SUNBEAMS AND THEIR WORK. 
37 
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 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 
Fie- 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 caught. 
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 
