th 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.
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
analysed that light. We get now a coloured band, similar to that of the
rainbow, and this band is called the spectrum (see Fig. 16). If we could
now run all these coloured rays together again, we should simply
reproduce white light. We can do this by catching the coloured band in
another prism, when the light now emerging will be found to be white.
Every part of that spectrum consists of homogeneous light, _i.e._ light
that cannot be further split up. The way in which the white light is so
unravelled by the prism is this: As the light passes through the prism
its different component coloured rays are variously deflected from their
normal course, so that on emerging we have each of these coloured rays
travelling in its own direction, vibrating in its own plane. It is well
to remember that the bending off, or deflection, or refraction, is
towards the thick end of the prism always, and that those of the
coloured rays in that analysed band, the spectrum, most bent away from
the original line of