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 Muller-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 _vexil