Improvements in Prisms for Optical Purposes.
Year: 1899

page_text: PROVISIONAL SPECIFICATION.
Improvements in Prisms for Optical Purposes.
I, HANS SCHMIDT, of The Photo Optical Co., 33, Haymarket, in the County of
London, Engineer, do hereby declare the nature of this invention to be as
follows:-
In almost all prisms now used for reflection, the faces forming the prism are
not made use of to the best advantage. For instance in the rectangular prism
with two equal catheti (Fig. 1) the light being incident upon one of them only
one face that is the hypothenusal face, is rearly made optical use of. Yet the other
two faces, those of the catheti, which in this case of reflection afford no direct
profit, require to be most accurately finished.
In case the light is incident upon the hypotheuse (Fig. 2) the effect is a more
profitable one as then two. of the faces are used for reflection. In the first case
the prism is so to say made use of to the extent of 33 per cent., in the second
case 66 per cent.
Various prisms lately used in optics for reflection, as for instance the quad-
rangular prism shown in Fig. 3 are used to the extent of only 50 per cent
advantage.
Considering the difficulties to be overcome in constructing and finishing the
faces of prisms it will be easily understood that an improvement in making
use of the different faces of the prism will be of great importance to the optician.
The prism described and shown in Fig. 4 is as to its form well known; but
with regard to the considerations mentioned before, greater advantage can be
derived from the use of this prism as well in quantity as in quality, than has
been hitherto known. This improvement is the subject of the present invention.
The prism shown in Fig. 4 is that used in the camera lucida. The only course
of rays known or practically used and which is the only one possible under the
circumstances in which the prism is at present used, is such that the ray is
vertically incident at 1 (see Fig. 4) undergoing total reflection upon face a b
meeting face be in 2 here again being totally reflected and emerging ver-
tically to e d.
Thus only two of the four faces of the prism are used with advantage, namely ab
and be, or figuratively speaking the advantage represents 50 per cent. The two
reflecting faces a band be being much inclined towards the direction of the ray, the
prism affords no great optical effect, as the field of view is under these circum-
stances very small.
By means of providing for a different course of rays and accordingly adjusting
the reflecting faces to it, a form of prism will be obtained similar to that shown
in Fig. 4 but the prism thus obtained is distinguished from the former by its
faces as well as by its qualitative and quantitative optical effect.
The prism as used in the camera lucida consists of four faces inclined to one
another at angles of 90° and 672° respectively. Two of its faces afford total
reflection, whereas the other two serve only for incidence and emergence of the
ray without being of any advantage in the reflection.

page_text: The prism shown in Fig. 5 being the subject of the invention consists also of
four faces with angles of inclination of 90° and 672° respectively. Two of the
faces (da and d c) possess no special characteristics, whereas the other two and
herein consists the great difference from the prism used in the camera lucida are
plated like a mirror, a circumstance which affords a much more rational course
for the rays in the prism. The ray is vertically incident at 1 (see Fig. 5) meets
face be at 2 under a rather small angle 22). In the prism as used in the
camera lucida the ray would emerge here but in the present instance it is
reflected by means of the plated face at an angle of 45°, arrives at 3 where it is
totally reflected by the unplated face c d on account of its inclined incidence,
meets face da which at first afforded no advantage having served only for
incidence of the ray, is totally reflected at 4 and arrives at 5 of the plated face ba
by which it is reflected at the angle of 45° and emerges vertically to face c d
which had already once before an optical effect upon the ray. Thus all faces of
the prism are used, and the advantage obtained represents 100 %. Besides this
the position of all reflecting faces to the ray is such a favourable one as to afford
a field of view much larger here than it can possibly be in the case of the prism
as used in the camera lucida.
The prism possesses the quality of reflecting the ray rectangularly without
causing any reversion of the image, vertically or laterally. In case of vertical
reversion being required one of the two faces (ab and b c) may be substituted by a
dihedron (that is two faces standing one to the other rectangularly). The sub-
stitution of one of the two faces (a bor be) by the dihedron has the natural
consequence that then only one face of the prism requires to be plated as the
dihedron always causes total reflection of any ray incident upon it.
If the reflection is required to produce a laterally reversed image, this single
prism can be combined with any other prism affording lateral reversion as does
for instance a rectangular triangular prism upon whose catheti the ray is incident.
It is certain that by proper combination, vertical and lateral reversion of the
image can be obtained at one time an example for which is given by the com-
bination of prisms shown in Fig. 6. Prism I is that in question combined with
the dihedron at the face b c. The effect of it is vertical reversion and reflection
of the ray under 90°. The effect of prism II is lateral reversion and again
reflection under 90°. The total effect resulting from the combination of prism I
and II is therefore, vertical and lateral reversion and parallel shifting of the ray.
Fig. shows another form of prism which is so made that all its surfaces
are operative in reflection. In plan it is an isosceles triangle having two equal
sides a c, a bat an angle of 45 degrees, and a base cb forming an angle of 67
degrees to each side a c, a b. The face bis either a mirror or a dihedron.
The ray is incident at 1 perpendicularly to face ( is reflected at 90° degrees
at 2 from face a b is reflected at 3 at 45 degrees from face cb (which being a
mirror or a dihedron gives total reflection) is again reflected at 4 at an angle of
90 degrees from face ac and emerges at 5 in face b.
The path of emergence is at 45 degrees to the path of incidence, and if a mirror
is used at be the image is laterally reversed while if a dihedron is used there,
vertical as well as lateral reversion occurs.
By combination of this prism with another prism of suitable kind there may
be obtained vertical or lateral reversion and angularity or parallelism between
the incident and emergent rays as desired.
As example of such a combination Figs. 8 and 9 shows a second prism d f g
in which the face d f is either a mirror or a dihedron.
These prisms may be used alone or in combination with any others or with
lenses for the various purposes of optics and photography.
Dated this 2nd day of June 1899.
HERBERT HADDAN & Co.,
Agents to Applicant.

