On the Alleged Use of Keplerian Telescopes in Naples in the 1610s
OOn the Alleged Use of Keplerian Telescopes in Naples in the 1610s
By Paolo Del Santo
Museo Galileo: Institute and Museum of History of Science – Florence (Italy)
Abstract
The alleged use of Keplerian telescopes by Fabio Colonna (c. 1567 – 1640), in Naples, since asearly as October 1614, as claimed in some recent papers, is shown to be in fact untenable and dueto a misconception.
At the 37 th Annual Conference of the Società Italiana degli Storici della Fisica edell'Astronomia (Italian Society of the Historians of Physics and Astronomy, SISFA), held inBari in September 2017, Mauro Gargano gave a talk entitled
Della Porta, Colonna e Fontana ele prime osservazioni astronomiche a Napoli . This contribution, which appeared in the
Proceedings of the Conference (Gargano, 2019a), was then followed by a paper —actually, veryclose to the former, which, in turn, is very close to a previous one (Gargano, 2017)— published,in the same year, in the
Journal of Astronomical History and Heritage (Gargano, 2019b). In bothof these writings, Gargano claimed that the Neapolitan naturalist Fabio Colonna (c. 1567 –1640), member of the Accademia dei Lincei, would have made observations with a so-called“Keplerian” or “astronomical” telescope (i.e. with converging eyepiece), in the early autumn of1614. The events related to the birth and development of Keplerian telescope —named afterJohannes Kepler, who theorised this optical configuration in his
Dioptrice , published in 1611—are complex and not well-documented, and their examination lies beyond the scope of thispaper. However, from a historiographical point of view, the news that someone was alreadyusing such a configuration since as early as autumn 1614 for astronomical observations, if true,would have not negligible implications, which Gargano himself does not seem to realize. As amatter of fact, it would mean to backdate of a couple of years the terminus ante quem for theactual realisation of the very first Keplerian telescope (the most ancient known written sourceconcerning a concrete example of this optical combination is a document of 1616, kept at theTiroler Landesmuseum Ferdinandeum, in Innsbruck; Daxecker, 2004: 14), and of at least fifteenyears its introduction in the observational praxis.Gargano’s claim is essentially based upon a passage of a letter that Fabio Colonna wroteto Galileo Galilei on the occasion of the solar eclipse of 3 October 1614. The letter contains sixsketches depicting the Sun’s surface, partially eclipsed by the Moon, with the sunspots. Theobservation was made by projecting the solar image through the telescope onto a sheet of paper,a method widely used, among others, by Galileo himself, and first suggested by his discipleBenedetto Castelli (Galileo, 1613: 52). In the above-mentioned letter, Colonna apologises forthe poor quality of his drawings, and invites Galileo to salvage as much as possible and to “turnthem right side up, since they came out from the telescope inverted [alla riversa]” (Colonna,1614). That is how Gargano (2019b: 54) has interpreted Colonna’s words: pon reading this letter it is evident that Colonna used a telescope and not a Galileanspyglass. Therefore, this was the first astronomical observation made from Naples using aKeplerian-like refractor.
But did Colonna really use a Keplerian telescope in his astronomical observations? Aswe shall see, the answer to this question is definitely no! Indeed, as it is well known and easy toprove, in the direct observation, a Galilean telescope provides upright images and a Kepleriantelescope upside-down ones, but, if used in projection, a Galilean telescope gives upside-downimages and a Keplerian telescope upright ones. Gargano therefore, seems not only to beunfamiliar with geometrical optics, but with the early history of the telescope as well.Otherwise, he would know the following passage from the renowned work by Galileo
Istoria edimostrazioni intorno alle macchie solari e loro accidenti , published in March 1613, i.e. oneyear and a half before Colonna’s letter:
It should be noted next that [using the telescope by projection] the spots exit the tubeinverted and located opposite to where they are on the Sun: that is, the spots on the rightcome out on the left side, and the higher ones lower, because the rays intersect each otherinside the tube before they emerge from the concave glass. But because we draw them on asurface facing the Sun, when turning back toward the Sun, we hold the drawing up to oureyes, the side on which we drew no longer faces the Sun but is instead turned away from it,and therefore the parts of the drawing on the right-hand side are already in their proper placeagain, corresponding to the right side of the Sun, and the left ones on the left, such that oneonly has to invert the upper and lower ones. Therefore, turning the paper over and thusmaking the top the bottom, and looking through the paper while facing the light, oneobserves the spots as they should be, as if we were looking directly at the Sun. And in thisappearance they must be traced and inscribed on another sheet in order to have themcorrectly positioned. (Galilei, 1613: 53; translation from Reeves and Van Helden, 2010:127).
