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The difficulties encountered in photometry at low levels of brightness are in a wide measure due to changes in the spectral characteristics of the eye as it becomes light- or dark-adapted. A study has been made of filter glasses suitable for correcting two types of multiplier photo-tubes used in photometric work where there is a wide variation in field brightnesses. Results of this study are described, and filter glass numbers as well as thickness specifications are given for filters used to correct RCA 1P21 and 1P22 tubes over brightness levels corresponding to the Purkinje range.
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W. S. Plymale (1947) studied this question.
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