THE MEAN ERROR OF PHOTOGRAPHIC PLATES by P. AHNERT, Sonneberg For a critical examination of small fluctuations of light, especially of irregular stars, it is desirable to know the uncertanties of the photographic layer. A priori, we can suppose that these errors might be smaller at mirrors and Schmidt-cameras than at lenses because the images of the stars will be unfavourably influenced by the chromatic deviations. Some observers assume that the mean error of a plate taken with the Schmidt-camera is about +-0.02m or at most +-0.03m. For the plates taken with our Tessar lenses, with a diameter of 47 and 140 mm, I found a considerably higher value. I have estimated the two Cepheids TX and VX Cygni on 45 plates of the 140 mm-Tessar seven times on different days. The magnitudes of the comparison stars have been derived from the S. A. 40 which was photographed on the same plates. Then the mean light curves were drawn by aid of the means of the seven estimations. If I suppose that the means of the estimations give the true brightness on the plate, further that the mean light curve gives the true brightness of the star at each phase, then the deviations of the single estimations from their means must give the mean error of the observations, and the deviations of the single estimations from the mean light curve must give the total mean error caused by plate and observer. The square root of the difference of squares of both mean errors will then nearly give the mean error, caused only by the condition of the plate. The mean error of the estimations has been found to be +-0.068m for TX and +-0.062m for VX Cygni. The mean error of all estimations in relation to the mean light curve was +-0.094 and +-0.107. The mean error of the plates alone then becomes +-0.065m and +-0.087m respectively. In the same manner I investigated two series of photographic observations of R Sge, got with the 47 mm and 140 mm Tessar cameras. Only four single estimations have been made. The results were as follows: Mean error of one estimation little camera big camera in relation to the mean value............ +-0.074m +-0.037m Mean error of all estimations in relation to the mean light curve......... +-0.101m +-0.075m Mean photographic error of one plate....... +-0.069m +-0.065m If I give half the weight to the observations of R Sge, the mean error of a photographic image on plates, taken with our Tessar cameras, becomes +-0.073m. These results have been confirmed by photoelectric measurements of TX and VX Cygni on the same plates. The mean error of one measurement in relation to the mean light curve became +-0.065m and +-0.095m respectively, in a near agreement with the values found above. The larger mean error of the VX Cygni observations possibly has been caused by the faintness of the images on some plates near the minimum.