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.
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