Merz, Munich 1864

Glass, brass;
162 mm diameter
refraction angle 12°

INAF – Rome Astronomical Observatory

Secchi ordered this instrument from Merz in 1864 and used it to deepen his studies on stellar spectra. The prism is described in the two main astronomical treatises by Secchi, Le Soleil and The stars; it was initially provided with a circular shield which, as a lid, would be accommodated on the objective of the Merz telescope, thus making its placement easier. This original support is now lost; the prism is currently placed inside a brass ring, through which it could be adapted to the Cauchoix telescope. With this instrument Secchi was able to extend even further his spectral classification, by adding a fifth class, which included stars whose spectra present Hydrogen emission lines such as those from γ Cassiopeae. [i.c.]

Bibl.: Calisi in Chinnici (ed.) 2009, p. 200; Secchi 1875-77, I, p. 460.