178 SEVENTEENTH CENTURY. pt. iii. 



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 exactly 

 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 sensation of the 

 note you call the middle C. 



Eefraction 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. But 

 if the wave goes into the glass obliquely (see p. 47), one end 

 of it will reach the glass first before the other, and will move 

 slowly, while the other end goes on unchecked, and so the 

 wave will swing round and will have its direction altered. 

 In the same way, when it passes out again from the glass, 

 one end will pass out first, and will move more easily in the 

 air than the end that is still in the glass, and so it will swing 

 round again and make another bend. 



You must not be disappointed if you do not understand 

 this at once, for it is very difficult ; to make it easier we will 

 borrow a very ingenious illustration given last year (Jan. i, 

 1874) by Mr. E. B. Tylor, in a periodical called 'Nature.' 

 Take two small wheels about 2 inches round, and mount 

 them loosely upon a stout iron axle measuring about half-an- 

 inch round. This will make a runner like two wheels of a 

 cart, and if you let it roll down a smooth board it will repre- 

 sent very fairly the crests or tops of the waves of light in 



