We have already shown how the complementaries of the spectrum colours can be found; the question is can we find the complementaries of pigments by the spectrum? There is one very self-evident way. We can place the three slits in the spectrum as given in chapter IX., and match in intensity the white light of the reflected beam, and note the apertures of the slits. We must then in the reflected beam place the pigment whose complementary colour is required, and match its colour with the light from the three slits, keeping, for the sake of convenience, the white light falling on the pigmented surface of unaltered intensity, and again note the apertures. If we deduct the last measures from the first, the difference of aperture will give the complementary colour. Thus it was found that with slits in a certain position in the spectrum, to make white light the following apertures in hundredths of a millimetre were required: { Red 165 (1) { Green 60 { Violet 100 Emerald green was placed in the patch and was matched by the light from the three slits, when it was found that it required { Red 4 (2) { Green 35 { Violet 25 Deducting one from the other we get as the complementary colour, { Red 125 (3) { Green 25 { Violet 75 This is a complementary colour, but like the green itself it is mixed with white light; but we can easily deduce what is the simplest complementary colour; for we have only to deduct the possible white light from the second measure. Now evidently the greatest amount of white light is when the whole of the green is taken as forming part of it, with the proper proportions of red and violet, and these we can obtain by taking the proportions of the colours in (1); therefore deduct-- { Red 69 (4) { Green 25 { Violet 41·5 and this would leave as the complementary colour without any admixture of white-- (5) { Red 56 { Violet 33·5 which is a purple as would be expected. Now to give the same dilution of white to the complementary that the emerald green has, we must take away from the emerald green all the white mixed with it, and add that quantity to the complementary. The white in the emerald green can be found by treating the whole of the red as going to form the white; we then have from (1)-- { Red 40 (6) { Green 14·4 { Violet 24 Deducting these from (2), we find that the colour of emerald green, less the white light, is 20·6 of green mixed with 1 of violet. To find the proper dilution of the complementary colour we must add the above proportions of the three colours, and as our final result we find the complementary colour, of equal impurity, is a mixture of-- { Red 96 (7) { Green 14·4 { Violet 57·5