18 The Theory of the Aether 



air propagates the vibrations of sound, but with far greater 

 velocity. 



This aether pervades the pores of all material bodies, and 

 is the cause of their cohesion ; its density varies from one body 

 to another, being greatest in the free interplanetary spaces. It 

 is not necessarily a single uniform substance : but just as air 

 contains aqueous vapour, so the aether may contain various 

 " aethereal spirits," adapted to produce the phenomena of 

 electricity, magnetism, and gravitation. 



The vibrations of the aether cannot, for the reasons already 

 mentioned, be supposed in themselves to constitute light. 

 Light is therefore taken to be " something of a different kind, 

 propagated from lucid bodies. They, that will, may suppose 

 it an aggregate of various peripatetic qualities. Others may 

 suppose it multitudes of unimaginable small and swift 

 corpuscles of various sizes, springing from shining bodies 

 at great distances one after another; but yet without any 

 sensible interval of time, and continually urged forward by a 

 principle of motion, which in the beginning accelerates them, 

 till the resistance of the aethereal medium equals the force of 

 that principle, much after the manner that bodies let fall in 

 water are accelerated till the resistance of the water equals the 

 force of gravity. But they, that like not this, may suppose 

 light any other corporeal emanation, or any impulse or motion 

 of any other medium or aethereal spirit diffused through the 

 main body of aether, or what else they can imagine proper for 

 this purpose. To avoid dispute, and make this hypothesis 

 general, let every man here take his fancy ; only whatever 

 light be, I suppose it consists of rays differing from one another 

 in contingent circumstances, as bigness, form, or vigour."* 



In any case, light and aether are capable of mutual inter- 

 action; aether is in fact the intermediary between light and 

 ponderable matter. When a ray of light meets a stratum of 

 aether denser or rarer than that through which it has lately 

 been passing, it is, in general, deflected from its rectilinear 

 * Royal Society, Dec. 9, 1675. 



