One Century of Solar Physics in Italy 1850-1950

Mem. S.A.It. Vol. 74, 556 °c SAIt 2003 Memorie della

One century of Solar Physics in Italy 1850-1950

A. Righini

Dipartimento di Astronomia e Scienza dello Spazio, Universit`adi Firenze, Largo E. Fermi, I-50125 Firenze e-mail: [email protected]

Abstract. In this paper we brieﬂy describe the story of Solar Physics in Italy during one century and we try to evaluate its international impact. At the beginning, in the serendipitous phase, we have e relevant contribution of italian solar physicist like Secchi and Tacchini. The choice of the Abetti father and son to build in Arcetri a Solar Tower, under the technical supervision of George Ellery Hale, could have given to italian solar physics the trust to compete in the international arena. However the lack of necessary technology, the war, and the choice to use the tower for patrol of the solar chromosphere kept italian solar physics from developing at the level of its competitors at the end of the ﬁrst half of the XX century.

Key words. History of science– Solar Physics – Italy

1. Introduction 2. Solar science in Italy in the XIX century Science should have no borders. However At the beginning of the XIX century as- the difficulties in communications, differ- tronomers had very good tools (theoretical ent political regimes and research policies and observational) to speculate about solar gave rise to national sciences at least until apparent motion in the sky, solar distance the end of the second World War. We can and all the mechanical parameters involved therefore speak of an Italian Solar Physics in the orbital motion of the Earth, but were which merges in the global science in the lacking of tools for the physical study of second half of the XX century. In this pa- our nearest star. Knowledge was a privilege per we briefly review its history from the of few, and among those we may consider first works of Secchi and Donati trying to Antonio Cagnoli. evaluate how the work of the italian scientists has been evaluated abroad.

Send oﬀprint requests to: A.Righini Correspondence to: Largo E. Fermi 2, I-50125 Firenze A. Righini: One century of Solar physics in Italy 1850-1950 557

