should make us see one colour, and the slow waves another. This
is a very difficult question, for we have a great deal still to
learn about the effect of light on the eye. But you can easily
imagine that colour is to our eye much the same as music is to
our ear. You know we can distinguish different notes when the
air-waves play slowly or quickly upon the drum of the ear (as we
shall see in Lecture VI) and somewhat in the same way the tiny
waves of the ether play on the retina or curtain at the back of
our eye, and make the nerves carry different messages to the
brain: and the colour we see depends upon the number of waves
which play upon the retina in a second.

Do you think we have now rightly answered the question - What is
a sunbeam? We have seen that it is really a succession of tiny
rapid waves, travelling from the sun to us across the invisible
substance we call "ether", and keeping up a constant cannonade
upon everything which comes in their way. We have also seen
that, tiny as these waves are, they can still vary in size, so
that one single sunbeam is made up of myriads of different-sized
waves, which travel all together and make us see white light;
unless for some reason they are scattered apart, so that we see
them separately as red, green, blue, or yellow. How they are
scattered, and many other secrets of the sun-waves, we cannot
stop to consider not, but must pass on to ask -

What work do the sunbeams do for us?

They do two things - they give us light and heat. It is by means
of them alone that we see anything. When the room was dark you
could not distinguish the table, the chairs, or even the walls of