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 pa