326 POPULAR LECTURES AND ADDRESSES. 



of the force required to move a tuning-fork 400 

 times a second, and compare that with the force 

 required for a motion of 400 million million times 

 a second. If the mass moved is the same, and 

 the range of motion is the same, then the force 

 would be one million million million million times 

 as great as the force required to move the prongs 

 of the tuning-fork it is as easy to understand 

 that number as any number like 2, 3, or 4. 

 Consider now what that number means and what 

 we are to infer from it. What force is there 

 in the space between my eye and that light? 

 What forces are there in the space between 

 our eyes and the sun, and our eyes and the 

 remotest visible star? There is matter and there 

 is motion, but what magnitude of force may 

 there be? 



I move through this " luminiferous ether " as 

 if it were nothing. But were there vibrations 

 with such frequency in a medium of steel or 

 brass, they would be measured by millions and 

 millions and millions of tons' action on a square 

 inch of matter. There are no such forces in our 



