This sextant has an aluminum frame, and a bubble-type artificial horizon
mounted below the horizon glass. Karl Schwarzschild designed the form around 1910. The U.S. Naval Observatory transferred this example to the Smithsonian in 1963.
August Spindler and Adolf Hoyer began in business in Göttingen in 1898, specializing in fine optics and precision engineering. Their instruments for optical research, sea and air navigation, and seismography have been used in Germany and around the world. They are today part of the LINOS Group, based in Göttingen.
Ref: K. H. Beij, "Astronomical Methods in Aerial Navigation," Report of the National Advisory Committee for Aeronautics 198 (1924), p. 16.