This is a conical glass lens on a brass base that would fit on an optical bench. The "Wm. Grunow, New York" inscription refers to WIlliam Grunow (b. 1830), a noted German-American optical instrument maker.
Ref: D.J. Warner, "Julius and William Grunow," Rittenhouse 3 (1989): 41-48.
National Museum of American History