44 THE FAIRY-LAND OF SCIENCE. 
those which pass into the wall, by giving motion to its 
atoms, lose their own vibrations. 
Into polished shining metal the waves hardly enter 
at all, but are thrown back from the surface ; and so a 
steel knife or a silver spoon are very bright, and are 
clearly seen. Quicksilver is put at the back of look- 
ing-glasses because it reflects so many waves. It not 
only sends back those which come from the sun, but 
those, too, which come from your face. So, when you 
see yourself in a looking-glass, the sun-waves have first 
played on your face and bounded off from it to the 
looking-glass; then, when they strike the looking- 
glass, they are thrown back again on to the retina of 
your eye, and you see your own face by means of the 
very waves you threw off from it an instant before. 
But the reflected light-waves do more for us than 
this. They not only make us see things, but they 
make us see them in different colours. What, you 
will ask, is this too the work of the sunbeams? Cer- 
tainly ; for if the colour we see depends on the size of 
the waves which come back to us, then we must see 
things coloured differently according to the waves they 
send back. For instance, imagine a sunbeam playing 
on a leaf : part of its waves bound straight back from 
it to our eye and make us see the surface of the leaf, 
but the rest go right into the leaf itself, and there 
some of them are used up and kept prisoners. The 
red, orange, yellow, blue, and violet waves are all 
useful to the leaf, and it does not let them go again. 
But it cannot absorb the green waves, and so it throws 
them back, and they travel to your eye and make you 
see a green colour. So when you say a leaf is green, 
