||T is now well known that the light of the night sky has little in common with the day sky. When the sun is 18° below the horizon, and the moon also below the horizon, night conditions may be considered to be established. A clear sky is of course necessary for the study of the luminosity. Unlike the day sky, it is found to exhibit very little polarisation. The intensity is considerably below the threshold of colour vision, and subjective impressions about its colour, which is sometimes described by imaginative writers as blue, have no basis in reality.