UNfVER; 



CH. xxi. VARIOUS THEORIES OF LIGHT. 175 



received a very good education, and wrote some able treatises 

 upon geometry when he was only two-and-twenty. From 

 this time he advanced very rapidly, both writing valuable 

 papers and making grand discoveries. In 1658 he invented 

 a peculiar kind of pendulum called the cycloidal pendulum, 

 which would keep accurate time when swinging over wide 

 spaces ; and he was also the first to apply pendulums to 

 clocks. In 1659 he made a telescope ten feet long, with 

 which he discovered one of Saturn's satellites, and described 

 accurately Saturn's ring, which Galileo had mistaken for two 

 stars. In 1660 he came to England and solved some 

 questions which the Royal Society had proposed about 

 the laws of motion. Then he was invited to settle in France, 

 and it was there, in 1678, that he read before the ' Academic 

 des Sciences ' the theory of light which we must now try to 

 understand. 



Undulatory Theory of Light, 1678. We have seen 

 that it had been known ever since the time of the Greeks, 

 that sound is caused by a trembling or vibration of the air, 

 so that when you strike the wire of a harp, the trembling of 

 the string shakes the air, and the quivering motion travels 

 along until some wave strikes the drum of your ear and pro- 

 duces the sensation we call sound. 



Now Huyghens said, 'We can only explain light by sup- 

 posing it to be a vibration like sound.' But here at the 

 very outset came a difficulty. We know that light is not a 

 vibration of the air, for if you draw the air completely out oi 

 a glass vessel, light will still pass across it ; and besides, we 

 get light from the sun and the distant stars, so that it has to 

 come across a great airless space before it reaches the at- 

 mosphere of our earth. And yet, if light is a vibration, it is 

 clear there must be something between the sun and us to 

 vibrate. To meet this difficulty Huyghens supposed the 



