o 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