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


“Report of Prof. G.B. Donati, Director of the R. Observatory of Florence”

Published in “Rapporti sulle osservazioni dell’eclisse totale di sole del 22 dicembre 1870 eseguite in Sicilia dalla Commissione Italiana, Palermo: Stabilimento Tipografico Lao, 1872” (see Na. 11). (Report on the observations of the total solar eclipse of December 22, 1870, made in Sicily by the Italian Committee, chaired by Prof. Giovanni Santini)

214 p., ill., tav.; 331 mm
tav. III, 331×241 mm

INAF-Library of the Arcetri Astrophysical Observatory

Scheme of a six-prisms spectroscope drawn by Donati and presumably built by the Officina Galileo for the observations of the total solar eclipse of Dec. 22, 1870 (see Sect. 3). The spectroscope consists of a collimator (A) to be fixed to the telescope, having a slit at one extreme. On passing through the collimator, the light beam is dispersed by a system of six prisms of flint glass of high refractive index, and is refracted toward the observation telescope (B), which can be rotated perpendicularly to the optical plane of the instrument, but is positioned almost longitudinally to the telescope as in a direct-vision spectroscope. The last prism could be rotated independently or integrally with the telescope to explore various sections of the solar spectrum. An additional tube (C) carried a graduated glass scale that was projected by a lamp on the outer face of the last prism and reflected into the telescope. The spectroscope combined high dispersion with a good extension of the spectrum, being able to cover the range from the Fraunhofer C line in the red to the F line in the blue by simply rotating the observing telescope around its axis. In 1876 the instrument was presented to the “special loan collection of scientific apparatus” at the South Kensington Museum in London. It is currently held in the Museo Galileo in Florence.

We also show Donati’s drawing of the light path through the six-prism system (INAF-Arcetri Astrophysical Observatory, Folders Donati).