A device for making comparator settings by photoelectric scanning of spectrum line images is described. The fiducial position is determined by matching mirror image oscilloscope traces of the line contour. This method combines the advantages of increased speed, precision, objectivity of setting, and decreased eye and nervous strain on the operator. The device is readily adaptable to comparators of the fixed microscope moving plate type, and conversion from photoelectric to visual setting can be accomplished in a few minutes.
FRANK S. TOMKINS and MARK FRED, "A Photoelectric Setting Device for a Spectrum Plate Comparator," J. Opt. Soc. Am. 41, 641-641 (1951)