This is a compound monocular designed for petrographic work. It has coarse and fine focus, analyzer in the tube, square stage with circular top graduated to degrees, sub-stage polarizer, sub-stage mirror, heavy horseshoe base, and wooden box. The inscription on the tube reads “Dr. E. Hartnack / Potsdam.” The serial number “19544” appears on a smaller wooden box that holds seven lenses. A brass plate on the box reads “C. Whitman Cross.”
Edmund Hartnack (1826-1891) was an accomplished microscope maker in Paris who received an honorary doctorate from the University of Bonn in 1868, moved to Potsdam in 1870, at the start of the Franco-Prussian War, and adopted the "Dr. E. Hartnack" signature in 1879.
Charles Whitman Cross (1854-1959) was an American geologist who graduated from Amherst College, studied in Göttingen, and received a PhD from the University of Leipzig. His dissertation was supervised by Ferdinand Zirkel, an early proponent of microscopical petrography, the practice of using a polarizing microscope to observe thin sections of rocks. Joining the U.S. Geological Survey, Cross specialized in the classification of igneous rocks. He became an active member of the National Academy of Sciences, and an Associate in Petrology at the Smithsonian Institution.
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