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COLOURS IN STILL WATER 83 other theory explains the colour by supposing that water has a selectzve absorption for the rays of the red end of the spectrum—¢hat water is in fact a blue transparent medium. Three different methods were adopted of testing the correctness of these rival theories, and all three proved the water of the Mediterranean to be blue by selective absorption, and show that light in passing through the water has the rays of the red end of the spectrum absorbed, and only those of the blue end transmitted.” In the same paper Dr. Aitken shows how, while the selective absorption of the water determines its colour, the brilliancy of colour is determined by the quantity of solid particles that the water carries in suspension. All the different colour phenomena of the Mediterranean are not only thus easily accounted for, but can even be imitated experimentally by means of a solution of Prussian blue and a fine white powder. “If the solution of Prussian blue is placed in a vessel, the bottom and sides of which are dark and reflect no light, then the coloured solution appears dark and colourless; but if a little of the white powder is added then the solution at once becomes brilliantly coloured. By varying the amount of the powder in the water all the varied colour effects of the Mediterranean can be reproduced, a little powder causing the solution to appear deep blue, and as more powder is added the brilliancy of the water increases, and its colour changes from blue to chalky blue-green.” The influence of the colour of the floating particles themselves on the appearance of the water is also pointed out, and the greenness of our northern seas attributed in part to the reflecting particles being yellow. Dr. Aitken’s investications included the examination of well waters, which were found to vary in colour from blue to yellowish-brown, and it was observed that the more transparent a water was, the nearer its colour was to blue. A series of experiments were also made to determine the colour of distilled water, which proved to be almost exactly that of Prussian blue. “As the