2.1. The physics of the sunspots reviews the results obtained in this ﬁeld

1 in a fortunate book edited first in French, Cagnoli , in his handbook of Astronomy ti- in Paris, and than in Italian in Florence tled Notizie Astronomiche (Cagnoli 1827) (Secchi 1884). This book was interna- writes: Pende ancora fra gli astronomi la tionally appreciated and it may be consid- soluzione del problema: cosa sieno le mac- ered the first textbook in solar physics. In chie del Sole; e forse penderàper non breve this book we may find a lot of interest- tempo avvenire...... Non èquindi appieno ing details, as the description of the the inverisimile per quanto riguarda la conget- structure of several spots and the discov- tura, che le macchie siano scorie o sostanze ery that sometime clouds, pink in color, poco gravi fluttuanti nella materia ignea del were present on the spots. The obvious Sole. conclusion was that these clouds could be The Notizie astronomiche were written the same red prominences observed at the for a cultivated reader (nobles or officers of limb of the Sun during total solar eclipses. the army), but we may consider these words Interesting results were obtained in the as representative of the general ignorance year 1852 on March 19 by Angelo Secchi about the facts of the Sun. The situations with a thermopile placed behind the projec- deeply changes in about fifty years thanks tion plane of the Cuchoix telescope of the 2 also to the work of Angelo Secchi who Osservatorio del Collegio Romano. Secchi was able to diaphragm the pinhole down 1 Antonio Cagnoli, astronomer, meteorolo- to one arc minute, obtaining a ratio about gist (1743-1816), as secretary of venetian am- two in potere calorico between the center bassador in Paris was able to meet Lalande and the limb (Secchi 1884). Secchi remarks who introduced him to astronomy. He de- that his measurements were the first of this veloped his astronomical interests using his type. own instrument first in Paris and than in his hometown, Verona where he built an observatory. In the year 1796 he was appointed presi- 2.2. Solar Corona and the prominences dent of the SocietàItaliana delle Scienze. The Repubblica Cisalpina, formed in northern Italy Another problem on stage in the middle of by Napoleone, transferred the Society in Milan the XIX century was the nature of Solar and the instruments of Cagnoli were moved corona and the location (Sun or Moon) of to the Brera Observatory. Cagnoli ended his solar prominences. Also in this field italian scientific career as professor of Matematica scholars, with Donati3 and and Secchi were Sublime at the military Academy in Modena. well in the front of the research. During 2 Angelo Secchi S.J. (1818-1878) born in Reggio Emilia, was educated by the Jesuits. the solar eclipse 1842 Arago extensively de- In 1835 he is at the observatory of Collegio scribed the Corona end prominences ob- Romano, the Jesuit main house in Rome, as served during the total solar eclipse, sim- student. In the year 1848 he is in Georgetown, close to physics. He was too modest, his contribution Washington working with Curley director of was outstanding and his fame rapidly spread that astronomical Observatory. Back to Rome out the narrow borders of Italy (Righini 1978). he succeeded in building a new and origi- 3 Giovan Battista Donati (1826-1873) stud- nal meteorological instruments which was ap- ied in Pisa and in 1852 is in Florence as as- praised with the Gold Medal at the Universal sistant of Giovan Battista Amici, His major Expo in 1867. Due to the poor mechanical sta- achievement is the first attempt of spectral bility of the astronomical instruments at the classification of stars. In 1860 he is in Spain to Observatory of the Collegio Romano Secchi, as observe a total solar eclipse and again in Sicily he writes, decided to study the physics of the for the eclipse of 1870. In 1872 he was able to Sun, and few months before his death, writ- dedicate the new observatory build in Arcetri, ing about his studies, he recognizes to have on a small hill south of Florence. (Bonelli given a contribution to the new science of Solar 1971) 558 A. Righini: One century of Solar physics in Italy 1850-1950 ilar observations were reported also by find that the maxima of the auroral phe- Vassenius since 1733 but Arago’s descrip- nomena propagated from east to west but tion had a worldwide resonance. The so- they were observed at about the same lo- lar eclipse in Spain, in 1860, was an ex- cal time. The conclusion of the paper was cellent occasion for the astronomers com- that a a new meteorology was needed that ing from all Europe to observe the black Donati defined as cosmic meteorology (we sun and among them we find Secchi and now would translate in space weather). Donati. Donati confirms the correspondence, between sunspots and prominences (Donati 1866). Secchi brought in Spain 2.4. The solar spectroscope and the the Chauchoix objective of the Collegio Memorie degli spettroscopisti Romano and, very wisely, did not use italiani any anlarging device for obtaining photo- graphic pictures of the eclipsed Sun. The It was october 20, 1868 when the news ar- focal lenght was of about two meters just rived in Europe that Janssen observed with allowing a good resolution on the plate. A the spectroscope the C line at the limb of comparison of these pictures with those ob- the solar disk where the day before dur- tained by De la Rue at Rivabellosa on the ing a solar eclipse there was an outstanding Atlantic side of eclipse path, clearly showed prominence. This result opened new per- the parallax effects on the Moon but the spectives in prominence observations, each identity of the position of the prominences, small refractor with just a simple spectro- confirming that they were solar in origin. scope, mounted at the focal plane, could be used to observe prominences both for 2.3. Space weather physical purposes and for just recording the geometrical parameters of the phe- The cyclic character of solar activity was nomenon. The