phere all day long is sorting out the waves. The blue
"sky" overhead means that the fine particles in the upper atmosphere
catch the shorter waves, the blue waves, and scatter them. We can make a
tubeful of blue sky in the laboratory at any time. The beautiful
pink-flush on the Alps at sunrise, the red glory that lingers in the
west at sunset, mean that, as the sun's rays must struggle through
denser masses of air when it is low on the horizon, the long red waves
are sifted out from the other shafts.

Then there is the varied face of nature which, by absorbing some waves
and reflecting others, weaves its own beautiful robe of colour. Here and
there is a black patch, which _absorbs_ all the light. White surfaces
_reflect_ the whole of it. What is reflected depends on the period of
vibration of the electrons in the particular kind of matter. Generally,
as the electrons receive the flood of trillions of waves, they absorb
either the long or the medium or the short, and they give us the
wonderful colour-scheme of nature. In some cases the electrons continue
to radiate long after the sunlight has ceased to fall upon them. We get
from them "black" or invisible light, and we can take photographs by it.
Other bodies, like glass, vibrate in unison with the period of the
light-waves and let them stream through.

Light without Heat

There are substances--"phosphorescent" things we call them--which give
out a mysterious cold light of their own. It is one of the problems
of science, and one of profound practical interest. If we could produce
light without heat our "gas bill" would shrink amazingly. So much energy
is wasted in the production of heat-waves and ultra-violet waves which
we do not want, that 90 per cent. or more of the power used in
illumination is wasted. Would that the glow-worm, or even the dead
herring, would yield us its secret! Phosphorus is the one thing we know
as yet that suits the purpose, and--it smells! Indeed, our artificial
light is not only extravagant in cost, but often poor in colour. The
unwary person often buys a garment by artificial light, and is disgusted
next morning to find in it a colour which is not wanted. The colour
disclosed by the sun was not in the waves of the artificial light.

[Illustration: ROTATING DISC OF SIR ISAAC NEWTON FOR MIXING COLOURS

The Spectroscope sorts out the above seven colours from sunlight (which
is compounded of these seven colours). If painted in proper proport