CERTAIN APPLICATIONS OF TOTAL OR PARTIAL REFLECTION OF LIGHT ON TRANSPARENT SURFACES, EITHER ALONE OR COMBINED WITH THE REFRACTION
Year: 1854

page_text: LETTERS PATENT to Ignace Porro, of Paris, and of 4, South Street, Finsbury,
London, late Colonel of Engineers in the Kingdom of Sardinia, Director
of the Optical and Technomatical Institute of Paris, for the Invention of
"CERTAIN APPLICATIONS OF TOTAL OR PARTIAL REFLECTION OF LIGHT ON
TRANSPARENT SURFACES, EITHER ALONE OR COMBINED WITH THE REFRACTION."
Sealed the 17th April 1855, and dated the 9th November 1854:
PROVISIONAL SPECIFICATION left by the said Ignace Porro at the Office
of the Commissioners of Patents, with his Petition, on the 9th November
1854.
IGNACE PORRO, of Paris, and of 4, South Street, Finsbury, London, late
Colonel of Engineers in the Kingdom of Sardinia, Director of the Optical
and Technomatical Institute of Paris, do hereby declare the nature of my
Invention of "CERTAIN APPLICATIONS OF TOTAL OR PARTIAL REFLECTION OF LIGHT
ON TRANSPARENT SURFACES, EITHER ALONE OR COMBINED WITH THE REFRACTION,"
to be as follows:-
As an application of the first part of my Invention, for reversion of images,
I make use of a piece of glass, composed of one or more parts, terminating.
with several planes and polished faces, in which the light enters and egresses
through plane and parallel surfaces under the same angles; and during its
passage into the interior it is subjected to four complete reflections, which are

page_text: alternate at right angles. The images seen through that glass are reversed,
the top being underneath and colorless.
It will be, therefore, sufficient to interpose between the eye and the object
glasses of an astronomical. telescope, a piece of glass, prepared as herein
described, to obtain a land telescope redressing the objects.
As an application of my Invention for magnifying objects, I obtain a perfect
achromatic, with great clearness, by combining together three prisms in the
following order The first is an, achromatic prism with a slightly curved
surface; it is rectangular, having its cathetus plan horizontal, and its hypo-
tenuse produces a comple reflection. The second prism is a simple rectangular
reversing prism; it is placed under the first at a variable distance, and is set
differently, according to the necessity of producing a direct or inverted sight,
receving only the light from the object when it has passed through the first
prism, and sending it to the third prism by means of the reversions, which is
the result of the double and complete reflection. The third prism is similar
to the first one, and is besides much more curved; it is placed by its side at
the same èlevation, but in an inverted position. By applying the eye to such
a system, the objects are seen far off, and magnified either reversed or upright,
according to the mode of setting up the second prism, exactly as in good
telescopes. The compass can be increased by adding a small lens. With
four prisms instead of three, all of them being achromatic, and placed so as to
operate on the light two by two in a reciprocal order, I obtain a double opera
glass, reduced to very small dimensions, having a vast compass, which increases
as well as the magnifying power by adding two small lenses. As an application
of the effect of direction and collineation, I make use of a simple trapezoid
prism, having three polished faces; that prism shews to the eye placed in the
extension of its largest face, the distant objects placed on the other side in the
same extension, and produces their inverted image. The points of. the object
which are common for the direct and reversed visions are evidently geome-
trical points, representing the precise line of extension of the reflecting plane.
I have applied that property to all the topographical measurement and
levelling instruments, by employing such mode of vision to the pinules,. and
even to the small glasses of some instruments.
As another application of the first part of my Invention to the decompo-
sition of movements, I use the same trapezoid prism. As an application of
the first part of my Invention, for the immobilisation, or fixing in images one
upon the other, I have constructed a small instrument, based on the princi-
ples herein-before described, and consisting in, a parallelopiped of glass, having
its two extremities- duly prepared, the property of which is, to fix for the

