Colour Measurement and Mixture

by Sir William de Wiveleslie Abney

Another couple of striking experiments which the writer devised to show these colours can be made with the colour patch apparatus, and on the same principle as that used for obtaining the intensity of the rays reflected from pigments, and transmitted through coloured transparent bodies. Instead of the small slit with a right-angled prism in front to deflect the beam from the top spectrum, where two spectra are produced (see [Fig. 16], p. 95), a single spectrum is used, with a right-angled prism of such a size that it deflects half of it, which is again reflected on to the screen by a mirror, and through a lens to form a second patch of equal size as the undeflected beam. A rod can be so placed in the path of the beams that two coloured stripes are formed, together with a white stripe caused by their overlapping. The two coloured stripes are complementary one to the other. By moving the prism along the spectrum various coloured stripes can be formed, in some cases one being much less luminous than the other, and yet they are complementary. If instead of the large right-angled prism a smaller one be used, the complementary colour due to a small part of the spectrum can be shown in the same manner.

It is customary to show the complementary colours diagrammatically by what is known as the chromatic circle. Roughly it is drawn as in the above figure ([Fig. 38]). The three colours, red, green and blue, which are taken for primary colours, are placed at 120° apart in a circle, and lines drawn from them through the centre, at which white is supposed to be situated. Where these lines cut the circumference is placed the complementary colour. Other colours can be placed round the circle with their complementary colours opposite, and so a fairly complete diagram of the spectrum can be made. But it must be remembered that this is really of no scientific value, as it conveys no idea of the luminosity of the spectrum colours, nor of the quantities which have to be mixed together to form the complementaries. Such a circle is, however, convenient as a sort of memoria technica, and can be filled up according to the fancy of the observer.

The following are pairs of most carefully selected complementary colours of pigments, as adopted by Professor Church.

{	Complementaries.	Pigments.
	Red	Madder red or crimson vermilion.
	and
	Green blue	Viridian, the emerald oxide of chromium with a little cobalt.
{
	Orange	Cadmium yellow, of full orange hue.
	and
	Greenish blue	Cobalt green.
{
	Orange yellow	Cadmium yellow, or deep chrome.
	and
	Turquoise	Cœrulium, or cobalt blue, with a little emerald green.
{
	Yellow	Lemon yellow, pale chrome, or aureolin.
	and
	Blue	Ultramarine from lapis-lazuli.
{
	Greenish yellow	Aureolin with a little viridian.
	and
	Violet blue	French ultramarine.
{
	Green yellow	Lemon yellow, with some emerald green.
	and
	Violet	French ultramarine with madder carmine.
{
	Yellowish green	Lemon yellow with much emerald green.
	and
	Purplish violet	Madder carmine with French ultramarine.
{
	Green	Emerald green with lemon yellow.
	and
	Purple	Madder carmine with French ultramarine.
{
	Emerald green	Emerald green alone.
	and
	Reddish purple	Madder carmine with a little French ultramarine.

As these pairs of pigments are complementary, it follows that if rotated together in proper proportions, they should make a grey which will be indistinguishable from a grey formed by rotating black and white sectors together. (See [chap. XV.])

It will probably happen that a good deal more of one of the pairs of the colours is required in the disc than of the other, and supposing that the two are each used of the full brightness which the pigments are capable of giving, it follows that in a diagram where equal areas are filled with the pigments as complementary, some means must be adopted to give the true depth of tone to each. The mixture of white will heighten the luminosity of either, or the admixture of black will lower it, but often alters the hue.

One of the most beautiful methods of observing complementary colours is by means of the polarization of light, which we need not describe in detail. What is known as Brücke's schistoscope is perhaps one of the most convenient. Dove's Iceland spar prism is also useful, when two pigments have to be worked on to paper, so as to be complementary. The two squares of pigmented paper are placed side by side, and two images of each are formed. One image of one colour can be caused to overlap the second of the other, and if the two when superposed appear of a grey they are complementary one to the other. If too much of one colour appears, it must be toned down till the grey is formed. This is a very simple piece of apparatus, and for experiments with pigments will be found to be very handy. When the right tint of each is secured in this manner, a further test may be made by making the pigmented surfaces into sectors, and rotating them together, when if the double-image prism gives correct results, the angular aperture of the sectors should be 180° each, to match a grey produced by a mixture by rotation of black and white.

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:

(1)	{
		Red	165
		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