The First Prismatic Military Telescope with Rangefinder, "Longue vue Napoleone III", Ignazio Porro, 1855

The First Prismatic Military Telescope with Rangefinder, "Longue vue Napoleone III", Ignazio Porro, 1855

"Longue Vue Napoleon III" prismatic telescope with rangefinder, made by Ignazio PORRO in 1855, in his optical laboratory "l'Institut Technomatique" located in Paris Boulevard d'Enfer n.10 (or Rue D'Enfer "Road to Hell", this was the very first optical instrument with double reflection prisms with rangefinder, designed and produced for military use, suitable for detecting distances.

In 1850, Porro created the Longue-Vue Biprismatique Cornet, a terrestrial telescope weighing 300 grams, measuring only 15 centimeters and with 10/12 magnification. Given the presence of the objective reticle, it allowed the determination of the distance of an infantryman or a soldier on horseback. This telescope, called Cornet because the shape and way of holding it recalled the acoustic horn of the time, while Longue-Vue stood for terrestrial telescope, was made of ivory and gilded copper and was presented to Emperor Napoleon III who, both he was enthusiastic that it became his inseparable tool on the battlefields, asking I. Porro to create an even smaller model for military use.

Thus followed, in 1855, the "Lorgnon" an improvement of the previous model even smaller and more compact that Porro himself defined the "Nec Plus Ultra" (no more further) of the small half collimators and optical measurers of distances for use by officers". The new instrument was called by Porro "Lorgnon Longue-Vue Napoleon III" from which he derived the term "lorgnette" for a small eyeglass used in the theater by ladies. In Italy they were also called military rangefinder or prismatic monocle.

The "Lorgnon Longue Vue Napoleon III" was dedicated and presented to the Emperor at the Palais des Tuilleries, royal residence, on February 22, 1855.

Porro with this new model had developed a double prismatic system which in the years to come will be known worldwide as the "Prisma Porro II" and used on all types of binoculars built up to now by all the world's optical industries.

For the Lorgnon Longue-Vue Napoleon III, the Leek obtained a gold medal during the Universal Exhibition of Agricultural, Industry and Fine Arts products held in Paris from 15 May to 15 November 1855.

On page 403 of the Reports of the joint international jury (1856) we find the motivation provided by the thirteen members of the jury of Class VIII^ relating to the Precision arts, science and education industries, presided over by the Marshal of France Jean Baptiste Philibert Vaillant ( 1790-1872): "M. Porro (n° 1920), in Paris (France), exhibited a large number of objects, including several unfinished instruments which were unable to undergo a complete examination.

The jury, having noticed what is ingenious in these various instruments, turned their attention to the small telescope that Mr. Porro calls the "Longue Vue Napoleon III" telescope. Mr. Porro, obviously through a game of various reflections, managed to build a bezel reduced to very small dimensions, very portable and very convenient for military reconnaissance.

Currently, only 3 specimens are known, including this one, kept in private collections.

PORRO, Ignazio – He was born in Pinerolo (Turin) on November 25, 1801; by Ignatius and Paola Lantery of Annecy.

He was admitted to the Military College of Turin and on 19 March 1815 he entered the Royal Schools of Artillery and Engineering as a cadet and pupil. With the commission of 18 December 1817 he became a supernumerary second lieutenant and pupil at the Royal Schools, 2nd class lieutenant on 19 September 1819, 1st class on 28 December 1821.

He was promoted to senior lieutenant on 27 January 1824 and again in January 1824 Porro, under the supervision of Captain Agostino Chiodo, teacher of descriptive geometry and fortifications at the Military Academy of Turin, would develop a complete semi-permanent fortification project and in the winter of same year he would have been a teacher in the complementary school created to hold 'academic meetings' and maintain emulation in the body of genius, reviving the love for study.

In that year he was in Savoy, where he began the first experiments on ceremonial measurement; in that period the first anallactic theodolite was also built, named Pantometro by Porro.

Porro's first invention that made his name stand out was the stereogonic telescope built in Turin in 1824; it had a lens with a diameter of 40 millimeters and a focal distance of 48 centimetres.

Recalled to the capital, on 25 January 1825 he was promoted to captain 2nd class with the task of instructing the subordinate officers of the Engineers. In 1826 he was in Nice and then in Sardinia as local director of the Engineers; he was promoted to captain 1st class on 3 December 1830. In 1833 he was judged «very distinguished in the physical-mathematical sciences, very expert in the use of geodetic instruments, capable of perfecting and executing them by himself as demonstrated by the useful inventions already made» ( State Archives of Turin, Ministry of War, Matriculation rolls, Corps of Military Engineers, Matriculation roll of officers, reg. 2662, f. 4).

Promoted to senior captain on 25 February 1834, in that same year he married Casulda Di Somma, of Spanish origin, who followed him with affectionate attention all his life.

In 1836 he was promoted to Major in the same Corps and in 1842 he asked for leave.

Free from the obligations of military service, he founded an optical-mechanical workshop in Turin in which not only instruments for geodesy but also everything that may have a bearing on progress in engineering and industry were to be built: from Fourneyron's turbine, as he he said, into the astronomer's telescope. His name was already known favorably and his merit was already recognized even outside the military environment. As early as 1839 he had already published his Essai sur les moteurs hydrauliques, in which he had proposed a turbine which was an innovation for that time and in 1845 he was commissioned by the Minister Count of Pralormo to report on Médail's project for a tunnel in the Cottian Alps to Fréjus hill. On this occasion he was the first to have the idea of using compressed air for the excavation of the tunnel and the movement of the drilling machines.

