Colour is merely an impression produced upon the retina, and therefore
on the brain, by various surfaces or media when light falls upon them or
passes through them. Remove the light, and colour ceases to exist. The
colour of a substance does not depend so much on the chemical character
of that substance, but rather and more directly upon the physical
condition of the surface or medium upon which the light falls or through
which it passes. I can illustrate this easily. For example, there is a
bright-red paint known as Crooke's heat-indicating paint. If a piece of
iron coated with this paint be heated to about 150 deg. F., the paint at
once turns chocolate brown, but it is the same chemical substance, for
on cooling we get the colour back again, and this can be repeated any
number of times. Thus we see that it is the peculiar physical structure
of bodies which appear coloured that has a certain effect upon the
light, and hence it must be from the light itself that colour really
emanates. Originally all colour proceeds from the source of light,
though it seems to come to the eye from the apparently coloured objects.
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