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Hemoglobinometry that compares a blood sample to a color chart, in use circa 1850 - 1950
A hemoglobinometer made by the American Optical Company. The light of a battery-driven lamp is travelling on two paths to the eyepiece. One path goes through the blood sample, the other one through an adjustable filter. When the brightness of the two paths is equal, the hemoglobin level can be read on the device. Such devices were made in the 1940s.