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.
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