GALILEO’S SHOPPING LIST: AN OVERLOOKED DOCUMENT ABOUT EARLY TELESCOPE MAKING

Giorgio Strano*

Introduction

During the first half of the seventeenth century, Italian telescope mak- ers usually kept maximum secrecy concerning their methods of grind- ing and polishing lenses. On the one hand, it was a point of honour to produce the best lenses and to prevent anyone from equalling their ability. On the other hand, rivals and plagiarists were always eager to reveal the procedures that could grant a leading position in the tele- scope market. A few examples may suffice to make this fact clear. Around 1637, the Neapolitan optician Francesco Fontana tried to sell telescopes to the Medici Court in Florence. Fontana, however, was told that his lenses were of inferior quality, or equivalent, or just marginally better than those already produced in Florence. Despite this criticism, the Medici Court made unsuccessful attempts—via Benedetto Castelli—to steal the secret of Fontana’s method of polish- ing the lenses.1 A few years later, in 1647, the natural philosopher and optician Evangelista Torricelli passed away suddenly at the age of 39. This event prompted the Grand Duke of Tuscany to set up investigations immediately to try to recall from the grave Torricelli’s lens polishing secrets. Witnesses that had had access to the departed’s house and could have observed him at work, were summoned to the Court and

* This article is the development of Giorgio Strano, “La lista della spesa di Galileo: Un documento poco noto sul telescopio”, Galilaeana 6 (2009), pp. 197– 211. I would like to thank Karen Giacobassi for her kind support in revising the English text. 1 Paolo Del Santo, “On an Unpublished Letter of Francesco Fontana”, Galilae- ana 6 (2009), pp. 236–238. 2 giorgio strano thoroughly questioned.2 No particular results, however, emerged from those investigations, except the great handcraft ability of Torricelli. Between 1663 and 1665, the clockmaker and optician Giuseppe Campani from Castel San Felice, near Spoleto, was the main char- acter in the telescope competition that was held in Rome under the supervision of the Florentine Accademia del Cimento.3 In addition to the results of a series of public comparisons of telescopes, Campani’s triumph over the rival optician Eustachio Divini, a native of San Severino Marche, was decided in the astronomical field. From 1664, Gian Domenico Cassini made noteworthy celestial discoveries by using Campani’s telescopes. In 1669, Cassini became director of the Paris Observatory and continued to prefer telescopes made by the same optician.4 Notwithstanding the stability of his position achieved in the optical market, for all of his life Campani kept his methods of grinding and polishing lenses a secret. As his work bench, moulds and tools have survived up to the present, such methods are only partially known today.5 These few examples outline the extensive secretive behaviour of early opticians. In particular, as we turn our attention back to the origin of the telescope, very dense clouds shroud the secret of grinding and polishing lenses. This fact is apparent especially in the case of Galileo Galilei (Pl. 1), whose celestial observations contributed to changing the meaning of astronomy. Even today, Galileo’s methods of improving upon the telescope remain at the centre of scholarly controversy.

2 Paolo Galluzzi, “Evangelista Torricelli: concezione della matematica e segreto degli occhiali”, Annali dell’Istituto e Museo di Storia della Scienza 1 (1976), pp. 84–95. 3 Maria Luisa Righini Bonelli, Albert Van Helden, Divini and Campani: A Forgotten Chapter in the History of the Accademia del Cimento, Florence, 1981, pp. 15–43. 4 P. Del Santo, G. Strano, “Il cimento degli astri”, in P. Galluzzi (ed.), Scien- ziati a Corte: L’arte della sperimentazione nell’Accademia Galileiana del Cimento (1657–1667), Florence, 2001, p. 34; P. Del Santo, “Italian Optics in the 17th Century: Fontana, Divini and Campani”, in Ileana Chinnici (ed.), Astrum 2009: Astronomy and Instruments: Italian Heritage Four Hundred Years after Galileo, Livorno, 2009, p. 45. 5 Campani’s moulds, tools and optical bench are the property of the Museum of Physics of the University of Bologna; see: Silvio A. Bedini, “The Optical Workshop Equipment of Giuseppe Campani”, Journal of the History of Medicine and Allied Sci- ences 16 (1961), pp. 35–36; P. Galluzzi (ed.), Galileo: Images of the Universe from Antiquity to the Telescope, Florence, 2009, p. 535; Chinnici, op. cit. (n. 4), p. 156.