Colour vision

by Sir William de Wiveleslie Abney

CHAPTER III.

Let me take you back again to matches of colour. We will now, however, make the matches with the primary colours in the guise of pigments. These colours themselves are complex colours, but as the eye cannot trace any difference, or at all events very little difference, between them and simple colours, a mixture of these complex colours should answer nearly as well as do mixtures of the simpler colours. We have here three discs, a red, a green, and a blue, and we can very closely match these colours by a red, a green, and a blue in the spectrum.

By having a radial slit cut to the centre of these card discs, we can slip one over the other so as to expose all three colours as sectors of a single disc. Then we can place the compounded disc on the axis of a rapidly rotating motor, and the colours will blend together, giving an uniform colour. Any proportions of the three colours can thus be mixed, and by a judicious alteration in them we now have them so arranged that they give a grey. By inter-locking together ([Fig. 9]) a black disc and a white disc, each with a diameter slightly larger than that of the other discs, but equal to each other, and rotating them on the same spindle behind the three colour discs, we can, by an alteration in the proportion of black to white, form a grey which will match that produced by the rotation of the three coloured sectors. In other words, white, though degraded in tone, can be produced by the three complex pigment colours, as we have seen can also be done by the mixture of the three simple spectrum colours.

Fig. 9.

The mixture of the three spectrum colours can match other colours than white. For instance, it can be made to match the colour of brown paper. By the colour discs also we can do exactly the same by introducing, if necessary, a small quantity of white or black, or both, to dilute the colour or to darken its tone.

Another application of the same principles enables us to produce an artificial spectrum by means of a red, a green, and a blue glass. By fixing these three glasses behind properly shaped apertures cut in a card disc at proper radial distances from the centre, and rotating the disc, we have upon the screen when light is passed through them a ring of rainbow colours. If the beam of light be first passed through a suitable rectangular aperture, the breadth of which is small compared with its length, placed close to the rotating disc, and an image of the aperture be focussed on the screen by a suitable lens, we shall have a very fair representation of the spectrum—every colour intermediate between the red and green, or the green and blue, being formed by mixtures of these pairs respectively.

We have now given a very fair proof that vision is really trichromic—that is, that it is unnecessary to have more than the sensations of three colours to produce the sensation of any of the others.

Fig. 10.

There is one colour, if it may be called so, that has not been shown you, and whether it is a simple colour or not cannot be stated. It seems, however, to be the basis of all other colours, since they all commence with it. It would, perhaps, be preferable to call it the first perception of light instead of a colour. We can exhibit this in a fairly easy manner by a little artifice. An incandescent lamp is before you, and a current from a battery passing through the carbon thread causes it to glow brightly. In the circuit, however, I have introduced what is known as a resistance, which consists of a very large number of square pieces of carbonized linen, pressed more or less tightly together. By means of a screw the pressure can be varied. When the pressure is somewhat relaxed, the resistance to the passage of the current is increased, and the carbon thread glows less brightly; and by a still greater release of pressure, the light can be made to disappear altogether. A beaker ([Fig. 10]) which we have here is covered with thin blotting paper, and when placed over the incandescent glow-lamp it appears as a luminous yellow cylinder, the colour being due to that of the light within it. We can next insert more resistance in the circuit, and it becomes red, due to the ruddy light of the thread. By inserting still more resistance into the circuit the red fades away, but in the darkness of this lecture theatre the beaker is still a luminous object, though faintly so. It has no colour, and the only sensation it provokes is one of light. Taking off the beaker, we see that the carbon thread is a dull red and nothing more. The passage of this light through the white blotting paper so reduces it that the red is non-existent, and the initial sensation is all we perceive.

Placing a piece of red, green, or blue gelatine round the lamp, we get the same effect, showing that the basis of all colour, be it red, green, or any other colour, is what appears to us to be colourless. This experiment is one which is full of interest, as it has a very distinct bearing on diagnosing our colour sensations, and a variation of it will have to be repeated under other conditions.

