Prism objective

Merz, Monaco di Baviera, 1864
glass, brass
Diameter 162 mm
Refraction angle 12°.
Museum dell'INAF-Astronomical Observatory of Rome, Monte Porzio Catone (Rome)

In 1869 Lorenzo Respighi experimented with the use of the prism lens (Figure 1), introducing in stellar physics the use of this technique which had initially been proposed and adopted, with poor results, by Joseph von Fraunhofer, German physicist and astronomer, father of spectroscopy.
In the same years, Secchi designed and commissioned to Merz the prism shown here, which he used by placing it in front of the objectives of the Cauchoix and Merz telescopes of the Roman College Observatorye.
A strong controversy over the authorship of the invention of the instrument broke out between Secchi and Respighi ; the first observations made with their prisms were in fact published almost simultaneously.
Secchi commissioned his instrument to Merz in 1864. The instrument is described in the two main astronomical treatises of Secchi, Le Soleil and Le Stelle; It was initially equipped with a circular armature that was inserted as a cover on the lens of the Merz telescope, facilitating its positioning. This original support has been lost; the prism is now inserted into a brass metal ring, used to adapt it to the Cauchoix telescope. Thanks to this instrument, Secchi was able to extend its original spectral classification, introducing up to a fifth class, comprising stars whose spectra have hydrogen lines in emission, such as Γ Cassiopeae.

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