Oldal 124 [124]
OCR
68 LIGHT AND WATER In anysheet of coloured water, therefore, the apparent brilliancy of its colour depends upon ¢he amount — of light coming to the eye from beneath the surface, the light which is reflected at the surface being, as we have said, unchanged in colour. Nowéhis amount obviously depends first on the amount of light which enters the water. For example, the sea on our coasts is generally of a greenish hue. This colour often reveals itself in the thin edges of the waves where they rise steeply or curl over before breaking, but, except where the crests become thus translucent, it is not always evident. With a cloudy sky comparatively little light enters the water, and therefore comparatively little is reflected from the floating particles of sand. The amount -of greenish light reaching the eye is thus very small compared with the amount of gray light from the clouds seen by surface-reflexion, and the water looks a dull gray. If, however, the clouds part and a beam of direct sunlight strikes the water, the eye is relieved by a gleam of emerald. | Secondly, it depends on the proportion of suspended particles. As we have seen, if the water is perfectly clear and very deep, it will look black, all light that enters it being absorbed. But where suspended particles are present, as explained above, the colour is brought out, and, up to a certain point, the more particles the more colour. Beyond this point the water commences to become turbid, and gradually loses its characteristic green colour, as the colour water. (Dr. John Aitken, in “Nature,” Vol. LIX, page 509). See Note at end of chapter, page 82.
