Now, when the light passes through the three-sided glass or prism, the waves are spread out, and the slow, heavy, red waves lag behind and remain at the lower end R of the coloured line on the wall (Fig. 7), while the rapid little violet waves are bent more out of their road and run to V at the farther end of the line; and the orange, yellow, green, blue, and indigo arrange themselves between, according to the size of their waves.

And now you are very likely eager to ask why the quick waves 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 the room.  Why?  Because they had no light-waves to send to your eye.  But as the sunbeams began to pour in at the window, the waves played upon the things in the room, and when they hit them they bounded off them back to your eye, as a wave of the sea bounds back from a rock and strikes against a passing boat.  Then, when they fell upon your eye, they entered it and excited the retina and the nerves, and the image of the chair or the table was carried to your brain.  Look around at all the things in this room.  Is it not strange to think that each one of them is sending these invisible messengers straight to your eye as you look at it; and that you see me, and distinguish me from the table, entirely by the kind of waves we each send to you?