Direct vision spectroscope

J. Hofmann, Parigi, 1862
Brass, glass
Length cm 40
Museum of INAF-Astronomical Observatory of Rome, Monte Porzio Catone (Rome)

The direct vision spectroscope, conceived by the Florentine astronomer Giovanni Battista Amici, consists of a series of glass prisms of different refraction grade, glued in succession, through which the beam of light to be analysed passes through. In 1862, on the occasion of stay of Jules Janssen in Rome , who used a similar one for his studies on the solar spectrum, Secchi had the opportunity to test the performance of the instrument. Secchi ordered the Parisian optician Hoffman a five-prisms spectroscope, shown here, which he used in 1863 for his first observations of stellar spectra. Later, Secchi commissioned Merz to produce another, better-quality, Direct-Vision Spectroscope, with which in 1867 he could achieve his spectral classification of stars.
The spectral classification of Secchi constituted the basis of successive classifications, in particular those carried on at the Harvard Observatory, in the United States, at the end of the nineteenth century. These studies have made it possible to determine the physical parameters and the chemical composition of the stellar atmospheres and, together with stellar photometry, have represented the observational basis of the models describing the structure and evolution of stars and of the stellar systems.
Various combinations of prisms were used by Secchi also for spectroscopic observations of the solar prominences (Figure 1). While Lorenzo Respighi, director of the Capitol Observatory, used the “enlarged slit” method- he was able to obtain panoramic images of the protuberances by placing the slit of the spectroscope tangent to the solar edge – Secchi preferred to apply the method of “narrow slit” (Figure 2), which allowed to do a more detailed scan of the prominence, observing from time to time small strips of the same and allowing to “rebuild by sight” the general image of the prominence.

Aldo Altamore, Università degli Studi “Roma Tre”
Marco Faccini, INAF-Osservatorio Astronomico di Roma

Altamore A. 2012, “La nascita della Nuova Astronomia”, in Angelo Secchi- L’Avventura Scientifica del Collegio Romano, a cura di A. Altamore e S. Maffeo, Ed. Quater, Foligno 2012, p. 128 e ss; ibid., pp.131 e ss;
Secchi A. 1877, “Le Stelle. Saggio di astronomia siderale”, F.lli Dumolard, Milano 1877.