Pure air is blue, because, according to Newton, the molecules of the
air have the thickness necessary to reflect blue rays. When the sky
is not perfectly pure, and the atmosphere is blended with perceptible
vapours, the diffused light is mixed with a large proportion of
white. As the moon is yellow, the blue of the air assumes somewhat
of a greenish tinge, or, in other words, becomes blended with
yellow.--_Letter from Arago to Humboldt_; _Cosmos_, vol. iii.