That is all. Then why,
say you, does that liquid look yellow if all the rest of those rays pass
through and enter the eye, namely, the blue-green with a trifle of blue,
the green, yellow, orange, and red? The reason is this: we have already
seen that the colours complementary to, and so producing white light
with red, are green and greenish-blue or bluish-green. Hence these
cancel, so to say, and we only see yellow. We do not see a pure yellow,
then, in picric acid, but yellow with a considerable amount of white.
Here is a piece of scarlet paper. Why does it appear scarlet? Because
from the white light falling upon it, it practically absorbs all the
rays of the spectrum except the red and orange ones, and these it
reflects. If this be so, then, and we take our spectrum band of
perfectly pure colours and pass our strip of scarlet paper along that
variously coloured band of light, we shall be able to test the truth of
several statements I have made as to the nature of colour. I have said
colour is only an impression, and not a reality; and that it does not
exist apart from light.
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