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COLOUR (Lat. color, connected with celare, to hide, the root meaning, therefore, being that of a covering) . The visual apparatus of the eye enables us to distinguish not only differences of form, size and brilliancy in the objects looked upon, but also differences in the character of the light received from them. These latter differences, familiar to us as differences in colour, have their physical origin in the variations in wave-length (or frequency) which may exist in light which is capable of exciting the sensation of vision. From the physical point of view, light of a pure colour, or homogeneous light, means light whose undulations are mathematically of a simple character and which cannot be resolved by a prism into component parts. All the visible pure colours, as thus defined, are to be found in the spectrum, and there is an infinite number of them, corresponding to all the possible variations of wave-length within the limits of the visible spectrum (see SPECTROSCOPY). On this view, there is a strict analogy between variations of colour in light and variations of pitch in sound, but the visible spectrum contains a range of frequency extending over about one octave only, whereas the range of audibility embraces about eleven octaves. Of all the known colours it might naturally be thought that white is the simplest and purest, and, till Sir Isaac Newton's time, this was the prevailing opinion . Newton, however, showed that white light could be decomposed by a prism into the spectral colours red, orange, yellow, green, blue, indigo and violet; the colours appearing in this order and passing gradually into each other without abrupt transitions. White is therefore not a simple colour, but is merely the colour of sunlight, and probably owes its apparently homogeneous character to the fact that it is the average colour of the light which fills the eye when at rest. The colours of the various objects which we see around us are not due (with the exception of self-luminous and fluorescent bodies) to any power possessed by these objects of creating the colours which they exhibit, but merely to the exercise of a selective action on the light of the sun, some of the constituent rays of the white light with which they are illuminated being absorbed, while the rest are reflected or scattered in all directions, or, in the case of transparent bodies, transmitted. White light is thus the basis of all other colours, which are derived from it by the suppression of some one or more of its parts. A red flower, for instance, absorbs the blue and green rays and most of the yellow, while the red rays and usually some yellow are scattered. If a red poppy is illuminated successively by red, yellow, green and blue light it will appear a brilliant red in the red light, yellow in the yellow light, but less brilliant if the red colour is pure; and black in the other colours, the blackness being due to the almost complete absorption of the corresponding colour.Bodies may be classified as regards colour according to the nature of the action they exert on white light. In the case of ordinary opaque bodies a certain proportion of the incident light is irregularly reflected or scattered from their surfaces. A white object is one which reflects nearly all the light of all colours; a black object absorbs nearly all. A body
The next class is that of most transparent bodies, which owe their colour to the light which is transmitted, either directly through, or reflected back again at the farther surface. A body
lighter tint than the substance in bulk, as the light is reflected back after transmission through only a few thin layers. For the same reason the powders of transparent substances are opaque. Polished bodies, whether opaque or transparent, when illuminated with white light and viewed at the proper angle, reflect the incident light regularly and appear white, without showing much of their distinctive colours. Some bodies reflect light of one colour and transmit that of another; such bodies nearly always possess the properties of selective or metallic reflection and anomalous dispersion. Most of the coal-tar dyes belong to this category. Solid eosin, for example, reflects a yellowish-green and transmits a red light. Gold appears yellow under ordinary circumstances, but if the light is reflected many times from the surface it appears a ruby colour. On the other hand, a powerful beam of light transmitted through a thin gold-leaf appears green.Some solutions exhibit the curious phenomenon of dichromalism (from &-, double
The blue colour of the sky (q.v.) has been explained by Lord Rayleigh as due to the scattering of light by small suspended particles and air molecules, which is most effective in the case of the shorter waves (blue). J. Tyndall produced similar effects in the laboratory. The green colour of sea-water near the shore is also due to a scattering of light. The colours of bodies which are gradually heated to white incandescence occur in the orderred, orange, yellow, white. This is because the longer waves of red light are first emitted, then the yellow as well, so that orange results, then so much green that the total effect is yellow, and lastly all the colours, compounding to produce white. Fluorescent bodies have the power of converting light of one colour into that of another (See FLUORESCENCE). Besides the foregoing kinds of colorization, a body may exhibit, under certain circumstances, a colouring due to some special
double
