Then, when they fell upon your eye, they entered it and excited
the retina and the nerves, and the image of the chair or the
table was carried to your brain. Look around at all the things
in this room. Is it not strange to think that each one of them
is sending these invisible messengers straight to your eye as you
look at it; and that you see me, and distinguish me from the
table, entirely by the kind of waves we each send to you?
Some substances send back hardly any waves of light, but let them
all pass through them, and thus we cannot see them. A pane of
clear glass, for instance, lets nearly all the light-waves pass
through it, and therefore you often cannot see that the glass is
there, because no light-messengers come back to you from it.
Thus people have sometimes walked up against a glass door and
broken it, not seeing it was there. Those substances are
transparent which, for some reason unknown to us, allow the ether
waves to pass through them without shaking the atoms of which the
substance is made.
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