A prototype comparator was constructed which enabled exact comparison of negative photographs taken of works of art. The comparator contained two channels, in one of which the photograph of the known work was inserted. The second was used to form a coincident image from the original work, photograph it, and then project the negative simultaneously with that in the first channel; coincidence of all details provided identification. Applications of the comparator to paintings, drawings, objects, and works of art in situ are described.