THE COLOUR-SENSE.
The phenomenon of Colour is one with which all who are not blind must of necessity be familiar. So accustomed, indeed, have we been to it throughout all our lives, that most of us are inclined to take it for granted, and probably trouble ourselves very seldom as to its true cause. A brief discussion, therefore, of the nature of the Colour-sense may, we trust, prove not uninteresting to our readers.
What, then, is colour? It is obvious that it may be considered in two ways; we may either discuss the impression it makes on the mind, or the real external causes to which it is due. Viewed in the first light, colour is as much a sensation as is that of being struck or burnt. Viewed from the latter stand-point, it is merely a property of light; hence, in order correctly to understand its nature, we must first briefly examine the nature of this phenomenon.
According to modern scientific men, light is not a material substance, but consists of a kind of motion or vibration communicated by the luminous body to the surrounding medium, and travelling throughout space with an enormous velocity. The medium, however, through which light-waves travel is not air, nor any of the ordinary forms of matter. Of its real nature nothing is known, and its very existence is only assumed in order to account for the observed phenomena. It must be very subtle and very elastic; but it is a curious fact that the nature of the vibrations in question would seem to preclude the supposition that it is a fluid, these being rather such as would be met with in the case of a solid. To this medium, whatever its true nature may be, the name of ether is given.
The sensation, then, which we know by the name of Light is to be regarded as the effect on the retina of the eye of certain very rapid vibrations in the ether of the universe. All these waves travel with the same swiftness; but they are not all of the same length, nor of the same frequency; and investigation has shown that it is to this difference of wave-length that difference of colour is due. In other words, the impression to which we give the name of a certain colour is due to the effect on the retina of vibrations of a certain frequency. This conclusion is arrived at by a very simple experiment, in which advantage is taken of the following principle. So long as a ray of light is passing through the same medium, it travels in one straight line; but in passing obliquely from one medium into another of different density, its path is bent through a certain angle, just as a column of soldiers has a tendency to change its direction of march when obliquely entering a wood or other difficult ground. Now, this angle is naturally greatest in the case of the shortest waves, so that when a ray of light is thus bent out of its course—or, as it is called, ‘refracted’—the various sets of vibrations of which it is composed all travel in different directions, and may be separately examined. In fact the ray of light is analysed, or broken up into its component parts. The most convenient apparatus to employ for this purpose is a prism of glass. It is found, as is well known, that if a beam of ordinary sun-light be allowed to pass through the prism and be then received on a screen, it is resolved into a band of colours succeeding one another in the order of those of the rainbow. Such a band of colours is called a ‘spectrum.’
Now, of the visible portion of the spectrum the red rays are those which undergo the least refraction, while the violet rays are bent through the greatest angle, the other colours in their natural order being intermediate. From what has been said above, it is evident that, this being the case, the portion of the light consisting of waves of greatest length and least frequency is that which produces on the eye the sensation of red, and that each of the other colours is caused by vibrations of a certain definite length. We are speaking now of the visible part of the spectrum. As a matter of fact, the waves of least and greatest frequency make no impression on the eye at all; but the former have the greatest heating power, while the latter are those which chiefly produce chemical effects such as are utilised in photography.
Having now arrived at the nature of colour, we are in a position to apply these facts to the discussion of coloured substances.
When light falls on a body, a portion of it is turned back or, as it is called, ‘reflected’ from the surface; another part is taken up or ‘absorbed’ by the substance; while, in the case of a transparent body, a third portion passes on through it, and is said to be ‘transmitted.’ Most bodies absorb the different parts of the light in different proportions, and hence their various colours are produced. The colour of a transparent substance is that of the light which it transmits; while an opaque body is said to be of the colour of the light which it reflects, or rather of that part of it which is irregularly scattered.
