SUNBEAMS AND THEIR WORK. 43 
that each one of them is sending these invisible mes- 
sengers 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 sec 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. In clear glass, for example, all 
the light-waves pass through without affecting the 
substance of the glass ; while in a white wall the 
larger part of the rays are reflected back to your eye, 
and 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 looking- 
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 your- 
self 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 
