Stranica 124 [124]
68 LIGHT AND WATER
In anysheet of coloured water, therefore, the appar¬
ent 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 sus¬
pended 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 sus¬
pended 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 gradu¬
ally 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.