two roman observatories: well known when a large aurora was the Collegio Romano and the Campidoglio, observed in 1872 in the night between were between the first institutions to be February 4 and 5 in a large part of equipped with the spectroscope (Monaco the northern hemisphere also at low lat- 2000). Obviously a new observation device itudes. Donati was persuaded that the triggered a large amount of observational causes of the aurora were solar (Donati routine work not very different from that 1873) or at least cosmic. In the first of the meteorologists and consequently the printed contribution the recently inaugu- need of keeping a log. Therefore the ital- rated Arcetri Observatory he published the ian community was forced to start a new results of an inquiry on the local time scientific society and a new journal: the of occurence of the maximum of the phe- Memorie degli spettroscopisti Italiani which nomenon as observed in diferent part of was mainly intended for the publication the world. He had the support of the ital- of the result of the spectroscopic measure- ian ministry of foreign affairs which sent to ments performed on the Sun, but not only. the italian consular authorities around the The first suggestion of founding the soci- world a questionnaire prepared by Donati. ety degli spettroscopisti italiani was com- Answers arrived from Shanghai, Tiensin, ing from Secchi as Tacchini writes in the Bombay, Taganrog, Damasco, Beirut,Saint Memorie (Tacchini 1872) il Secchi mi co- Petersburg, Alessandria, Smirne, Tunisi, municava l’idea di formare una societàdi Glasgow, Cadiz, Melbourne, Santiago. Spettroscopisti Italiani, i quali lavorando di From United States Donati received a de- comune accordo e secondo un programma tailed report from Joseph Henry, president stabilito, avrebbero dato in poco tempo le of the Smithsonian in Washington. As a re- richieste serie di regolari e continue osser- sult of all these reports Donati was able to vazioni per la sicura ed accelerata soluzione A. Righini: One century of Solar physics in Italy 1850-1950 559 di importanti problemi relativi alla fisica present in Secchi, Tacchini5. Riccò6. No solare. The international role of the ital- need of team work, and no need of large in- ian solar spectroscopists was reflected by strumentations. The research at this stage the structure of the Memorie. We find sev- is simple, not expensive and the intelligence eral papers of known foreign scholars and, and personal sacrifice of single scientist are sometimes, also translations of papers ap- enough to obtain universally acknowledged peared in other journals. At the end of results. The situation drastically changes in each volume of the Memorie it is possible the following fifty years. to find drawings of the shape of the observed prominences and some remarkable 3. Solar physics in Italy in the first engravings. The Memorie accepted also in- part of the XX century teresting discussions on several topics, as for instance, a project for the reform of 3.1. Antonio and Giorgio Abetti the italian observatories, reports of solar eclipse expeditions and spectroscopic stud- Italian Solar physics is in debt with 7 ies of wines for fraud detection. Antonio Abetti of the intuition that the astronomy was changing. His younger son, Giorgio was willing to work in Astronomy 2.5. The international impact of Italian and Antonio sent him several times over- Solar Physics at the end of XIX seas were new ideas were flourishing, es- century pecially due to the appassionate effort of George Ellery Hale, the Maestro, as Giorgio In the year 1885 A. M. Clerke published called him when he was in Mt. Wilson a fortunate history of Astronomy of the (Wright 1966). Giorgio met also Hale’s XIX century (Clerke 1885). The book, although written for non scientists, is very 5 Tacchini born in Spilamberto (Modena) in scrupulous in the citations and therefore it 1838 was a skillful observer and a very ac- is valuable to assess a criterion to evaluate tive scientist. In 1878 he observed in India the impact of the italian Solar Physics. We with the spectroscope the inner corona and dis- find quoted Secchi, Tacchini, Riccòin con- covered several lines in red end of the spectrum. Again Tacchini observed the coronal nection with solar arguments, other italians spectrum in 1883 in the Marquesas islands in are, obiously, Galileo, Riccioli, Donati (for the Souther Pacific and in many other solar the comet and for stellar spectra), Melloni eclipse expedition. He founded the Memorie for the lunar heat, Rosetti for the tempera- degli Spettroscopisti Italiani. He died in in ture of the Sun and Piazzi. The total quota- 1905. tion referring to solar arguments are about 6 Born in Modena in 1844 he was director sixty therefore we may define an impact of the Catania Astrophysical Observatory, He factor4 of about 0.07 over the universe of organized several eclipse expeditions and fol- solar physicist quoted by A.M.Clerke. We lowing and with hale he tried to make pho- may consider this number a very good re- tographs of the corona outside an eclipse from sult if we keep in mind the complex po- the ton of Etna using a spectroheliograph. He died in 1919 litical history of Italy in the XIX century. 7 However during large part of the XIX cen- Antonio Abetti 1846-1928 received a de- gree in engineering from the University of tury solar physics was in its serendipitous Padua. Than he was appointed astronomer at phase. In this phase all that is needed the observatory of that university and in 1894 is fantasy, intuition, and deep interest in becomes director of the Acetri Observatory in observations. All these qualities are well Firenze. Abetti’s activity was essentially de- voted to positional astronomy. In Arcetri he 4 Hereinafter, we will call impact factor the was able to carry out many observations on ratio of quoted italians over the universe quo- small planets, comets and fixed stars (Abetti tation in the considered review or book. 1970). 