Photo-electric cells of various types are becoming more widely used every day for the measurement of illumination, both natural and artificial. Their great convenience renders them specially suitable for the measurement and recording of daylight. A word of warning is, however, necessary. What the cells measure is not really “ light,” i.e. , the physiological effect produced on the eye by the radiation, but rather the physical effect produced on the cell by the same radiation. It has been argued with some justice that it is therefore unscientific to use the ordinary units, derived from visual measurements, to measure the effect on the cell. The argument would carry more weight if these units were strictly limited to the measurement of illuminations of definite spectral composition, and so did not attempt to compare effects which are essentially dissimilar. For practical convenience, however, we are driven to measure in “ candlepower ” illuminants differing widely from the standard candle in spectral composition.