Colour Measurement and Mixture

by Sir William de Wiveleslie Abney

Similar equations were formed as the above.

35 × 30 + 204 × 4·4 + 121 Y = (101 + 3·4 × 259) 100
whence Y = 77·6.

That is, the luminosity of the chrome yellow is ·78; the same as was obtained by direct measurement.

In the same manner the luminosity of any colour can be found. Thus that of a purple, or of green, can be ascertained; of the former by using the green disc with either the red or the blue disc, and the latter by the red and blue disc. From this it is apparent that we can check the luminosities derived from other means by this plan.

A taking experiment can be made with colour discs to imitate all the colours of the spectrum in their proper order, though diluted more or less by white light. This can be done by rotating V, E, and U together; but in order to get additional luminosity in the yellow, we can use chrome yellow as well. If a disc be made as in the figure (Fig. 44), it will on rotating give a fair imitation of the spectrum, if it be viewed through a slit held in front of the disc.

Fig. 44.—Disc arranged to give approximately all the Spectrum Colours.

The mixture of colours by means of rotating sectors is one which the artist cannot use for artistic purposes, and it might seem that for him any deductions made from this method are useless; but it is not so. Suppose we take black lines ruled closely together on paper, and examine the surface from such a distance that the lines are no longer distinguishable it will appear of a grey; and if we take the amount of black on the paper and amount of white, and prepare two sectors of black and white, whose angles are in these proportions, and rotate them alongside the ruled surface, it will be found that the grey of one matches the grey of the other. If instead of lines of black and white we have them of light yellow and cobalt blue, a grey is also produced when the surface covered by the blue is to that covered by the yellow in correct proportions, and may be matched by rotating sectors containing merely black and white. Now some artists employ stippling, filling up cross-hatching of one colour with dots of a totally different colour, or they place dots side by side. When seen from the distance at which the picture should be viewed, these various colours blend one into another, and form a tint which is the same as that which would be obtained by rotating these colours together in the proportion in which they cover the ground. Artists, however, generally mix their pigments together on the palette, and the resulting mixtures are often totally unlike those which are obtained by rotating the same colours together, a noteworthy example is that of yellow and blue. By rotation, and when in proper proportion, these two give a white, but when mixed on the palette a green results. What causes this difference? Experimental proof is always the most satisfactory proof, so let us have recourse to the spectrum apparatus to obtain an answer. Let a spectrum be thrown on the screen, and in it place a strip of paper painted with the yellow, and then another with the blue. With the first it will be seen that the blue rays are not reflected, but only the green and yellow and red, taking the spectrum as roughly made up of these four colours. With the latter the yellow is not reflected, and but very little red, but the blue and the green are reflected strongly. Now we have already said that the reflection of colour from a surface is indicative of the colours the particles of pigments when taken thin enough to be transparent would transmit; hence we may take it that the yellow pigment transmits the red, yellow, and green, and the blue pigment scarcely anything but blue and green. When we have a mixture of these fine particles of pigment on paper, some will underlie the others. But let us pay attention to what would happen if a yellow particle were at the top, and a blue one beneath it. White light would impinge on the yellow particle, but only red, yellow, and green would pass out or be reflected from it. This sifted light would next fall on the blue particle and—as we have seen—only blue and green can pass through or be reflected from it; but as the yellow particle has already deprived the white light of its blue component, the green light alone would pass to the paper, and be reflected either direct from the surface of the paper, or through the particles themselves to the eye. If the blue particle were on the top, precisely the same effect would be produced; it would only allow blue and green to pass to the yellow particle, and as the yellow is opaque to the blue, only green light again would pass. Similarly if side by side the same phenomena would occur, since the light reflected from one on to the other would be deprived of all colour except the green. A very pretty experimental proof of this is to place a yellow solution of dye in front of the slit of the colour apparatus, and having formed the yellow colour patch to place in it a piece of paper covered with a blue pigment: the latter becomes green. By placing a blue solution in front of the slit, and using a piece of yellow pigmented paper, the same result is obtained. The artist therefore in mixing his pigments calls into play the law of absorption, and from his mixtures very naturally assumes that blue and yellow make green. He makes a neutral tint of blue, red, and yellow, and as the red cuts off the green, this naturally follows from the above. Such experiments as these led him to the conclusion that red, yellow, and blue are the three primary colours, an assumption which had he used simple spectrum colours instead of compound colours, such as pigments, he would not have ventured to make.