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. Now, I can show you more, namely, that the
colour of the so-called coloured object is entirely dependent on the
existence in the light of the special coloured rays which it radiates,
and that this scarlet paper depends on the red light of the spectrum for
the existence of its redness. On passing the piece of scarlet paper
along the coloured band of light, it appears red only when in the red
portion of the spectrum, whilst in the other portions, though it is
illumined, yet it has no colour, in fact it looks black. Hence what I
have said is true, and, moreover, that red paper looks red because, as
you see, it absorbs and extinguishes all the rays of the spectrum but
the red ones, and these it radiates. A bright green strip of paper
placed in the red has no colour, and looks black, but transferred to the
pure green portion it radiates that at once, does not absorb it as it
did the red, and so the green shines out finely. I have told you that
sodium salts give to a colourless flame a fine monochromatic or pure
yellow colour. Now, if this be so, and if all the light available in
this world were of such a character, then such a colour as blue would be
unknown. We will now ask ourselves another question, "We have a new blue
colouring matter, and we desire to know if we may expect it to be one of
the greatest possible brilliancy, what spectroscopic conditions ought it
to fulfil?" On examining a solution of it, or rather the light passing
through a solution of it, with the spectroscope, we ought to find that
all the rays of the spectrum lying between an