This circle was designed to fit atop a U.S. Navy Standard Compass. The
Ritchie ledgers, now held by Ritchie Navigation, indicate that it was made on
April 10, 1941 and sold to T.S. & J.D. Negus, a New York firm that sold
navigational instruments. The U.S. Naval Observatory transferred it to the
Smithsonian in 1966.
This circle has two mechanisms for taking sights. In one, the rays of the sun are reflected from a cylindrical convex mirror to a right-angle prism on the
opposite side of the ring, and then through a cylindrical lens below, appearing
on the card as a bright bar of light. The other consists of sight vanes, hair line and reflector, for taking bearings of terrestrial objects.
E. S. Ritchie received his first patent (#49,157) for an azimuth circle in
1865, and another (#481,625) in 1892.
Ref: G. Dutton, Navigation and Nautical Astronomy (Annapolis, Md., 1948), pp. 33-34.
