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.