Hartmann Spectrocomparator Diagram
A diagram of Hartmann Spectrocomparator device to measure accurate the star's displacement. The photographed star is clamped on to the device. Then the series of light is reflected onto the photograph into the eyepiece to measuring the star's motion.
Encyclopaedia Britannica (11th ed., vol. 18) (New York, NY: The Encyclopaedia Britannica Company, 1910)
Courtesy the private collection of Roy Winkelman