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70 LIGHT AND WATER the relative amount of chalk powder or fine grains of sand floating in the water. Near the shore or over a sand bank it is almost always greener than in deep water, though close to the beach it may often, owing to excess of floating particles, assume a milky or a sandy colour. The colour of sea-water is also well brought out by foam, or sunken particles of air, as in the wake of a screw steamer.’ To repeat: the brilliancy of the colour is dependent, first, on the illumination from above, in other words, on the condition of the sky: secondly, on the condition of the water with regard to suspended particles. Having got these two very obvious conditions, let us inquire how it depends on the position of the observer. As in the consideration of reflexions in Chapter I, we will first suppose the water to be absolutely smooth. Leaving for a moment the question of colour, let the reader imagine himself to be standing on a stone lying in the shallow water at the edge ofa lake. The water is perfectly still and clear, and masses of white cloud are floating overhead. Looking down at his feet, he sees two pictures; (1) the bottom of brown mud or yellow gravel, and (2)—though he may not at first have noticed them—the sky and clouds also. These two pictures, the one caused by rays coming from beneath the surface, and the other by rays reflected at the surface of the water, are super" In very shallow water the brilliancy of colour depends of course on the reflective power of the bottom. The “silver sand” that occurs on some parts of our coasts, possesses this power in a high degree.
