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? Certainly; 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, you mean that the leaf does not want the green waves of
the sunbeam, but sends them back to you. In the same way the
scarlet geranium rejects the red waves; this table sends back
brown waves; a white tablecloth sends back nearly the whole of
the waves, and a black coat scarcely any.
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