174 SEVENTEENTH CENTURY. PT. HI. 



of 190,000,000 miles in 996 seconds, or about 190,000 

 miles in a second. This is seven million times as fast as the 

 quickest express train. 



Huyghens and Newton Theories of Light. The time 

 had now come when so much was known about the way in 

 which light behaves, that philosophers began to ask them- 

 selves, ' What is Light?' a question by no means so easily 

 answered as you may think ; for though it is by means of 

 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 are acted upon by 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 its surface. 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 



