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 and 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 a greenish shade, I should have added Croceine Orange, which on the fabric would absorb just those green and bluish rays of light necessary to radiate from and illumine that greenish part, and the greenish part would do the like by the orange rays; the effects would be neutralised, and all would fall together into black.