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.
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