CH. xxr. THE UNDULATORY THEORY. 177 



allow nearly all the waves of light to pass through them and 

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

 sequence been known to walk right up against glass doors 

 without seeing them. Bright polished surfaces, on the con- 

 trary, 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 

 absorbed as it is called, by the material of the leaf, and only 

 the green waves bound back upon your eye. In other words, 

 the vibrations of the ether coming from the leaf move ex- 

 actly fast enough to produce upon your eye the sensation 

 you call green, just as the vibration of the air caused by a 

 particular string of a harp produces on your ear the sensa- 

 tion of the note you call the middle C. 



Refraction of Light explained by Huyghens. But 

 we must now go back to Huyghens, and point out how beauti- 

 fully he explained by his undulatory theory the refraction or 

 bending-back of rays of which we have already spoken so 

 much. When a wave of light is travelling onwards, he said, 

 if it passes vertically into glass or any denser substance, the 

 wave will move more slowly, but it will still go straight on, 

 because both ends of the wave will be equally checked 



