The ur-phenomenon having once been discovered in the heavens, could then easily be found elsewhere in nature on a large or small scale-as, for instance, in the blue of distant hills when the air is sufficiently opaque, or in the colour of the colourless, slightly milky opal which looks a deep blue when one sees it against a dark background, and a reddish yellow when one holds it against the light. The same phenomenon may be produced artificially through the clouding of glass with suitable substances, as one finds in various glass handicraft objects. The aesthetic effect is due to the treated glass being so fashioned as to present continually changing angles to the light, when both colour-poles and all the intermediate phases appear simultaneously. It is also possible to produce the ur-phenomenon experimentally by placing a glass jug filled with water before a black background, illuminating the jug from the side, and gradually clouding the water by the admixture of suitable substances. Whilst the brightness appearing in the direction of the light goes over from yellow and orange to an increasingly red shade, the darkness of the black background brightens to blue, which increases and passes over to a milky white.