SUNBEAMS AND THEIR WORK. 43 
When the room was dark you could not distinguish 
the table, the chairs, or even the walls of the room. 
Why? Because they had no light-waves to send to 
your eye. But as the sunbeams began to pour in at 
the window, the waves played upon the things in the 
room, and when they hit them they bounded off them 
back to your eye, as a wave of the sea bounds back 
from a rock and strikes against a passing boat. Then, 
when they fell upon your eye, they entered it and ex- 
cited 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. 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 
