Measurements of illumination recorded by a Weston selenium rectifier cell under opal-flashed glass, standardized against artificial "mean noon sunlight," and simultaneously by a thermopile, showed that the radiation of a clear blue sky had a higher luminous efficiency than sunlight. The variations under different meteorological conditions were large, pure sunlight varying from 35.5 to 120 lumens/watt, and sky light from 48.5 to 158 lumens/watt. The usually higher values for sky are attributed to its poverty in the infra-red. Mixed sun and sky light values ranged from 81.1 to 119.4 lumens/watt. In expts. in which a Kodak special green filter was used under the opal glass to reduce the spectral sensitivity of the Weston cell to the visual scale, the sun on one occasion had a higher luminosity than the sky, which was deep blue. The filter and opal combination caused some additional absorption at the blue end of the spectrum. Measurements with a Na vacuum cell, sensitive to the blue end of the spectrum only, gave results comparable to those obtained with the Se cell for mixed daylight, but in separate determinations of sun and sky light the luminous efficiencies found differed considerably from the true visual values.