In this paper data are presented to show that for maximum photometric sensibility the brightness of the photometric field should be adjusted so that a brightness level of from 20 to 30 millilamberts is secured. At this brightness the difference fraction ΔB/B has a value of 1.37 percent. Further the precision of photometric observations as expressed by the average percentage deviation, from the mean, of a single observation is 0.19 percent at the above mentioned optimum brightness level. At this same field brightness the maximum percentage deviation from the mean of a single observation was found to be 0.41 percent.
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