On the Limit of the Solar Spectrum, the Blue of the sky, and the Fluorescence of Ozone

Abstract

THERE are two facts of particular interest which have been observed in connection with the light which we receive from the sun and the sky. First, though the ultra-violet spectrum of the sun is very well represented by the iron spectrum taken from the electric arc, yet its length is nothing like so great, and there is no fading away of feeble lines and a weakening of strong ones, which would be the case if the rays were affected by a turbid medium through which they were transmitted, but there is a sudden and sharp extinction which points to a very definite absorption. Secondly, light from the clearest sky, unaffected by aqueous vapour, is of a deep and beautiful blue colour, more of an indigo-violet tint than ordinary so-called sky-blue. There is nothing more beautiful in Nature than the blueness of the heavens.