(qq.v.). Composition of Colours.It has been already pointed out that white light is a combination of all the colours in the spectrum. This was shown by Newton, who recombined the spectral colours and produced white. Newton also remarks that if a froth be made on the surface of water thickened a little with soap, and examined closely, it will be seen to be coloured with all the" colours of the spectrum, but at a little distance it looks white owing to the combined effect on the eye of all the colours. The question of the composition of colours is largely a physiological one, since it is possible, by mixing colours, say red and yellow, to produce a new colour, orange, which appears identical with the pure orange of the spectrum, but is physically quite different, since it can be resolved by a prism into red and yellow again. There is no doubt that the sensation of colour-vision is threefold, in the sense that any colour can be produced by the combination, in proper proportions, of three standard colours. The question then arises, what are the three primary colours? Sir David Brewster considered that they were red, yellow and blue; and this view has been commonly held by painters and others, since all the known brilliant hues can be derived from the admixture of red, yellow and blue pigments. For instance, vermilion and chrome yellow will give an orange, chrome yellow and ultramarine a green, and vermilion and ultramarine a purple mixture. But if we superpose the pure spectral colours on a screen, the resulting colours are quitedifferent. This is especially the case with yellow and blue, which on the screen combine to produce white, generally with a pink
Thomas Young suggested red, green and violet as the primary colours, but the subsequent experiments of J. Clerk Maxwell appear to show that they should be red, green and blue. Sir William Abney, however, assigns somewhat different places in the spectrum to the primary colours, and, like Young, considers that they should be red, green and violet. All other hues can be obtained by combining the three primaries in proper proportions. Yellow is derived from red and green. This can be done by superposition on a screen or by making a solution which will transmit only red and green rays. For this purpose Lord Rayleigh recommends a mixture of solutions of blue litmus and yellow potassium chromate. The litmus stops the yellow and orange light, while the potassium chromate stops the blue and violet. Thus only red and green are transmitted, and the result is a full compound yellow which resembles the simple yellow of the spectrum in appearance, but is resolved into red and green by a prism. The brightest yellow pigments are those which give both the pure and compound yellow. Since red and green produce yellow, and yellow and blue produce white, it follows that red, green and blue can be compounded into white. H. von Helmholtz has shown that the only pair of simple spectral colours capable of compounding to white are a greenish-yellow and blue. Just as musical sounds differ in pitch, loudness and quality, so may colours differ in three respects, which Maxwell calls hue, shade and tint. All hues can be produced by combining every pair of primaries in every proportion. The addition of white alters the tint without affecting the hue. If the colour be darkened by adding black or by diminishing the illumination, g a variation in shade is produced. Thus the hue red includes every variation in tint from red to white, and every variation in shade from red to black, and similarly for other hues. We can represent every hue and tint on a diagram in a manner proposed by Young, following a very similar suggestion S B of Newton's. Let RGB (fig. r) be an MILITARY 729 through W. To vary the shade, the whole triangle must be uniformly darkened. The simplest way of compounding colours is by means of Maxwell's colour top, which is a broad spinning-top over the spindle of which coloured disks can be slipped (fig. 2). The disks are slit radially so that they can be slipped partially over each other and the surfaces exposed in any desired ratio. Three disks are used together, and a match is obtained between these and a pair of smaller ones mounted on the same spindle. If any five colours are taken, two of which may be black and white, a match can be got between them by suitable adjustment. This shows that a relation exists between any four colours (the black being only needed to obtain the proper intensity) and that consequently the number of independent colours is three. A still better instrument for FIG. 2.combining colours is Maxwell's colour box, in which the colours of the spectrum are combined by means of prisms. Sir W. Abney has also invented an apparatus for the same purpose, which is much the same in principle as Maxwell's colour box. Several methods of colour photography depend on the fact that all varieties of colour can be compounded from red, green and blue in proper proportions. Any two colours which together give white are called complementary colours. Greenish-yellow and blue are a pair of complementaries, as already men- tioned. Any number of pairs may be obtained by a simple device due to Helmholtz and represented in fig. 3. A beam of white light, decomposed by the prism P, is recompounded into white light by the lens 1 and focussed on a screen at f. If the thin prism p is inserted near the lens, any set of colours may be deflected to another point n, thus producing two coloured and complementary images of the source of light. Nature of White Light.The question as to whether white light actually consists of trains of waves of regular frequency has been discussed in recent
original
See also Newton's Opticks, bk. i. pt. ii.; Maxwell's Scientific Papers; Helmholtz's papers in Poggendorf's Annalen; Sir G. G. Stokes, Burnett Lectures for 88456; Abney's Colour Vision (1895). (J. R. C.) End of Article: COLOUR (Lat. color, connected with celare, to hide, the root meaning, therefore, being that of a covering) If you wish, you can link directly to this article.
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