CH. xxi. VARIOUS THEORIES OF LIGHT. 175 



the Undulatory (or Wave) Theory of Light ', has now been 

 found to be much more complete and satisfactory. This 

 last theory was first proposed by a Dutch mathematician 

 and astronomer named Christian Huyghens, the son of 

 Constantine Huyghens, Counsellor to the Prince of Orange. 



Christian Huyghens was born at the Hague, in Holland, 

 in the year 1629 ; when he was only thirteen years old he 

 was already passionately fond of mathematics, and ex- 

 amined every piece of machinery that fell in his way. He 

 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, ind 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. I must first tell 

 you that Newton, among his many other investigations, had 

 shown that sound is caused by a trembling or vibration of 

 the air. Thus, when you strike the wire of a harp, the 

 trembling of the string shakes the air, and the quivering 

 motion travels along like waves upon a pond, until some 

 wave strikes the drum of your ear and produces the sen- 



