Colour Measurement and Mixture

by Sir William de Wiveleslie Abney

Fig. 31.—Screen on which to match Gamboge.

If we wish to register the tint of any pigment, we have to slightly alter our mode of procedure. Suppose, for instance, we wish to register the colour of gamboge. In such a case we paint a small bit of card (Fig. 31) with the pigment, and divide the white space on which the colour patches are thrown into two parts, and cover one-half with the pigmented card, leaving the other half white. The reflected beam illuminates the pigment, and the spectrum patch the white. The widths of the three slits are then altered till the two tints agree, and the brightness matched by means of the rotating sectors.

There are certain sad and æsthetic colours which it might be considered cannot be matched by a mixture of three colours. A brown colour, or "eau de nil," might appear to come out of the range of matching. These colours, however, can be matched in precisely the same manner as the brighter colours are matched. Thus a brown pigment will be found to require red and a little green, and a trifle of blue; and the only difference between it and a brighter shade of the same colour, is that more total light has to be cut off from it to give the sombreness. A sad colour only means a pigment or dye which reflects but little light, and if that be so it can naturally be matched by using but very small quantities of the compounding colours.

There is one curious phenomenon to which attention may be called in this matching, which is worthy of remark. The match will be found to differ according as the patches are compared from a distance of a couple of feet, or from a considerable distance. More green will be required in the latter case than in the former. If matched at a distance of about six feet, and the eyes be then turned so that the edge of the patch falls on their centres, it will be noticed that the colour mixture appears of a green hue. This last experiment indicates that the retina is not equally sensitive for all colours throughout its area. Physiologists tell us that what is known as the yellow spot occupies a central position in the retina, and that it absorbs a part of the spectrum lying in the green. Now when the eyes are close to the patch, its image occupies a considerable part of the retina, and the colour is compounded as it were of the colour as seen on the yellow spot, and of that beyond it, for the yellow spot will take in an image of from six to eight degrees in angular measurement. When viewed at a distance we have the image of the patch falling almost entirely on the yellow spot, and hence a greater quantity of green is required, as it has to make up the deficiency caused by the absorption. When the eyes are turned a little on one side the image falls on the outside of the yellow spot, and the patch illuminated by the mixed light appears green, compared with the patch illuminated with the white reflected beam.

It is thus evident that when colour matches have to be made, the distance of the eye from the screen should always be stated, as also the dimensions of the patches viewed. It may be fairly asked why, if the half patch illuminated by the mixed colours appears greener when the eye is turned, the other should not equally do so. This is a very fair question to ask. It must be remembered that one strip is illuminated with white light, in which every coloured ray of light is compounded, whilst in the other only three rays are blended. The green ray chosen happens to be taken from that part of the spectrum which is absorbed by the yellow spot; but all of the green rays of the spectrum are not so much absorbed, hence in ordinary white light, in which all the green rays are present, only a small percentage of the total green in the spectrum is absorbed, compared with that absorbed from the single green ray with which the match is made. No doubt both patches are really greener when the eye receives the impression of their images outside the yellow spot, but one is much greener than the other, and it is thus comparatively green. It is possible to make a match with some colours with a blue-green in which the phenomenon described does not appear; but in cases where a match has to be made with colours in which but little blue is required, it would be impossible to make it, owing to the blue existent in such a green-blue ray.

We will now return to our compounding of three colours to make white. Why have we chosen the positions of the slits which we did in the spectrum for its formation? Would not other positions answer as well? Let us give our answer by experiment. Let us move the slit which is now in the green towards the red; we shall find that as we do so—and keeping the blue slit of the same width—that we shall have to close the red slit, and alter the aperture of the green slit itself. If we reason on this point we shall be forced to the conclusion that the green slit lets through more red light of some description, as less red from the red slit is required to make the match. If we move the green slit almost into the yellowish green, we shall find that the red slit has to be entirely closed, and that white light is formed of the two colours, yellowish green and violet. This shows us that the yellowish green colour here used is formed by a mixture of the red and green rays which passed through the two slits in their original positions. If we replace the slits in these positions and close the violet slit, we are at once able to verify it.

If we again form white light with the slits in their original positions, and move the green slit towards the blue, we shall find that, keeping the red slit at a constant aperture, the blue slit will have to be closed, and the green slit altered in width. The necessity of lessening the aperture of the blue slit shows that there is a certain amount of blue light coming through the green slit. At one point, when the slit has travelled into the blue-green, the blue slit may be entirely closed, and white light be formed of this and the red, showing that the blue-green colour is composed of the same proportions of blue and green which passed through the blue and green slits in their original position. The positions chosen were arrived at by the writer from experiments made in this manner, moving first one slit and then the others, and the position of the green slit was confirmed by a consideration of the neutral point which exists in a green colour-blind person's spectrum.

The method of mixing three colours together gives us a means of imitating all kinds of white light, as it does of coloured light. At page 110 we have already given a diagram of the relative amounts of spectrum colours in sunlight, skylight and gaslight. If we by any means throw a patch of the light which we wish to match on the patch formed by the colour patch apparatus, and interpose the rod, we can measure the apertures of the three slits, and thus arrive at the relative proportions of each colour present. In an experiment carried out, sunlight, the electric arc-light, and gaslight were compared in this manner. The following are the results, the red being near the C line, the green near the E line, and the violet near the G line of the solar spectrum.

	Sunlight.	Electric Light.	Gaslight.	Skylight.
Red	100	100	100	100
Green	193	203	95	256
Violet	228	250	27	760

Now from the above it might seem that as three simple spectrum colours will give us the colour of any pigment, that therefore two colours ought to give us the same colour as any intermediate simple colours in the spectrum which lie between them; for instance, that the simple blue-green ought to be obtained by mixing spectral green and spectral violet together. This can be ascertained with a single colour patch apparatus, by cutting a slit in the card that fills up the aperture between the two adjustable slits, and deflecting the beam transmitted through it by a right-angled prism, and back on to the screen through another similar prism, as described in chapter VIII. It is more convenient, however, to use a duplicate apparatus precisely similar to the first, with the exception that no collimator is required, placing them side by side, and mirrors making the reflected beam from the first traverse the second set of prisms. There will be a reflected beam from the second apparatus, which can be utilized in the same way as was that from the first apparatus, and the two spectra will vary together in brightness, as will also the new reflected beam, since they all are formed by the light coming through one slit. A patch of the colour intermediate between the two is thrown on the screen from the second apparatus, and the second patch from the first apparatus overlaps it. A rod placed in the usual manner throws two shadows, which are illuminated by the two different beams. If blue-green be a colour it is wished to match, it will be found that no matter in what part of the violet and green the slits are placed, no match can be effected. But if some very small quantity of red light be mixed with simple blue-green, that then a colour identical in every respect as regards the eye can be obtained from the violet and green of the first apparatus. It must be remembered that a mixture of red, green and violet form white, and that they are mixed in definite proportions. No matter how feeble in intensity the white may be, the same proportions will still obtain. In the above experiment, as the blue-green must contain violet and green, the small quantity of red must combine with the proper proportion of violet and green, and will form white light, so that the match is obtained by the residues of the violet and green mixed with the small quantity of white light, of which the red is the indicator.