The use of mural instruments fitted with telescope sights dates back to 1683 when a first instrument of this type was installed in the Paris Observatory. In 1704, when our instrument was built and put to use in the Marsili Observatory, it was therefore a still fairly recent observational procedure. It had been made possible by the enormous progress made in that period by clocks which had become so accurate as to compete with the direct and precise measurements obtained with the so-called astronomical sextants that computed the distance between two stars according to the difference between their transit times across the meridian.
Lusverg’s instrument, though smaller than instruments being used at that time in the observatories of London and Paris, was no less accurate. The latitude of Bologna, computed with its help in 1706, was only 8 tenths of an arcsecond off the real value, a typical error for the instruments made in the workshop of the Lusvergs which made them some of the best for the first half of the XVIIIth century.
It was with this instrument that Manfredi in 1731 was able to confirm for the first time Bradley’s theory of the aberration of light and provide evidence, unexpectedly found, of the reality of the Earth’s orbit around the Sun, predicted by the Copernican system (see Part I, par. 14).
It was set up in the Marsili Observatory on 1 August 1705 and removed May 28, 1709 (Reg. Oss. Marsiliano, Vol. I and III, Arch. Dip. Astron. Bo.).
The detailed description found in the Instrumentum donationis of 1712 suggests a provisional arrangement until the tower of the Specola of the Istituto delle Scienze was built with the Meridian Room where the semicircle was installed in December 1726 and rectified in March 1727 (Reg. Sp. Ist. Scienze, Vol. I, Arch. Dip. Astron. Bo.). We do in fact read:
"Semicirculus ad Meridianas observationes semid. ped. IV a Lusvergo elaboratus ferrea contignatione, limbo, & centro auricalceo. Dioptra est illi Telescopica, suis lentibus instructa circa cylindrum centro insertum volubis, in limbo divisiones indicans, quae Tychonica methodo peractae sunt. Marsiliana insignia ad umbilicum Semicirculi auricalco incisa spectantur. Ipsum Instrumentum oblique suspensum est in conclavi Astronomico, eadem verticali, & immobili positione, qua in Meridiani plano collocandum est. Sustinent illud vectes ferrei X diversae longitudinis pareti implantati. Eorum capita cochleatim contorta desinunt in semicirculi planum, quod inter alia duo helicum ferrearum, iis capitibus intortarum plana constringitur."A further rectification was made in 1735, bringing it nearer the meridian wall to improve stability and fitting a new object lens and eyepiece made by Francesco Vandelli, professor of military architecture at the Institute (note dated 6 October 1735 in Reg. Sp. Ist. Scienze, Vol. V, Arch. Dip. Astron. Bo.).
E. Baiada, A. Braccesi (1983), p. 86.
M. Daumas (1953), p.91.
E. Miotto, G. Tagliaferri, P. Tucci (1990), pp. 95 ff.
G. Tabarroni (1954), p. 43.
P. Todesco (1995).