LIGHT THE ETHER IN ITSELF 43 



and experimenters have measured it in a direct manner, 

 and fixed it at 186,000 miles. 



Two theories have been advanced as to the manner in 

 which light is propagated. The first, the emission theory 

 of Newton, represented its rays as made up of infinitesi- 

 mally small particles emitted by shining bodies; these 

 sped forward as arrows from a bow, stones from a sling, 

 bullets from a rifle. The sun would on this theory be 

 continuously sending forth from his body innumerable 

 bright particles, which, reaching and entering the eye, 

 produce the sensation of light. This view, though long 

 maintained, has been discredited. Observed facts con- 

 tradict it. The second is called the undulatory or wave 

 theory. It regards light as flowing from the sun to the 

 earth by means of a medium thrown into a state of wave 

 agitation. These waves spread through the medium in all 

 directions, after the manner of those caused by a stone 

 flung into the water. 



It has been found necessary to assume the existence of 

 a medium different from ordinary ponderable matter, to 

 which the name of ether has been given. Sound is 

 propagated through the particles of matter, and without 

 their presence and action no impression is made on the 

 ear. A bell hung in a vacuum cannot be made to give 

 forth sound. The necessary motions may be produced in 

 the metal by means of a hammer attached to clockwork ; 

 but these find no means of transmission outward, and 

 therefore are not transmitted, are not conveyed to the 

 ear. Light, on the other hand, passes through a vacuum 

 with the greatest facility. It finds in it, after the air has 

 been withdrawn, a medium of its own ready for perfect 

 action. It also passes through space, hastening to us 

 from the most distant stars, through regions where 

 atmospheres cannot be said to exist. There must, there- 



