This sextant belonged to Columbia University in New York and dates from
around 1880. The frame is brass. The silvered scale is graduated every 10
minutes from -5o to +180o and read by vernier with tangent
screw and swinging magnifier to 10 seconds of arc.
Ref: Deborah J. Warner, "American Octants and Sextants: The Early
Years," Rittenhouse 3 (1989): 86-112, on 108-109.