I 74 SEVENTEENTH CENTURY. FT. ill. 



light that we see everything, yet light in itself is invisible. 

 You will exclaim at once that you can see a sunbeam 

 shining through a crack in a window-shutter. But what you 

 see is not light itself, it is the particles of dust or smoke 

 which reflect the light so that they shine. There is one 

 very simple way of proving to yourself that rays of light are 

 not visible lines. When the moon is shining you know that 

 it is reflecting the light of the sun, therefore there must be 

 light crossing the sky and falling upon the surface of the 

 moon. But now look up some other night when the moon 

 is not there. All is darkness ; yet .the light must be there 

 just the same, and would have caused the moon to shine 

 if it had been there also, but as there is nothing to reflect 

 it to your eye it is invisible. 



What, then, is this light, invisible in itself, yet without 

 which we can see nothing? Newton thought that it was 

 composed of minute invisible particles of matter which darted 

 out in straight lines from luminous or light-giving bodies, 

 and falling upon our eyes caused the sensation which we 

 call light. This is called the Corpuscular, or sometimes the 

 Emission, Theory of Light. It was very ingenious, and 

 accounted for a great many of the facts, but there were 

 many others which it did not explain ; and I will not 

 attempt to describe it to you, because another theory, called 

 the Undiilatory (or wave) Theory of Light, lias now been 

 found to be much more complete and satisfactory. This 

 last theory was proposed by a Dutch mathematician and 

 astronomer named Christian Huyghens, the son of Constan- 

 tine 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 



