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° 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.
But without some elucidation this statement would appear as an enigma,
since it might be urged that the light of the sun as well as that of
artificial light is white, and not coloured. I hope, however, to show
you that that light is white, because it is so much coloured, so
variously and evenly coloured, though I admit the term "coloured" here
is used in a special sense. White light contains and is made up of all
the differently coloured rainbow rays, which are continually vibrating,
and whose wave-lengths and number of vibrations distinguish them from
each other. We will take some white light from an electric lantern and
throw it on a screen. In a prism of glass we have a simple instrument
for unravelling those rays, and instead of letting them all fall on the
same spot and illumine it with a white light, it causes them to fall
side by side; in fact they all fall apart, and the prism has actually