Stranica 128 [128]
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 depend¬
ent, 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 par¬
ticles. 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 ab¬
solutely 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.