The waves sent out on every side by the energetic electrons become
faintly visible to us when they reach about 1/35,000 of an inch. As they
become shorter and more rapid, as the electrons increase their speed, we
get, in succession, the colours red, orange, yellow, green, blue,
indigo, and violet. Each distinct sensation of colour means a wave of
different length. When they are all mingled together, as in the light of
the sun, we get white light. When this white light passes through glass,
the speed of the waves is lessened; and, if the ray of light falls
obliquely on a triangular piece of glass, the waves of different lengths
part company as they travel through it, and the light is spread out in a
band of rainbow-colour. The waves are sorted out according to their
lengths in the "obstacle race" through the glass. Anyone may see this
for himself by holding up a wedge-shaped piece of crystal between the
sunlight and the eye; the prism separates the sunlight into its
constituent colours, and these various colours will be seen quite
readily. Or the thing may be realised in another way. If the seven
colours are painted on a wheel as shown opposite page 280 (in the
proportion shown), and the wheel rapidly revolved on a pivot, the wheel
will appear a dull white, the several colours will not be seen. But
_omit_ one of the colours, then the wheel, when revolved, will not
appear white, but will give the impression of one colour, corresponding
to what the union of six colours gives. Another experiment will show
that some bodies held up between the eye and a white light will not
permit all the rays to pass through, but will intercept some; a body
that intercepts all the seven rays except red will give the impression
of red, or if all the rays except violet, then violet will be the colour
seen.
[Illustration: _Photo: H. J. Shepstone._