1 76 SEVENTEENTH CENTURY. PT. in. 



sation we call sound. The tighter and shorter the string is, 

 the more rapid the vibrations or waves will be, and the 

 more shrill will be the note which you hear. 



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 of 

 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 

 whole of space between our earth and the most distant stars to 

 be filled with an elastic invisible substance which he called 

 1 ether? He assumed this substance to be so fine and 

 subtle that it passes between the atoms, even of solid 

 objects, and that the sun and all luminous bodies cause it 

 to vibrate so that its undulations or waves strike upon our 

 eyes and give rise to the sensation we call light. 



Thus, according to this theory, when you look at the 

 sun, the invisible * ether ' filling the whole space between you 

 and it, is moving up and down in rapid vibrations, just as if 

 the sun held one end of a cloth and you the other, and the 

 sun was shaking the cloth so that the waves travelled along 

 it to your eye ; and every wave that hit you would cause 

 the sensation called light. 



This theory explains very well how light-waves may be 

 in the sky and yet we may not see them ; for if a stick 

 is moving rapidly to and fro in the air, and you go within 

 reach of it you feel pain, but if you keep out of reach no 

 pain is produced. In the same way, when the vibration of 



