A device for measuring photographic oscillogram coordinates is described. By means of a photoelectric balancing technique the instrument allows the locating of the ``middle'' of a photographic line with an error of the order of a few microns. The ``middle'' of a line is defined by the instrument as being the point of equal transmission, which is the location on the line at which there are equal amounts of light transmitted on each side. Since the photoelectric comparator is a null device, nervous strain on the operator is greatly reduced.