Straight away from the sun on all sides, never stopping, never resting, bat chasing after each other with marvellous quickness, these tiny waves travel out into space by night and by day. When our spot of the earth where England lies is turned away from them and they cannot touch us, then it is night for us, but directly England is turned so as to face the sun, then they strike on the land, and the water, and warm it ; or upon our eyes, making the nerves quiver so that we see Hght. Look up at the sun and picture to yourself that instead of one great blow from a fist causing you to see stars for a moment, millions of tiny blows from these sun-waves are striking every instant on your eye ; then you will easily understand that this would cause you to see a constant blaze of light.

But when the sun is away, if the night is clear we have light from the stars. Do these then too make waves all across the enormous distance between them and us ? Certainly they do, for they too are suns like our own, only they are so far off that the waves they send are more feeble, and so we only notice them when the sun's stronger waves are away.

But perhaps you will ask, if no one has ever seen these waves nor the ether in which they are made, what right have we to say they are there } Strange as it may seem, though we cannot see them we have measured them and know how large they are, and how many can go into an inch of space. For as these tiny waves 3re running on straight forward through the room, if we put something in their way, thev will have to run round it ; and if you let in a very narrow ray of light through a shutter and put an upright wire in the sunbeam, you actually make the waves run round the wire just as water runs round a post in a river; and they meet behind the wire, just as the water meets in a V shape behind the post. Now when they meet, they run up against each other, and here it is we catch them. For if they meet comfortably, both rising up in a good wave, they run on together and make a bright line of light ; but if they meet higgledypiggledy, one up and the other down, all in confusion, they stop each other, and then there is no light, but a line of darkness. And so behind your piece of

Fig. 5.

A, Hole in the shutter. B, Wire placed in the beam of light S S, Screen on which the dark and light bands are canghL wire you can catch the waves on a piece of paper, and you will find they make dark and light lines one side by side with the other, and by means of these bands it is possible to find out how large the waves must be. This question is too difficult for us to work

it out here, but you can see that large waves will make broader light and dark bands than small ones will, and that in this way the size of the waves may be measured.

And now how large do you think they turn out to be ? So very, very tiny that about fifty thousand waves are contained in a single inch of space ! I have drawn on the board the length of an inch,* and now I will measure the same space in the air between my finger and thumb. Within this space at this moment there are fifty thousand tiny waves moving up and down ! I promised you we would find in science things as wonderful as in fairy tales. Are not these tiny invisible messengers coming incessantly from the sun as wonderful as any fairies } and still more so when, as we shall see presently, they are doing nearly all the work of our world.

We must next try to realize how fast these waves travel. You will remember that an express train would take 17 1 years to reach us from the sun; and even a cannon-ball would take from ten to thirteen years to come that distance. Well, these tiny waves take only seven minutes and a half to come the whole 9 1 millions of miles. The waves which are hitting your eye at this moment are caused by a movement which began at the sun only 7J minutes ago. And remember, this movement is going on incessantly, and these waves are always following one after the other so rapidly that they keep up a perpetual cannonade upon the pupil of your eye. So fast do they come that about 608 billion waves enter your eye in one • The width of an inch may be seen in Fig. 12, p. 60.

single second.* do not ask you to remember these figures ; I only ask you to try and picture to yourselves these infinitely tiny and active invisible messengers from the sun, and to acknowledge that light is a fairy thing.

But we do not yet know all about our sunbeam. See, I have here a piece of glass with three sides, called a prism. If I put it in the Fig. 6. sunlight which is streaming through the window, what happens ? Look ! on the table there is a line of beautiful colours. I can make it long or short, as I turn the prism, but the colours always remain arranged in the same way. Here at my left hand is the red, beyond it orange, then yellow, green, blue, indigo or deep blue, and violet, shading one into the other all alone the line. We have all seen these colours Fig. 7 dancing on the wall when the sun has been shining brightly on the cut - glass pendants of the chandelier and you ^^^^^^^^^ spectrum thrown by a prism on the ' ^ . wail, may see them still d E, Window-shutter. F, Round hole in it. more distinctly if ABC, Glass prism. M N, Wall. you let a ray of light into a darkened room, and pass it through the prism as in the diagram (Fig. 7).

Light travels at the rate of 192,000 miles, or 12, 165, 120,000 inches in a second. Taking the averai^e number of wave-lengths in an inch af 50,000, then 12,165,120,000 X i;o.ooo = 608.256,000,000,000.

What are these colours ? Do they come from the glass ? for you will remember to have seen them in the rainbow, and in the soap-bubble, and even in a drop of dew or the scum on the top of a pond. This beautiful coloured line is only our sunbeam again, which has been split up into many colours by passing through the glass, as it is in the rain-drops of the rainbow and the bubbles of the scum of the pond.

Till now we have talked of the sunbeam as if it were made of only one set of waves, but in truth it is made of many sets of waves of different sizes, all travelling along together from the sun. These various waves have been measured, and we know that the waves which make up red light are larger and more lazy than those which make violet light, so that there are only thirty-nine thousand red waves in an inch, while there are fifty-seven thousand violet waves in the same space.

How is it then, that if all these different waves, making different colours, hit on our eye, they do not always make us see coloured light } Because, unless they are interfered with, they all travel along together, and you know that all colours, mixed together in proper proportion, make white.

I have here a round piece of cardboard, painted with the seven colours in succession several times over. When it is still you can distinguish them all apart, but when I whirl it quickly round—see !—the cardboard

looks quite white, because we see them all so instantaneously that they are mingled together. In the same way light looks white to you, because all the different colojred waves strike on your eye at once. You can easily make one of these cards for yourselves, only the white will always look dirty, because you cannot get the colours pure.

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 ver}- 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 4

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 wt 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 now, 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 'i 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 t(j 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 ?

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