Is it not curious to think that there is really no
such thing as colour in the leaf, the table, the coat,
or the geranium flower, but we see them of different
colours because, for some reason, they send back only
certain coloured waves to our eye?
Wherever you look, then, and whatever you see, all
the beautiful tints, colours, lights, and shades around
you are the work of the tiny sun-waves.
Again, light does a great deal of work when it falls
upon plants. Those rays of light which are caught
by the leaf are by no means idle; we shall see in
Lecture VII that the leaf uses them to digest its
food and make the sap on which the plant feeds.
We all know that a plant becomes pale and sickly if
it has not sunlight, and the reason is, that without
these light-waves it cannot get food out of the air,
nor make the sap and juices which it needs.
When you look at plants and trees growing in the beautiful
meadows; at the fields of corn, and at the lovely
landscape, you are looking on the work of the tiny
waves of light, which never rest all through the day
in helping to give life to every green thing that
grows.
So far we have spoken only of light; but hold your
hand in the sun and feel the heat of the sunbeams,
and then consider if the waves of heat do not do work
also. There are many waves in a sunbeam which
move too slowly to make us see light when they hit
our eye, but we can feel them as heat, though we cannot
see them as light. The simplest way of feeling
heat-waves is to hold a warm iron near your face.
You know that no light comes from it, yet you can
feel the heat-waves beating violently against your
face and scorching it. Now there are many of
these dark heat-rays in a sunbeam, and it is they
which do most of the work in the world.
In the first place, as they come quivering to the
earth, it is they which shake the water-drops apart,
so that these are carried up in the air, as we shall
see in the next lecture. And then remember,
it is these drops, falling again as rain, which make
the rivers and all the moving water on the earth.
So also it is the heat-waves which make the air hot
and light, and so cause it to rise and make winds
and air-currents, and these again give rise to ocean-currents.
It is these dark rays, again, which strike upon the
land and give it the warmth which enables plants to
grow. It is they also which keep up the warmth
in our own bodies, both by coming to us directly from
the sun, and also in a very roundabout way through
plants. You will remember that plants use up
rays of light and heat in growing; then either we
eat the plants, or animals eat the plants and we eat
the animals; and when we digest the food, that heat
comes back in our bodies, which the plants first took
from the sunbeam. Breathe upon your hand, and
feel how hot your breath is; well, that heat which
you feel, was once in a sunbeam, and has travelled
from it through the food you have eaten, and has now
been at work keeping up the heat of your body.
