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82 LIGHT AND WATER your head ; and thus the colours with which water is painted are an indication of the position of the spectator, and connected inseparably with the perspective of the shores. The most beautiful of all results that I know in mountain streams is when the water is shallow, and the stones at the bottom are rich reddishorange and black, and the water is seen at an angle which exactly divides the visible colours between those of the stones and that of the sky, and the sky is of clear, full blue. The resulting purple, obtained by the blending of the blue and the orange red, broken by the play of innumerable gradations in the stones, is indescribably lovely. All this seems complicated enough already; but if there be a strong colour in the clear water itself,as of green or blue in the Swiss lakes, all these phenomena are doubly involved ; for the darker reflections now become of the colour of the water. The reflection of a black gondola, for instance, at Venice, is never black, but pure dark 93 STC: «5:5 NOTE ON THE COLOUR OF WATER. The selective absorption theory of the colour of water is now generally accepted in preference to the selective reflexion theory, Dr. John Aitken, in his Paper “On the Colour of the Mediterranean and other Waters” (Proc. R. S. E. vol. xi, page 472), having shown conclusively that the former theory is the correct one. “ According to the selective reflexion theory the colour is due to the light reflected by extremely small particles of matter suspended in the water. These particles being so small they can reflect only the short waves of light, or those which belong to the blue end of the spectrum. The
