Sea-water is sometimes a rich profound blue, and sometimes a pale sickly blue. It is sometimes dull, sometimes brilliant. It is sometimes green, and sometimes almost black. For these variations, even while accepting the new explanation, it has been found needful to revert to the old theory in quest of added help. The sea is blue, because it really and truly _is_ blue. But that is not all. It is a deeper or less deep blue, a duller or brighter blue, a greenish blue or even a dull yellow, because of the vast supplies of dust floating in it. The Mediterranean was particularly examined, on account of its remarkable depth and brilliancy of colouring; and it was found to be exceptionally laden with dust, both fine and coarse, brought from land by innumerable rivers, and also pounded by breaking waves on the long surrounding coast-lines. Such floating particles, at all events the bigger ones, are no longer supposed, as once they were, to reflect only the tiny blue light-waves. On the contrary, they are believed to reflect all the waves of light indiscriminately, whether large or small; while the sea, by virtue of its own power as a Blue material, captures all those reflected waves except the blue, and allows the latter alone to reach our eyes. Near British shores the water has often a strongly green tint; and this may be due in part to floating particles of sand. The yellow of the sand, combining with the blue of the water, would naturally form green. Possibly also the sea, near shore, is sometimes affected in colour by floating sea-weeds, green-tinted. The blue of the sky is often reflected on the sea-surface, and sometimes so powerfully as to overcome the real ocean-colour. One hardly understands how intense such a sky-reflection may be, until one has watched the whole sea transformed into glowing crimson or gold from a radiant sunset. Much the same perplexity has been felt about the blue colour of air, as about the blue colour of the ocean; and much the same course of explanation has been followed. Its blueness was long maintained to be due only or mainly to the “scattering of sunlight” by infinite numbers of floating dust-specks, which also serve to account for the golden and crimson tints of sunset. It now appears that oxygen, an important element in the atmosphere, has a faint blue tint of its own, like water, which at least helps to answer for the blue of the air.