A PHOTOMETER is described with which the scotopic candlepowers of very feeble light sources (down to about 10−9 candle) can be measured. Small spots of radium paint, 2 or 3 mm. in diameter and having candlepowers in the range covered, are now used (Livingston) to plot the dark-adapted visual field, and the present photometer was designed primarily for their calibration. The radium paint emits green light but the instrument can be used for measurements of scotopic candlepowers of any colour. By tests on colour filters whose scotopic transmission factors can be calculated from their spectral transmission curves, it is shown that the photometer does, in fact, compare lights on the basis of the scotopic visibility curve of the eye, provided the conditions of measurement are those indicated.
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