Photometric calibration of magnitudes extracted from a two-exposure plate (archive ID 1, plate ID 20149). The observation was made with the Zeiss-Triplet telescope on May 23, 1910; exposure 1 lasted 610 s and exposure 2 was 190 s. Panel a shows the relation between raw and natural magnitudes for two exposures. Fitted calibration curves are drawn with solid lines. Dashed lines denote limiting magnitudes in the raw and natural magnitude scales. Panel b shows the distribution of plate magnitudes for both exposures.