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

TWENTY-FIVE-PRISMS SPECTROSCOPE

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