Absorption of Sunlight by Atmospheric Sodium

A photometer has been built which is sensitive to light only in a very narrow spectral region near the sodium D-lines. The light is scattered by sodium vapor at low density and the scattered light recorded by a photoelectric spectrometer. The latter resolves the two lines and allows the effect of stray light to be eliminated. With sunlight as the source, the radiation detected is at the bottom of the Fraunhofer lines and the residual intensity there may be measured. It is found that this intensity decreases slightly (relative to the continuum) as the sun approaches the horizon. This effect is attributed to atmospheric sodium; the amount found by this method agrees well with that deduced from twilight measurements. The thickness of the sodium layer in atoms/cm. 2 varies between 3 and 9 × 10 9 in winter and is about 1 × 10 9 in April. The measurements also give the residual intensities of the Fraunhofer lines in the incident sunlight, averaged over the whole of the sun's disk. These are (5.90 ± 0.46)% for D1 and (5.06 ± 0.24)% for D2.