iform green.
Hence the greenness of shoal water. You go to bed with the black water
of the Atlantic around you. You rise in the morning, find it a vivid
green, and correctly infer that you are crossing the Bank of
Newfoundland. Such water is found charged with fine matter in a state
of mechanical suspension. The light from the bottom may sometimes come
into play, but it is not necessary. The subaqueous foam, generated by
the screw or paddle-wheels of a steamer, also sends forth a vivid
green. The foam here furnishes a _reflecting surface_, the water
between the eye and it the _absorbing medium_.

Nothing can be more superb than the green of the Atlantic waves when
the circumstances are favourable to the exhibition of the colour. As
long as a wave remains unbroken no colour appears, but when the foam
just doubles over the crest like an Alpine snow-cornice, under the
cornice we often see a display of the most exquisite green. It is
metallic in its brilliancy. The foam is first illuminated, and it
scatters the light in all directions; the light which passes through
the higher portion of the wave alone reaches the eye, and gives to
that portion its matchless colour. The folding of the wave, producing,
as it does, a series of longitudinal protuberances and furrows which
act like cylindrical lenses, introduces variations in the intensity of
the light, and materially enhances its beauty.

We are now prepared for the further consideration of a point already
adverted to, and regarding which error long found currency. You will
find it stated in many books that blue light and yellow light mixed
together, produce green. But blue and yellow have been just proved to
be complementary colours, producing white by their mixture. The
mixture of blue and yellow _pigments_ undoubtedly produces green, but
the mixture of pigments is a totally different thing from the mixture
of lights.

Helmholtz has revealed the cause of the green produced by a mixture of
blue and yellow pigments. No natural colour is _pure_. A blue liquid,
or a blue powder, permits not only the blue to pass through it, but a
portion of the adjacent green. A yellow powder is transparent not only
to the yellow light, but also in part to the adjacent green. Now, when
blue and yellow are mixed together, the blue cuts off the yellow, the
orange, and the red; the yellow, on the other hand, cuts off the
violet, the indigo, and the blue. Green is the only colour to which
both a