d nearly to H and b (Fig.
16), _i.e._ all the bluish-violet, blue, and blue-green rays pass
through it unchanged, unabsorbed, whilst all the rest should be
completely absorbed. In like manner a pure yellow colour would allow all
the rays lying between orange-red and greenish-yellow (Fig. 16) to pass
through unchanged, but would absorb all the other colours of the
spectrum.

Now we come to the, for you, most-important subject of mixtures of
colours and their effects. Let us take the popular case of blue and
yellow producing green. We have seen that the subjective effect of the
mixture of blue and yellow light on the eye is for the latter to lose
sense of colour, since colour disappears, and we get what we term white
light; in strict analogy to this the objective effect of a pure yellow
pigment and a blue is also to destroy colour, and so no colour comes
from the object to the eye; that object appears black. Now the pure blue
colouring matter would not yield a green with the pure yellow colouring
matter, for if you plot off the two absorption spectra as previously
described, on to the spectrum (Fig. 16), you will find that all the rays
would be absorbed by the mixture, and the result would be a black. But,
now, suppose a little less pure yellow were taken, one containing a
little greenish-yellow and a trifle of green, and also a little
orange-red on the other side to red, then whereas to the eye that yellow
might be as good as the first; now, when mixed with a blue, we get a
very respectable green. But, and this is very important, although of the
most brilliant dyes and colours there are probably no two of these that
would so unite to block out all the rays and produce black, yet this
result can easily and practically be arrived at by using three colouring
matters, which must be as different as possible from one another. Thus a
combination of a red, a yellow, and a blue colouring matter, when
concentrated enough, will not let any light pass through it, and can
thus be used for the production of blacks, and this property is made use
of in dyeing. And now we see why a little yellow dye is added to our
coal-tar black. A purplish shade would else be produced; the yellow used
is a colour complementary to that purple, and it absorbs just those blue
and purple rays of the spectrum necessary to illuminate by radiation
that purple, and _vice versa_; both yellow and purple therefore
disappear. In like manner, had the black been of