So, if we did not know the laws of geometrical optics, in the light of this detaileddescription, following Gargano’s belief, we should be forced to think that Galileo was wrong, orthat Galileo himself had already abandoned the optical combination named after him, in favourof the Keplerian one, at the beginning of the 1610s!Besides, Gargano seems also not to know the passage of the
Rosa Ursina in which theGerman Jesuit Christoph Scheiner describes the projection technique by using both a concave diverging) lens, and, as an alternative to it, a convex (converging) one (Scheiner, 1630: 129 v ,130 r ). Therein, Scheiner correctly states that the telescope projects upside-down and right-sideup images, respectively.One could argue that the expression “alla riversa”, used by Colonna in his letter toGalileo, might refer not to the vertical (up-down) inversion, but to the horizontal (left-right) one.Actually, “riversa” is an Italian archaic term for the modern “rovescio” or “rovescia”, which,used in the locutions “al rovescio” or “alla rovescia”, have the generic meaning of “upsidedown”, “wrong side up”, “wrong way round” or even “back to front” (inside out). However,even though the optical paths of the Galilean and the Keplerian combination are completelydifferent, when used by projection, both produce mirrored images, namely, if one looks thescreen from the side of the eyepiece, left and right are inverted. This is why, in order to get allorientations correct, both Galileo, as we have seen above, and Scheiner (1630: 129 v , 130 r )recommend transposing the projected image onto the opposite surface of the sheet, tracing itagainst the light. Hence, the two optical configurations are indistinguishable on this account, andtherefore, even if Colonna really meant a mirror-like inversion of the image, this circumstancedoes not prove anything about the type of optical combination, Galilean or Keplerian, he used.We have already touched upon the implications of Gargano’s reconstruction on thechronology of the development of the Keplerian telescope, but he goes further: he claims thatthe new optical configuration “did no t derive from Kepler’s studies or those of Fontana, but wasthe result of the combined theoretical and practical skills of Della Porta and Colonna” (Gargano,2019: 54), who therefore, in Gargano’s opinion, would be the true originators of the Kepleriantelescope. Gargano’s statement is based on a letter from Della Porta to Galileo, dated 26September 1614, i.e. exactly one week before the aforementioned letter from Colonna to Galileoof the 3 rd of October. In his letter, Della Porta informs Galileo that he is working, together withColonna, to realize “a new kind of telescope, that will increase a hundredfold the performance ofthe usual ones” (Della Porta, 1614). However, Gargano (2019: 54) takes for granted that DellaPorta and Colonna were working on the development of a Keplerian telescope —which, as weshall see later, is more than unlikely, and, in any case, unproven— and that this new, unspecifiedinstrument was the one used by Colonna himself to observe the eclipse just a week later. As amatter of fact, neither in the letter to Galileo of the 3 rd of October nor elsewhere in the followingmonths, Colonna claims he is using a new kind of telescope, different from those (Galilean) thathe usually used in his observations. Nor is there evidence that suggests such a hypothesis.Frankly, I find rather odd that Colonna was satisfied with testing (furthermore, on his ownwithout Della Porta, his partner in the project) such an innovative instrument just with oneobservation, made returning home hurriedly, between two judicial hearing in the courthouse ofNaples, where he practised law for a living (Colonna, 1614). On the other hand, as far as weknow, Della Porta himself, who was to die a few months later, does not mention any more hismysterious telescope, and, in all likelihood, his project led to no result.In my opinion, the irrefutable evidence that Colonna did not use a Keplerian telescope in1614 is a letter —which Gargano evidently does not know (indeed, neglected by most historiansof the telescope)— from Colonna himself to Federeico Cesi, one of the founders of theAccademia dei Lincei. The letter, dated 19 September 1626, contains a reference to a smallelescope, made by the Neapolitan optician Francesco Fontana, which undoubtedly had aconverging eyepiece: This friend [Francesco Fontana] has also invented a […] telescope, long just a palm, whichshows objects upside-down, but magnifies them very much, and, what is most remarkable, itshows objects so near that those which are as far away as a musket shot are seen close to theeyes; I could not understand yet, because I cannot attend to that, due to my job, how it cando that, since two convex lenses show objects more distant and smaller than they actuallyare, but more sharply; when I get around to going to him, I will examine it, and I will obtainone to His Excellency. (Colonna, 1626) That passage shows conclusively that —twelve years after his letter to Galileo— Colonna stillcould not understand, both theoretically and practically, the way two convex lenses can form atelescope!In conclusion, there is no doubt that the blunder committed by Gargano is due to his lackof knowledge of the primary and secondary sources about the early history of the telescope (andof the laws of the geometrical optics as well), a history which, however, he aspires to rewrite.Nevertheless, Gargano probably is perpetrator and victim, at the same time, of a narration, oftenbased more on the imagination than on historiographic sources (although, to be fair to him, notwith equal sensationalism), chiefly due to Paolo Molaro and Pierluigi Selvelli, who, in severalpapers published over the last decade, have conceived and promoted: 1) The (wrong) idea thatthe telescope depicted in the
The Sense of Sigh, painted by Jan Brueghel the Elder and PeterPaul Rubens in 1617, is a Keplerian one, and therefore, that this optical combination wasalready pretty common at that time (Molaro and Selvelli, 20: 331-32); 2) The (wrong) idea —based on the (wrong) assumption that “[t]here are no apparent reasons to question FatherZupus’s declaration to have used Fontana’s [Keplerian] telescope in 1614”— that “the year of1608 does not seem so implausible as the birthdate of Fontana’s [Keplerian] telescope” (Molaro,2017b: 284-86; see also Molaro, 2017a: 227); 3) The (wrong) idea, consistent with the previousones, that, as early as around 1615, Fontana already enjoyed such a reputation as optician thatone of the greatest painter of his time, José de Ribera, would have portrayed him in his painting
Allegory of Sight (Molaro, 2017: 284-86). Well, if one believes all those things, probably, onewill consider plausible, even likely, that, in Naples, in autumn 1614, someone was alreadymaking astronomical observations by means of a Keplerian telescope...
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