Interference
colours are produced either by thin films or by very fine striae on the
surfaces of bodies, which cause rays of certain wave-lengths to
neutralise each other, leaving the remainder to produce the effects of
colour. Such are the colours of soap-bubbles, or of steel or glass on
which extremely fine lines have been ruled; and these colours often
produce the effect of metallic lustre, and are the cause of most of the
metallic hues of birds and insects.
As colour thus depends on molecular or chemical constitution or on the
minute surface texture of bodies, and, as the matter of which organic
beings are composed consists of chemical compounds of great complexity
and extreme instability, and is also subject to innumerable changes
during growth and development, we might naturally expect the phenomena
of colour to be more varied here than in less complex and more stable
compounds. Yet even in the inorganic world we find abundant and varied
colours; in the earth and in the water; in metals, gems, and minerals;
in the sky and in the ocean; in sunset clouds and in the many-tinted
rainbow.
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