Signedunder prism's arm: SOLEIL / RUE DE L'ODEON / A PARIS
Inscribedon base: 9 103
on base: 01
FunctionPrisms are used to create a light spectrum, an artificial rainbow. Wikipedia offers a nice description, with images, of how it works. See the following website.
This instrument is used for demonstrating and comparing the indices of refraction of different types of glass. It fact, this instrument produces seven spectra, the position and extension of which depend on the characteristics of each type of glass.
A video demonstration of a polyprism in use, recorded at the Fondazione Scienza e Tecnica in Florence, is available on youtube here.
Related WorksPaolo Brenni, "Soleil, Duboscq, and Their Successors," Bulletin of the Scientific Instrument Society 51 (1996): 7-16
Paolo Brenni, Gli Strumenti di Fisica dell'Istituto Tecnico Toscano: Ottica (Prato: Giunti, 1993), 82-83.