Giuseppe Poggiali e Giovan Battista Donati, Firenze, 1865

burnished brass, glass, length 530 mm

INAF-Brera Astronomical Observatory, Astronomical Museum and Brera Botanical Garden, inv. 1921 (7)

Donati recommended the purchase of this instrument to Schiaparelli, director of the Brera Observatory in Milan, where the spectroscope arrived in May 1866. In a letter to Schiaparelli, Donati described the instrument and its working in these terms: The main part of the spectroscope is a compound prism made of five prisms glued together that disperse the white light without diverting it from its direction. In front of this prism is a slit that can be expanded or tightened as needed […], in front of which is a cylindrical lens that collects the greatest amount of light on the slit […]. The spectrum of the light passing through the cylindrical lens and the slit, produced by the compound prism, is observed with a small telescope placed behind the prism. There are two lamps: one, filled with salty alcohol, can project the reference line of sodium in the field of the small telescope by means of a small prism placed in front of the slit; the other, containing lamp oil, illuminates a micrometer scale that appears bright on the dark field of the small telescope used to observe the spectrum. There is a piece […] to which the entire instrument is connected with a bayonet fitting, when the instrument is attached to the telescope in place of the ordinary eyepieces. The manufacturer’s label inside the wooden case reads: Workshop of optical and mathematical instruments directed by G. Poggiali under the scientific supervision of Professors G.B. Donati and T. del Beccaro. Specimens of this instrument were also sold to Otto Wilhelm Struve (1819- 1905), Annibale de Gasparis (see Na.07), and Gaetano Cacciatore, respectively Directors of the Observatories of Pulkovo, Capodimonte and Palermo. The Palermo specimen it is still preserved; another is at the Museo Galileo in Florence.

We also show a drawing by Donati with the light path through the Amici/Janssen direct vision prism used used in this spectroscope. (INAF-Arcetri Astrophysical Observatory, Historical Archive, Folders Donati).

Bibl.: Letter of Donati to Schiaparelli, Firenze, April 13, 1866, Brera Archives, Scientific Correspondence, fasc. 141, n. 39; MIOTTO ET AL. 1989; ABALAKIN 2009; CHINNICI 2000.