Colour Measurement and Mixture

by Sir William de Wiveleslie Abney

The colour which the fundamental green sensation excites in the normal eye has probably never been seen, nor can be seen. This is due to the fact that all three sensations overlap in the green; that is, that the pendulum which answers to the green colour in the spectrum also affects, but with much less energy, the other two pendulums, which respond to the red and violet sensations.

The word pendulum has been used advisedly, for it may equally as well apply to a molecular aggregation as to one which is visible and measurable. Without entering into the physiological structure of the eye, we may say that it has usually been assumed that the pendulums are the ends of nerves which vibrate with the waves of light; but this seems rather doubtful. Gross matter, such as these ends are, compared with the molecules of which they are built up, cannot, as a rule, vibrate with waves of light, and there seems to be no reason why there should be an exception in the case of the eye. It seems much more probable that a chemical decomposition takes place in some substance attached to them, and where such decomposition takes place electricity of some kind must be produced. In other sensations of the body the nerves act as telegraph wires, carrying messages to the brain, and it is not improbable that the nerves of the eye are employed in somewhat the same manner. Professor Dewar has shown that when light acts on an extirpated eye, a current of electricity does traverse the nerves, and of such an amount that it can be shown to a large audience. This experiment is not, however, conclusive, as the effect may be mistaken for the cause. This idea, however, is only hypothetical, as is indeed the hypothesis of the mechanical action of light on the gross matter of which the rods and cones attached to the retina are composed.

We have in a previous chapter stated that there are some eyes in which the sensation of some colour is altogether absent, and in others in which it is more or less deficient. Thus some eyes appear to be lacking wholly in the sensation of red, others of green, and some very few of violet; and there have been cases known in which two sensations, the red and violet, have been totally absent. In the first case, where the sensation of red is entirely absent, what is known to the normal-eyed as white can be matched with a mixture of blue and green, and there is a place in the spectrum that is recognized as white. Similarly white can be matched by a green blind person with a mixture of red and blue.

To those who may be curious to see the colour which red and green blind persons would call white, a very simple means is at hand to demonstrate it. Using the colour patch apparatus with the three slits inserted in the slide, and in the positions we have indicated in the violet, green, and red, and forming white light for ourselves on the screen, if we cover up the red slit entirely we shall have a patch of sea-green colour, which a red blind person would call white; and if we cover the green slit, uncovering of course the red, we shall have a brilliant purple, which to a green blind person would be white. They both would call white what the normal-eyed person sees as white, for the simple reason that either the red or the green mixed with the remaining colours would be unperceived. The examination of colour-blind people is of prime importance for testing any theory of colour vision. For instance, if it were asserted that the fundamental sensations did not overlap as shown in the diagram above, then it would follow that at some place in the spectrum there would be a dark point. If they do overlap, it must follow that both for the red and for the green colour blind person there must be some place in the spectrum where what is white light to them is produced.

Colour-blind people were tested with the colour apparatus. The reflected beam and the colour patch were made to cast shadows as before, and the rotating sectors placed in the path of the former. A slide with one slit was passed across the spectrum, and the position noted where it was said that the two shadows were illuminated with white light; to the normal-eyed person one shadow of course appeared illuminated with the sea-green colour, or bluish green, according as the observer was red or green colour blind. The ray in the spectrum which to the red colour blind is white, has a wave-length of about 4900, and that for the green colour blind a wave-length of 5020, which corresponds to the position in which we usually place the green slit when a normal-eyed person is making colour matches.

It may be further remarked, that if the maxima of all the three colour sensations are taken, as in the diagram, as of equal value, that the place in the spectrum where the white light is perceived by the colour-blind is where the two sensations are of equal strength, that is, where the two curves cut one another, and are of equal height. By obtaining the proportions of the different colours with colour-blind persons which make up what to them is white light, the curves for the two sensations can be worked out in the form of simple equations.

The experiments carried out with colour-blind people are of the most interesting character, and a good deal remains to be done with the data already obtained from them.

To the popular mind a colour-blind person is usually thought a strange creature, and it is a matter of wonderment, if not of amusement, that they cannot distinguish between the red of cherries and the leaves of the cherry tree. The physicist, studying the theory of colour, views the matter quite differently, and he looks upon an intelligent observer of this class as a boon. It may be remarked that both the red-blind and the green-blind persons would be unable to distinguish between the cherries and the leaves. The red-blind person would see the cherries as green, as also the leaves; whilst the green-blind person would see both as red. Without regarding form it is probable that the red-blind would see the leaves as a bright green, whilst the green-blind would see them as darker red than the cherries. Failure to distinguish between the two is more likely to occur with the green of leaves, and the red of such fruits as cherries, since the former contains a marked proportion of red in it, and the latter a small proportion of green.

One highly-educated gentleman was led to know his deficiency in colour sense, by hearing a companion on a tour going into raptures over a sunset. He saw but little difference between it and that to be seen at midday. Testing his vision it appeared that he was totally blind to the sensation of green, and that white and purple would consequently be mistaken by him for one another. The crimson on the clouds, illuminated by the setting sun, would appear to him as only slightly different to the white clouds which he would see at midday; in fact he would be always seeing what to us would be a sunset. For this gentleman to mix spectrum colours to match others would evidently be no guide to normal-eyed persons.

We believe that amongst us in our daily life we have many persons who are blind to some colour, but who are not aware of it, or if they are aware of it, hide their defect as far as possible. That some are ignorant of it to a late period of their life we know.