page_text: observer who looks through it, by means of the reversing of two right angles,"
the images of the objects set on the back of those in front, so that all the:
coincidences take place between the objects, the squaring of which passes
through the eye of the observer.
By combining two pantagonal prisms, producing by two. complete obtuse
reflections half a reversion, and having their reflecting faces turned on the
same side, I construct an instrument by which I measure micrometrically any
inaccessible distance at one vision, and without the dimensions of the objects
being known:
As an example of the application of the second part of my Invention, by.
which several movements of light are employed to prove the relative position
of different pieces between them and fixed lines, produced by means of the
combinations of artificial light and of partial reflection upon the transparent
surfaces, I use a very thin and small trapezoid prism to introduce laterally
some artificial light into the micrometer of the glass. This has the effect of
causing to appear directly along the optical axis some images of illuminated
micrometer, into which images are even accurately delineated.
A transparent piece of glass, and in some cases a fixed plane miror, which.
is added to the above system, serves to determine the relative position of the
visual with a fixed line. The construction of the object glasses are submitted.
to different stages of curving, according to the effects required for each especial
purpose.
SPECIFICATION in pursuance of the conditions of the Letters Patent, filed"
by the said Ignace Porro in the Great Seal Patent Office on the
9th May 1855.
TO. ALL TO WHOM THESE PRESENTS SHALL COME, I, IGNACE
PORRO, of Paris, and of 4, South Street, Finsbury, London, late Colonel of
Engineers in the Kingdom of Sardinia, Director of the Optical and Techno-
matical Institute of Paris, send greeting.
WHEREAS Her most Excellent Majesty Queen Victoria, by Her Letters
Patent, bearing date the Ninth day of November, in the year of our Lord
One thousand eight hundred and fifty-four, in the eighteenth year of Her;
reigu, did, for Herself, Her leirs and successors, give and grant unto me,
the said Ignace Porro, Her special license that I, the said Ignace Porro,
my executors, administrators, or assigns, or such others as I, the said Ignace
Porro, my executors, administrators, and assigns, should at any time agree
.

page_text: 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 of "CERTAIN APPLICATIONS OF TOTAL
OR PARTIAL REFLECTION OF LIGHT ON TRANSPARENT SURFACES, EITHER ALONE OR
COMBINED WITH THE REFRACTION," upon the condition (amongst others) that I,
the said Ignace Porro, by an instrument in writing under my hand and seal,
should particularly describe and ascertain the nature of my 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 iminediately after the date of the said Letters Patent.
NOW KNOW YE, that I, the said Ignace Porro, 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
thereof, that is to say:-
My Invention consists, first, in certain new combinations of the reflection
and refraction of light upon transparent surfaces; and, secondly, in the appli-
cation of partial reflection of light upon the surfaces of achromatic objectives,
and to the transparent liquids which I employ as a means of accurate measure-
ment in astronomical, and geometrical, and other instruments.
REVERSION AND MAGNIFYING OF DISTANT OBJECTS.
First Example. Effect of the Reversion of Images (Objects).
I make use of a piece of glass, composed of one or several pieces, having a
number of plane and polished surfaces, in which the light is admitted and
emitted by two plane and parallel surfaces at the same angle; in passing to
the interior it undergoes four distinct and alternate reflections at right angles.
Objects viewed through this glass appear reversed, upside down, and colourless.
It will suffice, then, to interpose between the eye-piece and the lens of an
astronomical telescope a piece of glass of this construction, to convert the
instrument into an ordinary geographical telescope re-reversing the object.
The form given to the re-reversing glass is slightly complicated, but it is free
from the inconvenience of the mode at present employed. My system gives
a much higher degree of transparency as the light passes through the glass
in the usual position; it also has the advantage of shortening the telescope.
Second Example-Magnifying Effects.
Amicis combination of four or eight prisms magnifies distant objects two or.
four times; but this combination is imperfectly achromatic, and deficient in

