ions
on a wheel, as shown in the coloured illustration, and the wheel be
turned rapidly on a pivot through its centre, only a dull white will be
perceived. If one colour be omitted, the result will be one colour--the
result of the union of the remaining six.]

Beyond the waves of violet light are the still shorter and more rapid
waves--the "ultra-violet" waves--which are precious to the photographer.
As every amateur knows, his plate may safely be exposed to light that
comes through a red or an orange screen. Such a screen means "no
thoroughfare" for the blue and "beyond-blue" waves, and it is these
which arrange the little grains of silver on the plate. It is the same
waves which supply the energy to the little green grains of matter
(chlorophyll) in the plant, preparing our food and timber for us, as
will be seen later. The tree struggles upward and spreads out its leaves
fanwise to the blue sky to receive them. In our coal-measures, the
mighty dead forests of long ago, are vast stores of sunlight which we
are prodigally using up.

The X-rays are the extreme end, the highest octave, of the series of
waves. Their power of penetration implies that they are excessively
minute, but even these have not held their secret from the modern
physicist. From a series of beautiful experiments, in which they were
made to pass amongst the atoms of a crystal, we learned their length. It
is about the ten-millionth of a millimetre, and a millimetre is about
the 1/25 of an inch!

One of the most recent discoveries, made during a recent eclipse of the
sun, is that light is subject to gravitation. A ray of light from a star
is bent out of its straight path when it passes near the mass of the
sun. Professor Eddington tells us that we have as much right to speak of
a pound of light as of a pound of sugar. Professor Eddington even
calculates that the earth receives 160 tons of light from the sun every
year!

ENERGY: HOW ALL LIFE DEPENDS ON IT

As we have seen in an earlier chapter, one of the fundamental entities
of the universe is matter. A second, not less important, is called
energy. Energy is indispensable if the world is to continue to exist,
since all phenomena, including life, depend on it. Just as it is humanly
impossible to create or to destroy a particle of matter, so is it
impossible to create or to destroy energy. This statement will be more
readily understood when we have considered what energy is.

Energy, like matter