That mechanical workshop, however, did not find favorable ground in Turin and was abandoned by Porro in 1847. In the same year, at his request, he was retired with his rank of Major of the Engineers and moved to Paris, where in 1850, under the protection and partly with the money of Count Eugene of Richemont, he founded the Institut Technomatique with Laboratory in Paris Boulevard d'Enfer n.10

Most of the instruments used in Piedmont and France for tacheometric surveying had the stereogonic telescope, through which the measured distances were counted by the optical center of the objective lens. It was convenient, and had already been noted since 1824, that the distances were counted from the center of the azimuthal circle of the instrument used. This new improvement was introduced with the construction of the centrally anallactic telescope, that is, with the telescope having an objective composed of two converging lenses, such that the first main focus (anallactic point) of the composite system coincided with the center of the horizontal circle of the instrument of which it was part.

The stereogonic telescope, which was nothing more than a telescope, having the anallactic point in the optical center of the objective lens disappeared forever.

His first book, Tacheometry, was a masterly work of admirable clarity. It gives the foundations of the new survey method he advocated and all the rules that must be used in practice.

With very few notes and additions it could be reproduced today and replaced with advantage to many celerimensura treatises that have followed over time.

From the aforementioned book it could be deduced that the centrally anallactic telescope was not made before 1850, since it is not mentioned at all in it. However, this is not correct if one takes into account a memoir by the engineer De Senarmont printed in 1849 in the "Annales des mines".

All those who have talked about Ignazio Porro have never mentioned the solution of a very important practical problem, namely: the shortening of the telescope, which he did since 1850 by means of double reflection prisms. He folded the focal distance into two or three parts and first made the longue-vue cornet and later in 1855 the longue-vue Napoléon III. Both dedicated to the French army and very comfortable to carry even as soldiers on horseback. These very useful and elegant telescopes were almost ignored in Italy and completely forgotten in France.

It took the exhumation of them by the famous Zeiss company of Jena, which reduced them to prismatic binoculars, for them to now be known and appreciated throughout the world. Nonetheless they are less elegant than those made by Porro".

Porro presented numerous memoirs to the Parisian Académie des sciences which were always received with interest. However, his goal was to return to Italy. In August 1861 he settled in Florence and attempted to initiate what Alberto Meschiari has defined as «his premature and chimerical project of an Italian Technological Society» (2005, p. 893), while he was teaching speed measurement in the local technical institute.

His project was the construction of a large factory for precision instruments, but the idea (of which there is a trace in his essay Dei compound microscopes, which appeared in the Rediconti of the R. Lombard Institute of Sciences and Letters, III, Milan 1866, pp. 285 ff.) was not successful, indeed it did not develop perhaps due to the difficult character of Porro, who in 1863 left Florence to move to Milan.

In this city he tried in vain to give life to a mechanical workshop, the project then taken up by others gave life to the Tecnomasio. Finding himself in difficulty, help came to him from Francesco Brioschi, the founder of the Royal Technical Institute (the future Polytechnic), who called him to teach celerimensura. Meanwhile I. Porro founded the Filotecnica, this too with a difficult life, as Salmoiraghi later noted: «his broad innovative ideas did not yet correspond to the maturity of the times, his little administrative, organizational aptitude was complicated by the lack of material means that were at his disposal.

In 1867 he had the satisfaction of being accepted as an honorary member of the newly formed Society of Engineers and Industrialists of Turin. On February 18, 1869 he presented to the members his latest creation, the cleps-cycle theodolite.

Giovanni Virginio Schiaparelli recalls the visit to the "astronomical park" of Porro with the large achromatic telescope with an aperture of 52 cm and a focal distance of 15 meters «la plus grande lunette du monde» (Schiaparelli, 1910, pp. 299 s.). Also very well known are the two telescopes with double reflection prisms and rangefinder that Porro developed and called: longue-vue cornet made in 1850 and the longue-vue Napoleon III, made in 1855 then taken up again many years later by Carl Zeiss of Jena which he reduced them to prismatic binoculars.

In the last years of his life, in bad health and oppressed by severe financial difficulties, he sold the rights to exploit his inventions to his pupil and later collaborator Angelo Salmoiraghi. He died in Milan on October 8, 1875.

The commemoration on his grave in the Monumental cemetery of Milan was held on October 10 by Salmoiraghi, continuer of his work and future senator of the Kingdom. La Gazzetta Piemontese (later to become La Stampa) also mentioned it in the issue of 11 October (p. 2).

Ignazio Porro was one of the great Italian scientists of the nineteenth century. His discoveries revolutionized the way surfaces were measured and represented graphically. His interventions were useful in every sector he had to deal with, even if he was not able to exploit his discoveries industrially, perhaps engaged on too many fronts at the same time. He lacked the ability to synthesize and to identify the elements that could be profitable from a financial point of view, and he did not find collaborators capable of making up for his shortcomings. An in-depth study deserves his production of books and articles, which has about 150 titles.

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