560 A. Righini: One century of Solar physics in Italy 1850-1950 coworker and could work with the new in- Hale to G. Abetti October 18, 1919 struments that Hale had built on top of the .....This brings the question of the design mountain; coming back to Florence, he felt of the coelostat and second mirror...... I that to give a new impulse to italian solar think the scheme indicated on the enclosed physics, a Solar Tower was needed and, as blueprint is worthy of your careful consid- early as 1919, we find the first letters writ- eration...... One of the reasons I cabled you ten by Antonio (director of the observatory to advise that work be stopped was the fact until 1924) asking for money to start the that in your drawings the second mirror building of the tower. is shown north of the coelostat instead of south, where it should be placed, if you had put in the foundations of the tower, they 4. George Ellery Hale, Giorgio Abetti would have been in the wrong position with and the Arcetri Solar Tower reference to the well [I advise you not to put the foundations and build the house at In Arcetri a large part of the correspon- foot of the tower until you have completely dence between Abetti and Hale has been all the drawings of the coelostat and spec- preserverd and catalogued thanks to the ef- trograph (and spectroheliograph)9] forts of the library staff (Baglioni et al. Hale to G. Abetti October 20, 1919 I 1999). We shall leave that the letters talk: now enclose the revised drawing, which I Hale to G. Abetti Nov. 1915 .... I think is very satisfactory...... Hale sends to am much interested to hear of Professor 8 Abetti the blue prints of the Mt. Wilson 13 Garbasso’s plans for astrophysical re- foot spectroeliograph on January 20, 1920, search in Arcetri, and shell be glad of any on February 20, 1920 Hale writes to Abetti further information you can give on this the dimensions of the optical parts. The subject...... tower was completed about 1924 and Hale Hale to G. Abetti August 26, 1919 .... writes to Abetti on October 9, 1925 ....I It was a great satisfaction to learn that con- hope your spectroheliograph is now work- struction of a tower telescope has been de- ing well and am anxious to see the first re- cided upon, and I am looking forward with sults with it. The tower does not work at its great interest to the first draft of the tower best, due to the bad quality of the gratings and location, which you promised to send and to the periodic errors of the driving in the near future.... mechanism of the spectroheliograph. Hale, Hale to G. Abetti October 10, 1919 ....I however, is favorably impressed by the very have cabled you to recommend that work be preliminary results obtained by Abetti on stopped on the well and foundations for the the large Evershed downdrafts he observed tower telescope, because I think a number above the sunspots with the spectrograph. of changes should be made in the design... The frontier of solar research was mov- .....In your drawing the second mirror is ing and the new problem was the mea- shown north of the coelostat, but it should surements of magnetic fields. Abetti is very be south...... anxious to obtain some results in this field, It is clear that Giorgio Abetti was com- but Hale in the letter dated November 23, pletely dependent on Hale about the tech- 1926 writes ...... I doubt if you can do nical characteristics of the telescope, it is much work on the magnetic fields in sun- strange that he passed a drawing with a so spots, as your solar image is too small for big error i.e.: the secondary mirror north of any but the larger spots and the dispersion the coelostat. of a 13 foot soectrograph is too low. In fact, 8 Antonio Garbasso was professor of physics as I found with our 30 foot spectrograph, a at the Regio Istituto di Studi Superiori, which much higher dispersion is really needed for will become in 1926 the Universitàdegli Studi di Firenze 9 Handwritten by Hale in a typed letter. A. Righini: One century of Solar physics in Italy 1850-1950 561 this work, and this is why we built the large effect11 and by the scarce popularity of tower. Abetti was obviously not very happy scientific magazines in which italians were of what his Maestro was writing to him used to publish. Using this list the risk and tries hard to obtain better results with could be to overestimate the italian con- the tower. Hale to Abetti October 11, 1927 tribution but a punctual analysis of the .....the spectroheliograph pictures you sent papers shows the great equilibrium used me seem to be very good, except of for the by Abetti in its quotations. In Abetti’s re- lack of good definition. I believe however, view we find about 13 % of citations refer- that they are as good as I shall be able to ring to italian scientists (including Galileo, obtain here in my solar laboratory10, where Secchi, and one work of Millosevich on his- the seeing is certainly not very promising. tory of Astronomy). Many of them refer This letter may be considered the closing to old classical solar topics as the height end of the early history of the Arcetri Solar of solar chromosphere, the size and dis- tower. A blue print of an old instrument, tribution of prominences, and some to his the first solar tower of Mt. Wilson, built in own work on the Evershed effect. We find Italy under the supervision of Hale and in some quotations also to results about the part with the money of the Hale founda- spectra of prominences obtained by Taffara tion. We must say that Abetti had great (Taffara 1929) in the Solar eclipse expe- difficulties to collect all the money needed dition. As soon as the review proceed to to build the tower, and that he would not describe the magnetic phenomena on the be able to build a larger one. Moreover he Sun we do not find any quotation of ital- had not the key technology, i.e.: he was ian work. This is obvious, the Arcetri so- not able to build gratings and he had to lar tower was not able to provide any re- accept the grating that were not good for sult in this field, as it did in the case of Mt. Wilson