Terrestrial telescope (for land use) with intermediate micrometer
Amici described his terrestrial telescope with intermediate, double-image micrometer in a long letter to Baron von Zach in 1823. This latter immediately published this information under the title Nouveau micromètre intermédiaire (download pdf) in volume IX of “Correspondance astronomique”.
The Royal Topography Office of Naples commissioned two of these instruments from Amici on 11 July, 1828. On 7 August, replying to Colonnel Desauget, then Head of that office, Amici proposed: “Since you have authorized me to change the dimensions of the distance measurers, I would say that one of these could become smaller and closer to what I described in Zach’s Astronomic Correspondence. A two-foot terrestrial telescope is certainly superior to a fifteen inch one, but since the extension of the scale of the micrometer is diminished, that is, since one cannot measure as large an angle with it as with a smaller telescope, its superiority for visual power does not mean a proportionate advantage for the determination of a terrestrial base” (cf. A. Meschiari, Giovanni Battista Amici e il Reale Officio Topografico di Napoli. Corrispondenza con i Colonnelli Visconti, de Sauget, Melorio, “Physis”, 1-2002).
The two terrestrial telescopes, together with two microscopes commissioned at the same time, were only ready at end March 1831 and they were shipped to Naples on 10 May.
They are most probably the instruments which are now in Florence in the collection of the Istituto Geografico Militare (Military Geographic Institute) and pictured here in two photographs kindly furnished by the same Institute (cf. A. Meschiari, Giovanni Battista Amici, il Reale Officio Topografico di Napoli e la collezione di strumenti scientifici dell’I.G.M., “L’Universo”, journal of the Istituto Geografico Militare, Florence, 1-2003).
In the Catalogo generale descrittivo degli strumenti geodetici e topografici dell’Istituto Geografico Militare al 27 ottobre 1922 <General descriptive catalog of the geodetic and topographic instruments of the Military Geographic Institute as of 27 October, 1922> (Barbèra, Florence, 1922), the Amici Terrestrial Telescope no. 62 is described as follows: This instrument is founded on the same principle as the Lugeol telescope, a common terrestrial telescope, but it differs from it both in the frame and for some notable construction details. “The micrometric apparatus is not separate, that is, mountable or dismountable from the telescope, but is one with it; it is not applied to the objective but can be found at the other extreme of the extension tube of the eyepiece; what is more, it is made in its essential part not by two circular lenses but by two strips of crystal, in perfect contact, with their width equal to a diameter of the tube, functioning as the objective cut in two of the other micrometer. One of these, the lower one, slightly longer than the diameter of the tube, is fixed and fulfils the same function of the fixed half lens of the other. The higher one, 10 cm long, is moveable with the same system of the Lugeol micrometer, and like it is graduated and it has a vernier. The instrument can also rotate by hand around the axis of the telescope, and a small ring connected to it, divided in degrees with a fixed index, can provide the inclination. The graduation of the arm with its vernier can give angles up to a maximum width of 10' with the approximation of 1". The telescope has a 43 mm objective with a 0.68 m focal distance and 25 of magnification. It is all in brass with a metal ring onto which it can be screwed; pillar and feet are hinged and can be moved as necessary and inclined. It has “Amici – Modena” written on it, without the indication of the year it was constructed”.The micrometer of the Amici Terrestrial Telescope no. 63 “fully equal to the micrometer on the preceding Amici telescope, except that it does not have the vertical ring, is also situated between the eyepiece and the objective, but on the tube of this and at a distance of only 10 cm from it. Adaptation to distance is done by moving the objective with rack and pinion wheel. The lens aperture is 30 mm with 0.34 m of focal distance and 23 of magnification. It is all in brass like the preceding one and it has the same form, but it is much smaller. It has no indication of the workshop from which it comes”.