page_text: I, HANS SCHMIDT, of 33, Haymarket, London, Engineer, 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 prisms used for optical-purposes.
In almost all prisms now used for reflection, the faces forming the prism are not
made use of to the best advantage. For instance in the rectangular prism with
two equal catheti the light being incident upon one of them, only one face, that
is, the hypothenusal face is made optical use of. Yet the other two faces, those
of the catheti, which in this case of reflection afford no direct profit, require to
be most accurately finished. In case the light is incident upon the hypothenuse
the effect is a more profitable one as then two of the faces are used for reflection.
In the first case the prism is so to say made use of to the extent of 33 per cent.
in the second case 66 per cent.
Various prisms lately used in optics for reflection as for instance the quad-
rangular prism hereinafter referred to are used to the extent of only 50 per cent,
advantage.
Considering the difficulties to be overcome in constructing and finishing the
faces of prisms it will be easily understood that an improvement in making use
of the different faces of the prism will be of great importance to the optician.
In the annexed drawing
Fig. 1 illustrates a quadrangular prism of a known form and the path of the
light rays as such prism is generally used.
Fig. 2 shows the same prism as improved by this invention and the path of
the light rays.
Fig. 3 illustrates the same with a dihedral face for vertical reversion.
Fig. 4 illustrates the improved prism in combination with an ordinary triangu-
lar prism for causing parallelism of incident and emergent rays.
Fig. 5 illustrates a modified form of the improved prism.
Fig. 6 shows the same with a dihedral surface.
Figs. 7 and 8 illustrate combinations for causing parallelism of incident and
emergent rays with the modified form of prism.
The prism described and shown in Fig. 1 is as to its form well known; but
with regard to the considerations mentioned before, greater advantage can be
derived from the use of this prism, as well in quantity as in quality, than has
hitherto known. This improvement is the subject of the present invention.
The prism shown in Fig. 1 is that used in the camera lucida. The course of
rays known or practically used and which is the only one possible under the cir-
cumstances in which the prism is at present, is such that the ray is vertically
incident at 1 (see Fig. 1) undergoing total reflection upon face a bat 2, meeting
face be at 3, here again being totally reflected and emerging vertically to c d
at 4.
Thus only two of the four faces of the prism are used with advantage namely a b
and be, or figuratively speaking the advantage represents 50%. The two
reflecting faces a band be being much inclined towards the direction of the
ray the prism affords no great optical effect as the field of view is under these
circumstances very small.
By means of providing for a different course of rays and accordingly adjusting
the reflecting faces to it, a form of prism will be obtained similar to that shown