There are three colours in the solar spectrum which are called ‘primary,’ owing to the fact that they cannot be produced by mixtures. These are red, violet, and deep olive green. The generally-received idea that red, blue, and yellow are primary colours, is by recent scientific authorities not regarded as tenable; it arose from observations on mixtures of pigments rather than of coloured light. For instance, objects seen through two plates of glass, one of which is blue and the other yellow, appear green; but this by no means justifies us in saying that a mixture of blue and yellow light is green. For remembering that the two glasses do not appear coloured by reason of their adding anything to the light, but rather through their stopping the passage of certain rays, we shall see that the green light which is finally transmitted is not a mixture of yellow and blue at all, but is rather that portion of the light which both of the glasses allow to pass. The blue glass will probably stop all rays except blue, violet, and green; the yellow glass, all but green, yellow, and orange. The only light, therefore, which can pass through both glasses is green. The same remark applies to mixtures of pigments, each particle being really transparent, though the whole bulk appears opaque. It is easy, however, to obtain real mixtures of coloured lights by employing suitable arrangements, of which one of the simplest consists of a disc painted with alternate bands of colours and rapidly rotated. By such means it is found that a mixture of blue and yellow is not green, but white or gray, and that yellow can itself be produced by a mixture of red and green in proper proportions. The late Professor Clerk Maxwell made an interesting series of experiments on colour mixtures by means of an apparatus known as Maxwell’s Colour-box, by which any number of colours could be combined in any required proportions.
It would, however, be beyond the scope of the present paper to discuss the many important results which followed from his investigations. Helmholtz believed the three primary colour sensations to be clue to the action of three sets of nerves at the back of the retina, each of which is excited only by vibrations within a certain range of frequency; and this theory is now generally held. In the case of some persons, the sensation corresponding to red is wholly absent, and the spectrum appears to consist of two colours with white or gray between. The nature of these colours is, for obvious reasons, difficult to determine; but one doubtless nearly corresponds to our sensation of blue, while the other is a deep colour, probably dark green. Persons thus affected are usually called ‘colour-blind;’ but this epithet is a misnomer, and the term ‘dichroic vision’ has been suggested for the phenomenon instead.
We have already remarked that our range of vision is comparatively narrow, the extreme portions of the spectrum making no impression on the retina. But we have no reason to think that these limits have been the same in all ages. The evidence would rather tend to show that the human eye is undergoing a slow and gradual development, which enables it to distinguish between colours which the ancients regarded as identical, and may in future render it able to perceive some portions at least of the parts of the spectrum which are now invisible. The Vedas of India, which are among the most ancient writings known, attest that in the most remote ages only white and black could be distinguished.
It would seem as if the perception of different degrees of intensity of light preceded by a long time the appreciation of various kinds of colours. After weighing the evidence, Magnus has come to the conclusion that red was the first colour to become visible, then yellow and orange; and afterwards, though at a considerable interval, green, blue, and violet in order. Various passages in the Old Testament have been cited as proof that the ancients failed to perceive all the colours seen by us, one of the most remarkable being in Ezekiel i. 27 and 28, where the prophet compares the appearance of the brightness round about the fire to that of the ‘bow that is in the cloud in the day of rain’—which passage has been cited by Mr Gladstone in his article in the Nineteenth Century for October 1877, as indicating a want of appreciation of distinct colours among the ancients. This is not quite clear, however, as the appearance round about the supernatural fire might have assumed auroral or rainbow tints. But the most important evidence on the apparent want of capacity among the ancients to discriminate between colours is that afforded by the writings of Homer, who, in the opinion of Magnus, could neither have perceived green nor blue. The point has been carefully examined by Mr Gladstone, who comes to the conclusion that this estimate is quite within the mark. Inquiring in detail into each of Homer’s colour-epithets, he shows that almost all must be in reality regarded as expressing degrees of intensity rather than of quality, and that the few exceptions are all confined to red and yellow. The brilliant blue sky of the southern climes where Homer lived must have appeared to him as of a neutral gray hue. Of course, the suggestion that the writings usually assigned to Homer were in reality the productions of many authors, does not invalidate the reasoning at all, as we do not attribute any defect in vision to the poet which was not equally manifested by his contemporaries.
It is curious that the distinction between green and blue is not yet perfectly developed in all nations. Travellers tell us that the Burmese often confuse these colours in a remarkable manner. This and other facts suggest that the development of the colour-sense is not yet completed; and that in the future our range of perception may be still further enlarged, so that the now invisible rays may be recognised by the eye as distinct colours.