SUNBEAMS AND THEIR WORK. 45 
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. 