page_text: in Fig. 1 but the prism thus obtained is distinguished from the former by its
faces as well as by its qualitative and quantitative optical effect.
The prism as used in the camera lucida consists of four faces a b, bv, c d, d a.
Of these cd, da are inclined to one another at an angle of 90° and d a, a b,
and d 0, cb are inclined to one another at an angle of 672 ° respectively, and eb
is inclined to ba at an angle therefore of 135°. Two of its faces a b, b c afford
total reflection whereas the other two cd, da serve only for incidence and emer-
gence of the ray without being of any advantage in the reflection.
The prism shown in Fig. 2 being the subject of the invention consists also of
four faces with angles of inclination of 90° and 672° respectively. Two of the
faces (and d) possess no special characteristics whereas the other two ab, bc
(and herein consists the great difference from the prism used in the camera
lucida) are plated like a mirror, a circumstance which affords a much more
rational course for the rays in the prism. In the drawing this mirror coating is
indicated by a cross hatched space of appreciable thickness but it will be under-
stood that the thickness is exaggerated. The ray is vertically incident at 1 (see
Fig. 2) meets the face bc at 2 under such an angle that in the prism as used in
the camera lucida the ray would emerge here but in the present instance it is
reflected by means of the plated face which it meets at an angle of 672° arrives
at 3 where it is totally reflected by the unplated face c d on account of its inclined
incidence, meets face da which at first afforded no advantage having served only
for incidence of the ray, is totally reflected at 4 and arrives at 5 on the plated
face ba to which it makes an angle of 67 degrees and by which it is reflected
and emerges vertically to face c d which has already once before had an optical
effect upon the ray. Thus all faces of the prism are used reflectively and the
advantage obtained represents 100%. Besides this the position of all reflecting
faces to the ray is such a favorable one as to afford a field of view much larger
here than it can possibly be in the case of the prism as used in the camera lucida.
The prism possesses the quality of reflecting the ray rectangularly without
causing any reversion of the image, vertically or laterally. In case of vertical
reversion being required one of the two faces (a b and b c) may be replaced by a
dihedron (that is, two faces standing one to the other rectangularly). The sub-
stitution of one of the two faces (a 6 or 6 c) by the dihedron has the natural con-
sequence that then only one face of the prism requires to be plated as the
dihedron always causes total reflection of any ray incident upon it.
If the reflection is required to produce a laterally reversed image, this single
prism can be combined with any other prism affording lateral reversion as does
for instance a rectangular triangular prism B (Fig. 4) upon whose catheti the ray
is incident.
It is certain that by proper combination, vertical and lateral reversion of the
image can be obtained at one time.
Figs. 5 and 6 show another form of prism which is so made that all its surfaces
are operative in reflection. In plan it is an isosceles triangle, having two equal
sides a cab, at an angle of 45 degrees and a base c b forming an angle of 67%
degrees to cach side a cab. The face a bis either a mirror (Fig. 5) of a dihedron
(Fig. 6).
The ray is incident at 1 perpendicularly to face ac falls on face a bat an
angle of 45°, is reflected at 2 from face a 6 falls on face c b at an angle of 672°,
is reflected at 3 from face cb (which being a mirror or a dihedron gives total
reflection) strikes face at an angle of 450, is again reflected at 4 from face a c
and emerges at 5 in face a b.
The path of emergence is at 45° to the path of incidence and if a mirror is
used at the image is laterally reversed while if a dihedron is used there,
vertical as well as lateral reversion occurs.
By combination of this prism with any other prism of suitable kind there may
be obtained vertical or lateral reversion and angularity or parallelism between
the incident and emergent rays as desired.

page_text: As examples of such a combination Figs. 7 and 8 show a second prisni d f g
in which the face d f is either a mirror or a dihedron. The angles f g =
gf
=
45, and d f g = 112 degrees respectively.
22½
These prisms may be used alone of in combination with any others or with
lenses for the various purposes of optics and photography.
Having now particularly described and ascertained the nature of the said
invention and in what manner the same is to be performed, I declare that what I
claim is:-
1. A prism adapted to receive an incident ray and to reflect the same internally
from each of its faces before the emergence of the ray.
2. A prism having non-coated faces adapted to receive an incident ray, and
permit emergence of the emergent ray respectively and having mirror coated
surfaces to internally reflect the said ray on those of the remaining faces where
the ray would otherwise emerge.
.
3. A prism having a clear incident face, a clear emergent face and a mirror
coated or dihedral reflecting face or faces so arranged to one another that the
ray is reflected from all the faces before emergence.
4. A prism having all its faces adapted to act as internal reflecting surfaces,
those of said surfaces on which the ray intended for reflection falls at such angle
that it would emerge being mirror coated.
5. A prism having two clear faces a d, dc and two mirror coated or dihedral
faces ab, bc, the latter lying at an angle of 135° to one another and adapted to
reflect a ray from each of the surfaces as described.
6. A prism having two clear faces a b, a c; at an angle of 45° to one another and
a mirror coated or dihedral face bc at an angle of 67 2° to the faces a bac
respectively.
7. A combination of prisms consisting of a prism having two clear faces a b, a c,
at an angle of 45° to one another and a mirror coated or dihedral face b c at an
angle of 67° to the faces a b, a c respectively and a prism having two clear
faces dy, fg and mirror coated or dihedral face d f substantially as described.
Dated this 1st day of March 1000
HERBERT HADDAN & Co.,
Agents to Applicant,
18, Buckingham Street, Strand, W.C.