Part 41

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 which reflects only a portion of the light, but which exhibits no predominance in any particular hue, is called _grey_. A white surface looks grey beside a similar surface more brilliantly illuminated.

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 which transmits all the visible rays equally well is said to be colourless; pure water, for example, is nearly quite colourless, though in large masses it appears bluish-green. A translucent substance is one which partially transmits light. Translucency is due to the light being scattered by minute embedded particles or minute irregularities of structure. Some fibrous specimens of tremolite and gypsum are translucent in the direction of the fibres, and practically opaque in a transverse direction. Coloured transparent objects vary in shade and hue according to their size; thus, a conical glass filled with a red liquid commonly appears yellow at the bottom, varying through orange up to red at the upper part. A coloured powder is usually of a much 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 _dichromatism_ (from [Greek: di-], double, and [Greek: chrôma], colour), that is, they appear of one colour when viewed in strata of moderate thickness, but of a different colour in greater thicknesses (see Absorption of Light).

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 order--red, 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 physical conditions rather than to the specific properties of the material; such as the colour of a white object when illuminated by light of some

## particular colour; the colours seen in a film of oil on water or in

mother-of-pearl, or soap-bubbles, due to interference (q.v.); the colours seen through the eyelashes or through a thin handkerchief held up to the light, due to diffraction (q.v.); and the colours caused by ordinary refraction, as in the rainbow, double refraction and polarization (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 quite different. This is especially the case with yellow and blue, which on the screen combine to produce white, generally with a pink tint, but cannot be made to give green. The reason of this difference in the two results is that in the former case we do not get a true combination of the colours at all. When the mixed pigments are illuminated by white light, the yellow particles absorb the red and blue rays, but reflect the yellow along with a good deal of the neighbouring green and orange. The blue particles, on the other hand, absorb the red, orange and yellow, but reflect the blue and a good deal of green and violet. As much of the light is affected by several particles, most of the rays are absorbed except green, which is reflected by both pigments. Thus, the colour of the mixture is not a mixture of the colours yellow and blue, but the remainder of white light after the yellow and blue pigments have absorbed all they can. The effect can also be seen in coloured solutions. If two equal beams of white light are transmitted respectively through a yellow solution of potassium bichromate and a blue solution of copper sulphate in proper thicknesses, they can be compounded on a screen to an approximately white colour; but a single beam transmitted through both solutions appears green. Blue and yellow pigments would produce the effect of white only if very sparsely distributed. This fact is made use of in laundries, where cobalt blue is used to correct the yellow colour of linen after washing.

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.

[Illustration: FIG. 1.]

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, 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 of Newton's. Let RGB (fig. 1) be an equilateral triangle, and let the angular points be coloured red, green and blue of such intensities as to produce white if equally combined; and let the colour of every point of the triangle be determined by combining such proportions of the three primaries, that three weights in the same proportion would have their centre of gravity at the point. Then the centre of the triangle will be a neutral tint, white or grey; and the middle points of the sides Y, S, P will be yellow, greenish-blue and purple. The hue varies all round the perimeter. The tint varies along any straight line 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 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.

[Illustration: FIG. 2.]

[Illustration: (After Müller-Pouillet's _Lehrbuch der Physik_, 1897.) FIG. 3.]

Any two colours which together give white are called _complementary_ colours. Greenish-yellow and blue are a pair of complementaries, as already mentioned. 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 l 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 years by A. Schuster, Lord Rayleigh and others, and it has been shown that even if it consisted of a succession of somewhat irregular impulses, it would still be resolved, by the dispersive property of a prism or grating, into trains of regular frequency. We may still, however, speak of white light as compounded of the rays of the spectrum, provided we mean only that the two systems are mathematically equivalent, and not that the homogeneous trains exist as such in the original light.

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 1884-5-6_; Abney's _Colour Vision_ (1895). (J. R. C.)

COLOURS, MILITARY, the flags carried by infantry regiments and battalions, sometimes also by troops of other arms. Cavalry regiments and other units have as a rule standards and guidons (see FLAG). Colours are generally embroidered with mottoes, symbols, and above all with the names of battles.

From the earliest time at which men fought in organized bodies of troops, the latter have possessed some sort of insignia visible over all the field of battle, and serving as a rallying-point for the men of the corps and an indication of position for the higher leaders and the men of other formed bodies. In the Roman army the eagle, the _vexillum_, &c. had all the moral and sentimental importance of the colours of to-day. During the dark and the middle ages, however, the basis of military force being the individual knight or lord, the banner, or other flag bearing his arms, replaced the regimental colour which had signified the corporate body and claimed the devotion of each individual soldier in the ranks, though the original meaning of the colour as a corps, not a personal distinction, was sometimes maintained by corporate bodies (such as trade-gilds) which took the field as such. An example is the famous _carroccio_ or standard on wheels, which was frequently brought into the field of battle by the citizen militia of the Italian cities, and was fought for with the same ardour as the royal standard in other medieval battles.

