iy6 SEVENTEENTH CENTURY. PT. in. 



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

 be filled with an elastic invisible substance which he called 

 Aether' 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 traveller* 

 along it to your eye ; and every wave that hit ycr- 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 

 this invisible ether strikes your eye you feel light ^ but 

 though the waves may be travelling rapidly across the sky, 

 so long as they do not fall upon your eye, no light will be 

 produced to you. 



But suppose you were not looking at the sun, but at the 

 ground, why should you still see ? Because the waves from 

 the sun which strike the ground cannot travel on so easily 

 through the solid earth as through the pure ether, so a great 

 number of them bound off and vibrate back along the ether 

 again, from the ground to your eye ; and as they vibrate dif- 

 ferently according as the ground is rough or smooth, hard or 

 soft, wet or dry, they make a different impression upon your 

 eye, and cause you to see a picture of the ground as it is. 



Clear white glass and other perfectly transparent bodies 