To go back, however, a little way, how does it arise that only three sensations are necessary to give the impression of all colours? One can understand that some definite period of the ether waves might be in unison with the possible swing of one apparatus in the eye, and another with another, but it is somewhat difficult at first sight to conceive that more than one can be made to answer to wave motion of a period with which it is out of tune, so to speak. A couple of illustrations taken from physical experiments may help to suggest how this can happen.

Fig. 11.

[Fig. 11] is a double pendulum arranged as shown. The pendulum A is heavily weighted, whilst the pendulum B is light, being only a string with a small weight attached. This difference in weight was made designedly, to prevent any great effect of the movement of B being shown on A, though that of A must necessarily exercise a great influence on B. The two pendulums are now of the same length. A is set in motion, and as it swings, B also begins to swing, and soon is oscillating with greater motion than A, and continues to do so. The length of the pendulum B is next shortened, and A is again set in motion. B takes up the motion, and increases its swing more and more, but now the two pendulums are in opposite phases, and the motion of A tends to diminish the swing of B, and continues to do so till, after an interval of time, B is once more at rest, when it again will start swinging. The fact is, that when A commences to swing, B also commences; and as long as B and A are moving in the same direction the impulses tend to make B increase its swing, but when they are moving in the opposite direction, or rather, perhaps it should be said, when A begins to start from the highest point of its swing downwards whilst B is travelling upwards, the swing of B will gradually diminish. This, of course, must happen when B is shorter or longer than A, since their times of oscillation are then different. We can now picture to ourselves that when in the perceiving apparatus in the retina the moving parts—probably molecules or atoms—arrive at a certain amplitude, there is then an impression of light, and that it is quite possible that not only those waves whose motion is exactly of the same period as that of the apparatus will set them in motion, but also those waves which are actually of a very different period. If such be the case, it can be seen that waves of light of some periods may set each of the three kinds of perceiving apparatus in motion, and that possibly the resulting impressions given by the sum of all three for a wave out of tune with any of them may be even greater than when the wave period is absolutely the same as one of them. For in the last case a maximum effect may be produced on one apparatus, and the effects on the other two may be insignificant; whilst in the first case the effects on two of them may be so large that their combined effects may have a larger value.

Fig. 12.

The following diagram ([Fig. 12]), made on the principle of Lissajou’s figures, shows graphically the motion of the pendulum. The pendulum, with a pen attached, was started by an independent pendulum, which had a different period, and the amplitude of the former registered itself on paper which moved by clockwork round the axis of suspension. As the two pendulums had different periods, the amplitude, as shown by the traces made, first increased and then diminished till there was no motion, and then started again. The trace is very instructive, and deserves attention. It will be noticed that the amplitude, or length of swing, increased rapidly at first, and then very gradually attained a maximum. Having attained this maximum, the amplitude diminished very slowly for some time, and finally came rather rapidly to zero, and the pendulum for an instant was at rest.

Fig. 13.

The top figure is the red sensation on the Young theory; the middle is the green sensation, and the lowest the violet or blue sensation.

With the notion in our minds that the perceiving apparatus might act in the way that the pendulum acts, we naturally apply it to the theories which early investigators on colour vision propounded. Thomas Young, whose name has already been mentioned, had propounded a theory of vision, which depended on the existence of only three colour sensations, and Von Helmholtz adopted it and explained the action of the three sensations in reference to the spectrum as shown in the diagram. These figures do not pretend to be absolute measures of the sensations, but only of the form which they might take ([Fig. 13]). The height of the curve at each part of the spectrum is supposed to represent the stimulation given to each apparatus by the different colours. Looking at the figures we see that each sensation has a place of maximum stimulation, and that the stimulation falls off more or less rapidly on each side of this maximum. It will, however, be noticed that whilst the green sensation takes very much the form of the pendulum amplitudes ([Fig. 12]) between its periods of rest, the other two differ from it. In the case of the red sensation, the stimulation falls very rapidly in the red as it reaches the limit of visibility of the spectrum, and in that of the blue sensation the steep descent is towards the extreme violet. When the three sensation theory is examined in the light of the careful measurements that have been made, the results tell us that these diagrams can only be taken as suggestive.