274 ON LIGHT. 



ently great to destroy the whole velocity of the luminous 

 particle, and to generate an equal one in the opposite 

 direction, in the time occupied by the particle in travers- 

 ing forwards and backwards the thickness of our stratum 

 of reflecting force. Now the velocity of light, as we have 

 seen, is 186,000 miles per second. To destroy and re- 

 produce this velocity iri a projectile shot directly upwards, 

 by the force of gravity on the earth, supposed uniform or 

 undiminished by distance, would require its action to bo 

 continued for 706 days, or very nearly two years, while 

 the same effect has to be produced by the reflecting force 

 (also supposed uniform), in that inappretiable instant of 

 time in which the act of reflection is performed a tinvj 

 which would be extravagantly overrated at the billionth* 

 part of a second. After this we need hardly trouble our 

 readers with any estimation of the intensity of the re- 

 fracting forces. The sturdiest philosophy may fairly 

 be staggered at such a postulate as the foundation of a 

 physical theory. 



(58.) According to the "undulatory theory" light con- 

 sists in an undulatory or vibratory movement propagated 

 through an elastic medium pervading all space, not even 

 excepting what is occupied, or seems to be occupied, by 

 what we call material bodies that is, such as have ?*' 

 and which, to us, constitute the visible and tangible uni- 

 verse of things. It therefore resembles sound, which is 

 not a travelling entity, but a propagated motion in the air, 

 analogous to the tremulous movement which runs from 



* A billion is a million times a million. The French milliard is 

 a thor.sand millions. 