Solar Tower. How difficult was the Evershed effect. At the end of Thirties for him to put the solar tower in opera- Unsöldpublished a book (Unsöld 1938) tion comes out clearly reading the tower which has been generally recognized as fun- logbook. The tower however played a key damental for a generation of astrophysicists role in the development of Solar Physics in working from the Fourties to the Fifties Italy until it was used as a research instru- of XX century. We find quoted the review ment and not for routine observation. by Abetti in Handbuch der Physik, some other works by Abetti (Abetti 1932) and Calamai (Calamai 1934) on doppler shift of 5. Solar physics in thirties of the XX lines in prominences, a statistic on latitude century in Italy drift of prominences during the solar cycle by Bocchino (Bocchino 1933), a paper by The extended and well documented re- G. Righini (Righini 1935) on the center view by G. Abetti (Abetti 1929) in the limb-variation of sodium D line, written on Handbuch der Phisik is very useful to un- data obtained at the Arcetri solar Tower, derstand the quality and the international and some old papers by Mascari on active impact of the italian solar physics at the regions and Riccòon prominences. We note beginning of the thirties of the XX century. The review by Abetti is comprehen- 11 Language was a great barrier in the spread- sive of every aspect of solar physics ad it ing of the scientific results obtained by ital- has been universally acknowledged, in later ian researchers; they published in italian and reviews, as well documented and complete. mainly in the observatory series, which were The reference list of this review should exchanged by observatories, but were not so therefore be considered, from the italian attractive for the reader as could be the point of view, as unbiased by the language Astrophysical Journal and the Zeitshrift für Astrophysik. Perhaps this provincial choice 10 In Pasadena. might be dictated by political constraints. 562 A. Righini: One century of Solar physics in Italy 1850-1950 that in Unsöld’shandbook no other italians Also the observations by Fracastoro on the were quoted and all those who Unsöldcon- flash spectrum and on the variation of the sidered worth of quotation were solar physi- height of the chromosphere deserve quota- cists. If we want to evaluate a sort of impact tion (Fracastoro 1939), Fracastoro (1941). factor of italian astrophysics from Unsöld In the contribution on solar activity written point of view we do not go further than a by Kiepenheuer we find again quoted ital- scarce 1%. ian studies of the Evershed effect (Abetti 1932), about the spectra of prominence (Calamai 1934) and about the fluctuations 6. Italian Solar physics at the end of of solar diameter (Cimino et al. 1946). In the fifties of XX century the other reviews, enclosed in the book, In the year 1952 Gerard P. Kuiper signed about line classification, radioastronomy, the introduction to the first volume of solar observing techniques, we do not find a series of four intended to give a sys- any quotations of italian work. tematic and comprehensive account of our present knowledge concerning the sun and 7. Conclusions the other members of the solar system (Kuiper 1953). The first volume deals with Italian Solar Physics well reflects the limits the Sun, and the different topics are re- of the italian scientific community in the viewed by outstanding personalities in their considered period. As soon as the results field presumably well informed on contri- of the research depend only on the qual- butions given by researchers all over the ity of the single person we find outstand- world. Leo Goldberg, in the introduction ing achievements as in the case of Donati, on the Sun, quotes and appraises the re- Secchi, Tacchini. We find also interesting view by Abetti (Abetti 1929) and two in- results from the systematic observations of teresting works by Cimino (Cimino 1944) the Sun as the studies on the height of (Cimino 1946) in connection with the the chromosphere and on solar diameter. Secchi Rosa effect, Strömgren,in his re- When the research needs large equipments view, dealing about the Sun as a star does and advanced technologies, Italy is not able not find any italian work worth to be cited, any more to compete (as in the case of so- while Minnaert, reviewing the solar atmo- lar magnetic fields), and researchers, if they sphere quotes several works as those on want to succeed, the should migrate as the the Evershed Effect (Abetti et al. 1935) italian nuclear physicists did at the end of or on spectral line profiles (Barocas et al. the thirties. 1951), (Righini 1933), (Righini 1946) and The story of the Solar Tower in Arcetri (Rosino 1939) or on a discussion on the tells that, notwithstanding the fact that the sharpness of solar limb(Righini 1943). Van instrument was a copy of an aged proto- der Hulst, dealing with the Solar Corona, type, it could have given results if used shows the importance of the italian activity in a different way, i.e. as spectrograph, in in coronal studies, especially of the fortu- that role the instrument was really out- nate expedition in Siberia for observing the standing ap to the beginning of the fifties. 1936 Solar Eclipse. We found quoted stud- Unfortunately the tower was dedicated to ies on minimum and maximum of the fre- the routine work of taking Ca II K and quency of high latitude prominences in con- Hα spectroheliograms preventing de facto nection with ellipticity of coronal isophotes its use for reseach. Abetti (1938) Biozzi (1939), while the Acknowledgements. Part of this work was sup- observation of the coronal spectrum and ported by Universitàdegli Studi di Firenze. its depression at λ 3850 A˚ is aknowl- The author thanks Giannina Poletto for sug- edged, although non exclusively, to ital- gesting this work. The author also whishes ians Doplicher (1946), Righini (1942). to thank Antonella Gasperini and Francesca A. Righini: One century of Solar physics in Italy 1850-1950 563

