PROPAGATION OF LIGHT. 



the path of a projectile in the air ; or about which we reason, 

 when we determine the course of a planet in its orbit. Mo- 

 tions of the latter kind, too, are everywhere taking place 

 around us. When the surface of stagnant water is agitated 

 by any external cause, the particles of the fluid next the origin 

 of the disturbance are set vibrating up and down, and this 

 vibratory motion is communicated to the adjacent particles, 

 and from them onwards, to the boundaries of the fluid surface. 

 All the particles which are elevated at the same instant con- 

 stitute what is called a wave ; and that this wave does not 

 consist of the same particles in two successive instants may be 

 seen in the movements of any floating body, which will be 

 observed to rise and fall as it is reached and passed by the 

 wave, but not to advance, as it must necessarily do if the 

 particles of the fluid on which it rested had a progressive 

 motion. The phenomena of sound afford another well-known 

 instance of the motion of vibration. The vibratory motion 

 is communicated from the sounding body to the ear, through 

 all the intervening particles of the air, though each of the 

 aerial particles moves back and forwards through a very nar- 

 row space. 



Each of these modes of propagated motion has been ap- 

 plied to explain the phenomena of light ; and hence the two 

 rival theories the theory of emission and the wave-theory. 

 In the former the luminous body is supposed to send forth, 

 or emit, continually, material particles of extreme minuteness, 

 in all directions. In the latter, the same body is supposed 

 to excite the vibrations of an elastic ether, which are commu- 

 nicated from particle to particle, to its remotest bounds. This 

 ethereal medium is supposed to pervade all space, and to be 

 of such extreme tenuity as to afford no appreciable resistance 

 to the motions of the planets. 



Such are the two systems, some traces of which may 

 be found even in the recorded opinions of the ancients. 