The application of the word "colour" to such insignia, however, dates only from the 16th century. It has been suggested that, as the professional captain gradually ousted the nobleman from the command of the drilled and organized companies of foot--the man of gentle birth, of course, maintained his ascendancy in the cavalry far longer--the leaders of such bodies, no longer possessing coat-armour and individual banners, had recourse to small flags of distinctive colour instead. "Colour" is in the 16th century a common name in England and middle Europe for the unit of infantry; in German the _Fähnlein_ (colour) of landsknechts was a strong company of more than 300 foot. The ceremonial observances and honours paid nowadays to the colours of infantry were in fact founded for the most part by the landsknechts, for whom the flag (carried by their "ensign") was symbolical of their intense regimental life and feeling. The now universal customs of constituting the colour guard of picked men and of saluting the colours were in equal honour then; before that indeed, the appearance of the personal banner of a nobleman implied his actual presence with it, and the due honours were paid, but the colour of the 16th century was not the distinction of one man, but the symbol of the corporate life and unity of the regiment, and thus the new colour ceremonial implied the same allegiance to an impersonal regimental spirit, which it has (with the difference that the national spirit has been blended with the regimental) retained ever since. The old soldier rallied to the colours as a matter of habit in the confusion of battle, and the capture or the loss of a colour has always been considered a special event, glorious or the reverse, in the history of a regiment, the importance of this being chiefly sentimental, but having as a very real background the fact that, if its colour was lost, a regiment was to all intents and purposes dissolved and dispersed. Frederick the Great and Napoleon always attached the highest importance to the maintenance at all costs of the regimental colours. Even over young troops the influence of the colour has been extraordinary, and many generals have steadied their men in the heat of battle by taking a regimental colour themselves to lead the advance or to form up the troops. Thus in the first battle of Bull Run (1861) the raw Confederate troops were rallied under a heavy fire by General Joseph Johnston, their commander-in-chief, who stood with a colour in his hand until the men gathered quickly in rank and file. The archduke Charles at Aspern (1809) led his young troops to the last assault with a colour in his hand. Marshal Schwerin was killed at the battle of Prague while carrying a regimental colour.

In the British army colours are carried by guards and line (except rifle) battalions, each battalion having two colours, the king's and the regimental. The size of the colour is 3 ft. 9 in. by 3 ft., and the length of the stave 8 ft. 7 in. The colour has a gold fringe and gold and crimson tassels, and bears various devices and "battle honours." Both colours are carried by subaltern officers, and an escort of selected non-commissioned officers forms the rest of the colour party. The ceremony of presenting new colours is most impressive. The old colours are "trooped" (see below) before being cased and taken to the rear. The new colours are then placed against a pile of drums and then uncased by the senior majors and the senior subalterns. The consecration follows, after which the colours are presented to the senior subalterns. The battalion gives a general salute when the colours are unfurled, and the ceremony concludes with a march past. "Trooping the colour" is a more elaborate ceremonial peculiar to the British service, and is said to have been invented by the duke of Cumberland. In this, the colour is posted near the left of the line, the right company or guard moves up to it, and an officer receives it, after which the guard with the colour files between the ranks of the remainder from left to right until the right of the line is reached.

In the United States army the infantry regiment has two colours, the national and the regimental. They are carried in action.

In the French army one colour (_drapeau_) is carried by each infantry regiment. It is carried by an officer, usually a _sous-lieutenant_, and the guard is composed of a non-commissioned officer and a party of "first class" soldiers. Regiments which have taken an enemy's colour or standard in battle have their own colours "decorated," that is, the cross of the Legion of Honour is affixed to the stave near the point. Battle honours are embroidered on the white of the tricolour. The _eagle_ was, in the First and Third Empires, the infantry colour, and was so called from the gilt eagle which surmounted the stave. The _chasseurs à pied_, like the rifles of the British army, carry no colours, but the battalion quartered for the time being at Vincennes carries a colour for the whole arm in memory of the first _chasseurs de Vincennes_. As in other countries, colours are saluted by all armed bodies and by individual officers and men. When the _drapeau_ is not present with the regiment its place is taken by an ordinary flag.

The colours of the German infantry, foot artillery and engineers vary in design with the states to which the corps belong in the first instance; thus, black and white predominate in Prussian colours, red in those of Württemberg regiments, blue in Bavarian, and so on. The point of the colour stave is decorated in some cases with the iron cross, in memory of the War of Liberation and of the war of 1870. Each battalion of an infantry regiment has its own colour, which is carried by a non-commissioned officer, and guarded as usual by a colour party. The colour is fastened to the stave by silver nails, and the ceremony of driving the first nail into the stake of a new colour is one of great solemnity. Rings of silver on the stave are engraved with battle honours, the names of those who have fallen in action when carrying the colour, and other commemorative names and dates. The oath taken by each recruit on joining is sworn on the colour (_Fahneneid_).

The practice in the British army of leaving the colours behind on taking the field dates from the battle of Isandhlwana (22nd January 1879), in which Lieutenants Melvill and Coghill lost their lives in endeavouring to save the colours of the 24th regiment. In savage warfare, in which the British regular army is more usually engaged, it is true that no

## particular reason can be adduced for imperilling the colours in the