CH. xxi. THE UNDULATORY THEORY. 177 



this invisible ether strikes your eye you feel light, but 

 though the waves may be travelling rapidly across the sky, 

 so long as they do not fall upon your eye, no light will be 

 produced to you. 



But suppose you were not looking at the sun, but at the 

 ground, why should you still see ? Because the waves from 

 the sun which strike the ground cannot travel on so easily 

 through the solid earth as through the pure ether, so a great 

 number of them bound off and vibrate back along the ether 

 again, from the ground to your eye ; and as they vibrate dif- 

 ferently according as the ground is rough or smooth, hard or 

 soft, wet or dry, they make a different impression upon your 

 eye, and cause you to see a picture of the ground as it is. 



Clear white glass and other perfectly transparent bodies 

 let nearly all the waves of light pass through them and send 

 hardly any back to your eye ; and people have in conse- 

 quence been known to walk right up against glass doors with- 

 out seeing them. Bright polished surfaces, on the contrary, 

 like steel and mercury, turn nearly all the waves back again, 

 and this is why we see our own faces reflected so clearly in 

 a looking-glass, where it is the mercury at the back which is 

 the real mirror. 



If we had room we might follow out these light-vibrations 

 in a very interesting manner. For instance, why does a 

 leaf look green and a soldier's coat red? Because, as in 

 sound the kind of note you hear depends upon the quick- 

 ness of the vibrations of the air, so in light it depends upon 

 the quickness of the vibrations of the ether what colour you 

 see. The vibrations which produce violet, indigo, blue, 

 green, yellow, orange, and red, have travelled all together as 

 white light through the ether, but they are differently treated 

 by the leaf. All except the green waves are quenched, or 



