21 March – 21 June 2016 in Florence, Rome, Naples, Palermo, Padua


Officina Galileo, Florence 1872

tarnished brass, glass

Florence, Museo Galileo, inv. 1396

This instrument consisted of 25 prisms, 16 of which being made of extra-dense flint glass. It used 3 series of direct-vision prisms (each consisting of 5 prisms), two positioned between the slit and the lens in the collimator (long tube) and one between the lens and the eyepiece of the telescope (short tube); nowadays only one survives. A spiral pattern with 10 flint glass prisms is located inside the main body of the instrument. Due to its high dispersive power, the instrument allowed to observe only a limited portion of the spectrum. Initially it was calibrated to observe the region of the Fraunhofer C line in the red part of the spectrum; after changing the internal layout, in June and July 1872 it was used to observe other lines, such as the D line in the yellow and the E in the green (belonging respectively to the spectrum of sodium and iron). In particular, in July 1872 Donati used a version with 20 prisms, probably obtained by removing one of the three series of direct-vision prisms. Even in this case, despite the reduced dispersion with respect to the full 25 prisms layout, the instrument allowed to observe at once only a portion of 50 Å of the spectrum. This instrument was presented by the Officina Galileo to the Universal Exhibition in Vienna in 1873. The drawing is from Officina’s catalog for 1881.

Bibl.: DONATI 1872a, 1872b; G. B. Donati, “Solar spectroscopic observations with the small equatorial, Fraunhofer objective and a 20-prism telecospe, July 1872”, manuscript, Historical archives of the Museo Astronomico e Copernicano, Fondo Donati.

Foto: Museo Galileo, Firenze – Archivio Fotografico