page_text: transparency. I obtain any magnifying power with perfect achromaticity and
a high degree of transparency by combining three prisms in the following,
order: The first is an achromatic rectangular prism, of which one surface is
slightly curved. Its plane axis is horizontal, its hypotenuse, affords a total
reflection. The second is a simple reversing rectangular prism, and is placed
below the first at a variable distance. It is set differently, according as a
direct or inverted vision is required. The light coming from the object is only
received by it after having passed through the first prism, when it transmits to
the third by the reversion which results from the double total reflection. The
third prism is similar to the first; it is more curved, and is placed at the side
of the first at the same elevation, but inversely. On applying the eye to this
combination, distant objects are scen magnified, and reversed or straightened,
according to the setting of the reverser, as in a good telescope; the field
may be extended by the addition of a small lens.
The length of the instrument, that is to say, in the direction of the line
of vision, for a magnifying power of ten or fifteen times, scarcely exceeds
one inch, while its vertical dimensions are about four inches, being extremely
convenient for the hand.
By employing four instead of three prisms I obtain a double glass case; the
four prisms are achromatic, and set in such a manner as to act on the light
two by two, in reciprocal order. This double glass is of small dimensions,
and has an extensive field. I in fact thus obtain a binocular telescope,
which, like the preceding instrument, scarcely exceeds one of each in length.
Here also one or two small lenses may be added to increase the field and
magnifying power.

page_text: of a tower, or any object of which the edges are seen. It also permits of
dividing an angle into two equal parts by a simple sight when suspended by a
thread, so as to retain a horizontal position. This prism becomes à simple
and convenient pocket level. Two of these prisms, fixed crosswise in a small

page_text: oblique reflections, a half reversion, and of which the reflecting surfaces are
turned in the same direction, from the base of an instrument of much utility,
with which all inaccessible distances may be micrometrically measured by a
single sight, and without known dimensions of the object. This problem, so
much studied has never vot boon solved.
SECOND PART.
ACTION OF LIGHT FOR THE DETERMINATION OF THE RELATIVE POSITIONS OF
DIFFERENT OBJECTS BETWEEN EACH OTHER, AND WITH GIVEN FIXED
LINES BY
THE COMBINATION OF ARTIFICIAL LIGHT, AND PARTIAL
REFLECTION ON TRANSPARENT SURFACES.
Demonstration of the Optical Axis of Astronomical Telescopes, and of the
Positive Vertical by the Illumination of the Threads.
With a very small and thin trapezoid prism I introduce artificial light
laterally into the micrometer of the telescope; immediately the images of the
illuminated micrometer appear along the optical axis. In these images the
threads are sharply defined; & if, for instance, the situation of the verniers of a
circle were to coincide with the images, the position of the telescope, or of the
sight line with reference to the fixed 'given lines, would be determined in a
manner independent of all portulatum upon the fallacious matter. If, as a
partially reflecting surface, the in-fero-superior surface of a transparent liquid
were interposed, a sight would be obtained invariably directed towards the
zenith. A transparent glass, and in certain cases even a mirror, fixed plane,
which I add to the system, permits the determination of the relative position
of the sight with a fixed immoveable line. To obtain all these advantages, I
5 employ in the construction of the lenses curves ruled by the effects which
I wish to obtain in each particular case.
And having now described the nature of my said Invention, and the manner
in which the same is to be carried into effect, I would observe that I do not
confine myself to the precise details or arrangements which I have described,
O as many variations may be made therefrom without departing from the
principle of the said Invention; but what I claim is,-
First, the combined use of the refraction and reflection of light emitted from
transparent bodies.
Secondly, the mixed employment of the artificial light, and of the light
5 transmitted by objects reflected and refracted on the transparent surface of

page_text: achromatic objectives, and other glasses, and on the surface of transparent
liquids.
In witness whereof, I, the said Ignace Porro, have hereunto set my
hand and seal, this Fifth day of May, in the year of our Lord One
thousand eight hundred and fifty-five.