Colour Measurement and Mixture

by Sir William de Wiveleslie Abney

The fact that red, yellow, and blue cannot be primary colours has been mentioned, as even now it is sometimes taught that they are so. As long as the theory of colour principally lay with artists there was reasonable ground for their assumption, since they worked with impure colours, viz. those of pigments; and as we shall see later on the truth of the assumption agreed with such experiments as they would make. When, however, the question was taken up by the physicist with more exact methods of experimenting, and with pure colours, the falsity of the old triad was soon capable of proof.

To return from our digression: how it is that three mixed colours can give the sensation of white light is at first sight hard to understand; but a reference to the action of light on a photographic salt helps us in some degree. In the case of a sensitive salt, such as the bromo-iodide of silver, we find that a chemical decomposition is caused by the violet end of the spectrum, and is only feebly affected by any other part, though with prolonged exposure even the red will cause it. The annexed figure (Fig. 33) gives the idea of the relative action of different parts of this violet portion.

Fig. 33.—Curve of Sensitiveness of Silver Bromo-iodide.

The height of the curve shows the relative effects produced. Now this curve is not symmetrical, but has a maximum effect nearer to the violet end of the spectrum than to the red. The atomic composition of the silver bromo-iodide is probably two atoms of silver and one of bromine and one of iodine oscillating together, and we can conceive of some one atom, the period of whose swings in its molecule is isochronous with some wave-length of light. Further, we can conceive that, like a pendulum whose vibrations are increased in magnitude by well-timed blows, the swing of the atom is also increased, and that eventually it gets beyond the sphere of the attraction of its parent molecule, leaves it, and is attracted to some neighbouring molecule of different constitution, and that thus a chemical change is induced. This we can conceive, but how can other waves, which are not isochronous with the rhythmic swing of the atoms, alter the composition of the molecule? If we have an impulse given to a pendulum exactly timed with the period of oscillation, there is no doubt that the swing is increased. If we have one nearly in accord, it will be found that though the swings are not increased in amplitude so greatly as when there is perfect accord, yet an increased swing is given, and as exact accord is removed further and further, so the increase in the swing of the pendulum gets smaller and smaller. In somewhat the same manner it is possible that many series of waves, differing in wave-length, and therefore in periods of oscillation, may be capable of increasing the amplitude of a swing, and with the photographic salt this probably occurs, with the result which we see in the above figure. Suppose in the eye we have three such sensitive pendulums which are capable of responding to the beats of waves of light, it requires no great imagination to see that one such pendulum will respond not only to that wave of light which is isochronous with it, but also with waves shorter and longer than that particular wave. The same pendulum indeed may respond to the whole of the visible spectrum, but when far off from the maximum the response would be very small indeed. We may therefore assume that though each pendulum may have its maximum increase of oscillation at one part of the spectrum, yet at other parts not only it alone answers to the beating of the waves, but that the other pendulums are also affected by the same, and thus the whole spectrum is recognized by the swings more or less long, of either one, two, or of all three.

To Thomas Young is usually attributed the three-colour theory, though it seems to have been promulgated in an incomplete state some time before; Clark-Maxwell and Helmholtz revived it in later years, and it is usually known as the Young-Helmholtz theory. It should be remarked that the three fundamental colour sensations are not of necessity the same sensations as are given by the three primary colours, as we shall see further on. The following figure (Fig. 34) is taken from Helmholtz's physiological optics, as diagrammatic of the three sensations.

Fig. 34.—Curves of Colour Sensations.

To this diagram there is an objection, in one respect, viz. that it gives the same luminosity-value to the blue of the spectrum as it does to the red and green. It has been seen that if we call the luminosity of the yellow 100, that of the blue is about 5. The objection does not hold if it is remembered that the three maxima of impressions are taken as equal. If the ordinates were increased, so that the maxima were of the same height as that of the photographic curve, the resemblance between them and this last would be very marked. It will be noticed that each of the three colour sensations is not only excited by a limited portion of the spectrum, but by all of it, the height of the curves being a measure of their response.

Now assuming that this is the case, since a certain degree of stimulation given simultaneously to the three sensations causes an integral sensation of white light, it follows that the colour perceived in every part of the spectrum is due to the excess of stimulation of either one or two of the fundamental sensations, together with the sensation of white light. If this diagram were correct, at no point in the spectrum is one fundamental sensation excited alone, but we believe that the diagram obtained by Kœnig ([Fig. 35]), from colour equations (which will be explained in our next chapter), is more exact, and that it is probable that in the extreme violet and extreme red of the spectrum the only sensations which are stimulated are the violet and red respectively. Our measures in the red and violet of the spectrum make it appear that each of the two sensations can be perceived unaccompanied by any others, and the fact that the red colour blind person perceives a shortened spectrum in the red end, is a further proof of this deduction, so far as the red is concerned.