This is a conical glass prism mounted on a brass base that, in turn, would fit on an optical bench. The “J. DUBOSCQ / A PARIS” inscription refers to Jules Duboscq (1817-1886), an optical instrument maker who began working on his own in the early 1850s, and who noted that this instrument produced a circular spectrum. This item was used by cadets at the U.S, Military Academy.
Ref: Jules Duboscq, Catalogue Systématique des Appareils D'Optique (Paris, 1870), p. 19.
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