as specified.
3. An optical prism or block of the kind specified, whereby the image is
reversed and displaced laterally and in which the sum of the three angles of
reflection is equal to two right angles as specified.
4. The modified form of optical prism or block of the kind specified, whereby
the reversal of the image is effected in two directions perpendicular to each other,
with or without lateral displacement and in which any one of the planes of
reflection is replaced by a dihedron of 90° whose arris is parallel to the said plane
as specified.
5. An optical prism or block of the kind specified, having the unused portions
of the reflecting faces incised with saw-cuts or grooves filled with black varnish
or other substance capable of arresting the useless light rays as specified.
6. The application of optical prisms or blocks such as herein specified, to
optical instruments generally and in particular to binocular stereoscopic or direct
vision instruments in such manner that by virtue of the lateral displacement of
the images the distance apart of the objectives may be greater than that of the
eye-pieces so as to produce an increased stereoscopic effect substantially as herein
specified.
Dated this 28th day of Wal 1800
A. M. & WM. CLARK,
Chartered Patent Agents, 53, Chancery Lane, London.