BornSan Severino(Marche, Italy), 4 October 1610
Eustachio Divini was one of the leading telescope makers of the seventeenth century. Divini’s mother, Virginia Saracini, died when he was four, and his father, Tardozzo Divini, died when he was 11. Eustachio was brought up by his elder brothers, Vincenzo and Cipriano, who started him off on a military career. After abandoning the army, Divini went to Rome where he began attending Benedetto Castelli ’s lessons in mathematics at the University La Sapienza. Here he met many scholars who would become famous scientists, such as Evangelista Torricelli , Giovanni Borelli , and Bonaventura Cavalieri , and he would develop his passion for astronomy and optics.
In the early 1640s, Divini established himself in Rome as a clockmaker. In 1646, he began making lenses and constructing compound microscopes and long-focus telescopes. Many of his instruments have survived in museums in Florence, Rome, Padua, and elsewhere. Between 1662 and 1664, Divini’s lenses and instruments competed with those of Giuseppe Campani , and a bitter rivalry between the two developed into a feud that involved Pope Alexander VII. Divini was still in Rome in 1674 but soon moved back to his native town, where he spent his last years comfortably, thanks to his wealth.
Divini was among the first to develop technology for the production of scientifically designed optical instruments – he produced long-focus telescopes, some as long as 72 Roman spans (about 16 m), and he was probably the first to use a reticule for the telescope, an important step toward the micrometer. By the 1650s, his telescopes were well known all over Europe – a Divini telescope was used by Antonio de Reitha . Sir Kenelm Digby took six of them, one of which he probably gave to Pierre Gassendi in 1653. Many Divini instruments were bought by high prelates of the Roman curia.
In 1649, Divini published a copper-engraved map of the Moon as a separate broadsheet dedicated to the Grand Duke of Tuscany Ferdinand II, primarily to advertise the quality of his lenses. There are many similarities between Divini’s map and that of Johannes Hevel (made in 1647). It is evident that Divini had one eye at the telescope and the other on the work of his predecessor. However, there are also enough differences to indicate that Divini did make many observations on his own. In the broadsheet published in 1649, around the big picture of the Moon, there are a crescent, a Saturn with its “handle” (as it was observed between 1646 and 1648), a horned (cornigera) Venus, and two pictures of Jupiter with the four Galilean satellites. Divini’s map was then twice included in printed books – first in Athanasius Kircher ’s Mundus subterraneus(1665) and subsequently in Otto von Guericke’s Experimenta nova(1672).
In 1659, Christiaan Huygens published his Systema Saturnium, in which he asserted that there was a ring around Saturn. He affirmed, among other things, that his own telescopes were the best and underlined that he saw Saturn much better than Eustachio Divini did. Divini’s answer came with a pamphlet (Brevis annotatio in Systema Saturnium Christiani Eugenii, July 1660), probably not written by Divini himself but most likely by Honoré Fabri, a Jesuit astronomer in Rome. This short treatise spoke ill of Huygens’s telescopes, described his ring theory as fantastic, and argued in favor of the theory that Saturn was accompanied by four satellites. After Huygens’s rejoinder (Brevis assertio Systematis Saturnii Sui, September 1660) and a second Divini-Fabri pamphlet (Pro sua annotatione in Systema Saturnium Christiani Eugenii adversus eiusdem assertionem, 1661), the prestigious Accademia del Cimento in Italy performed a series of experiments with models and found that Saturn’s appearance was explained most satisfactorily with Huygens’s ring theory, but the question was not definitively solved.
From 1662 to 1665, there was another quarrel between Divini and Campani. Both worked in Rome, so some rivalry between them was inevitable. In those years, however, the rivalry became a hot dispute. Many “comparisons” were made between the instruments of these rivals, which Divini mentioned in his letter to Count Antonio Manzini (1666). The first public comparison took place at the end of October 1663 in the garden of Mattias de’ Medici, in the presence of some famous astronomers like Giovanni Cassini . The contest ended in a draw since they acknowledged that Campani’s telescope had better focusing but Divini’s had bigger magnification. Many other comparisons were made in the following months, but they virtually ended in July 1665, when Campani’s 50-span-long telescope was unanimously judged as the best ever constructed. Even after the bad end of the quarrel with Campani, Divini’s instruments continued to be appreciated and esteemed, so he did not stop his work.