Prototype Coincidence Rangefinder, Ignazio Porro, Institut Technomatique, circa 1860

Prototype Coincidence Rangefinder, Ignazio Porro, Institut Technomatique, circa 1860

This is the prototype of the first coincidence telemeter, created by Ignazio PORRO, when at the helm of the Institut Technomatique à Paris, of which he was the founder, he produced innovative and technological optical and mechanical instruments between 1847 and 1860, which characteristics for many of them have remained unchanged to this day.

It is very likely that Ignazio Porro, encouraged by Napoleon III, amazed by the two Long-Vue telemetry scopes that Porro built under his commission, undertook the study aimed at creating an even more complex rangefinder optic by making full use of its innovative prismatic glasses.

Thus it was that he began the development, after the Longue-Vue Cornet (made in 1850) and the Lorgnon Longue-Vue Napoleon III (made in 1855), of this even more innovative coincidence rangefinder which he certainly made, given the notable improvements made to the two previous models, between 1857 and 1860.

What supports the thesis that it is a prototype of the first coincidence rangefinder, is contained precisely in its construction, i.e. in the detection distance which this exact specimen is able to operate: in fact, there is an optical-mechanical device which allows measure distances up to 16 meters. It is also well known that any military target placed at a distance of 16 meters has no reason to be telemetered.

The practical operation takes place, in coincidence rangefinders, by making two images presented in the eyepiece, often identical, materially coincide, often the two halves of the same image.

Once the image coincides, the instrument gives the distance in meters directly in the guide knob, this data is significant of the study applied to the prototype which, once developed and perfected with larger dimensions, would have been produced with scales for metric measurements extremely superior.

Nonetheless, this instrument, so innovative in terms of concept and era, remained only at the prototype level as the Institut Tecnhnomatique Porro à Paris saw its activity end in 1860.

In fact, the telemeter does not have any number or inscriptions such as to provide evidence of the filing of its patent.

The beginnings of this INCREDIBLE and innovative discovery are to be found, in the mind and intentions of Porro, in 1824, the year of the invention of the Stereogonic Telescope (objective with a diameter of 40 millimeters and a focal distance equal to 48 centimetres) which was adapted to a ancient graphometer to obtain the zenithal distances necessary to reduce the distances on the horizon.

Porro introduced for the first time in Kepler's astronomical telescope the "Anallactic Lens", a fixed converging lens, placed between the objective and the reticle, so that its prime focus coincided with the optical center of the objective.

It was an ingenious modification of the Montanari-Green-Reichembach ordinary distance meter, which made it possible to determine the distance of the staff from the optical center of the objective by precisely adding a lens.

Official experiments were made with it on 15 August 1824 by order of the General of Engineers Marquis Boyl by a Commission composed of Captains Passera, Siry and Porro.

The measurements were made on Viale del Re, measuring a known length of 12476.14 with ranges ranging from 80 to 120 metres.

The gap between the maximum and minimum value is 0.190; and therefore a relative error of about 1/4470.

This success led him to a new awareness which led him, in 1850, to invent the "Centrally Anallactic Telescope". Porro fixed the convergent anallactic lens in such a position that its first focus could fall before the objective and in such a way that the extension inside the telescope of the emerging rays directed at the stadia passed through the instrumental centre.

His interest in topography and the calculation of distances led him, with the invention of the prism which occurred to his genius in about 1850, to develop optical-mechanical systems which had, as a result, the invention of this extraordinary and unique prototype of "Coincidence Prismatic Telescope". The theoretical dynamics that lead him to the creation of this instrument are, to date, unknown.

Its original wooden box to contain it is present, where the following inscription is shown on the lid: "Institut Technomatique Porro à Paris".

In the years that followed the death of Ignazio Porro, scientists of the caliber of Ernst Karl Abbe and Carl Zeiss, enlightened by the operating principle of this prototype and also thanks to the deep friendship that bound them to Porro who shared their knowledge, were able to studying and deepening the characteristics that Porro concentrated in this small but surprising optical instrument and they developed, shortly thereafter, what would become the "Modern Military Rangefinder" also keeping its operating principles completely unchanged.

Excellent storage conditions.

Unique specimen known.

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