But when the sun is away, if the night is clear we have light from the starts.  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 not 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 are running on straight forward through the room, if we put something in their way, they 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.  Fir 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 higgledy-piggledy, 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 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 171 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 91 millions of miles.  The waves which are hitting your eye at this moment are caused by a movement which began at the sun only 7 1/2 minutes ago.  And remember,