Brunetti for their help in the search of biblio- Donati G.B. 1866, Annali R. Museo di graphical sources in the Arcetri Library and in Fisica e Storia Naturale per il 1865, the Arcetri historical archive. Firenze Donati G.B. 1873, Pub. Oss. Arcetri, 1 References Doplicher, V. 1946 Atti Acad. Naz dei Lincei, cl. sci. fis. ser VIII, 1, 63 Abetti G. 1929 Solar Physics in Hanbuch Kuiper G. 1953 The Sun The University of der Astrophysik, 6, 57 Chicago Press, Chicago Abetti, G. 1932 Publ. R. Osserv. Arcetri, Fracastoro M.G. 1939 Reale Accad. d’Italia 50, 47 Rend. cl. sci. fis., 1, 134 Abetti G. 1970 Dictionary of scientific bi- Fracastoro M.G. 1941 Reale Accad. d’Italia ography, 1, 19 Schribner’s, New York Rend. cl. sci. fis., 2, 979 Abetti G, Castelli, R. 1935 Pub. R. Osserv. Monaco G. 2000 L’astronomia a Roma, Arcetri, 53, 25 dalle origini al novecento Pubbl. Abetti G. 1938 Pub. R. Osserv. Arcetri, 56, Osservatorio Astronomico di Roma 53 Righini G. 1933 Publ. R. Osserv. Arcetri, Baglioni R., Gasperini, A., Grisendi, T. 51, 57 Arcetri Technical report 3/99 Righini G.: 1935, ZS. f. Ap., 10, 344 Barocas V. Righini G. 1951 Ap, J., 114, 443 Righini G. 1942 Zs, f. Ap, 21, 158 Biozzi M. 1939 Pub. R. Oss. Arcetri, 57, 5 Righini G. 1943 Publ. R. Osserv. Arcetri, Bocchino G. 1933, Publ. R. Osserv. Arcetri, 61, 67 51 Righini G, 1946 Publ. R. Osserv. Arcetri, Bonelli M.L. 1971 Dictionary of scientific 63, 61 biography, 4, 161 Schribner’s, New York Righini G. 1978 Padre Angelo Secchi: lo Cagnoli A. 1827, Notizie astronomiche di scienziato, commemorazione nel cente- Antonio Cagnoli adattate all’uso comune nario della morte, lecture given in Reggio Reggio, per Pietro Fiaccadori Emilia, Feb. 2, 1978 Calamai G. 1934 Pub. Oss. Arcetri, 52, 39 Rosino L. 1939, Mem, Soc, Astron. It., 12, Cimino M. 1944 Comm.Pont. Acad. Sci., 8, 233 485 Secchi A. 1884 Il Sole, Firenze Cimino M. 1946 Atti Acad. naz. Lincei, 1, Tacchini P. 1872 Mem, Soc. Spett. It., 1, 1 624 Taffara L. 1929 Mem, Soc. Astr. It., 4, 395 Cimino M., Armellini G. 1946 Proc. Nat. Unsöld A. 1938 Physik der Acad., 1, 624 Sternatmosphären, Berlin, Verlag Clerke A.M. 1885, A popular history of von Julius Springer Astronomy during the nineteenth cen- Wright H. 1966 Explorer of the Universe, tury, Adam & Charles Black, Edimburgh a biography of George Ellery Hale, E.P. Dutton & Co., Inc., New York