LIGHT THE ETHER IN ITSELF 45 



around. The appearance is as if the water were advancing, 

 as if it were rushing along the surface ; but there is no 

 such advance. A piece of wood floating on it simply rises 

 and falls in its place, and shows very little motion forward, 

 as it would do if there were a real onrushing of the water. 

 When a molecule has ascended above, and descended to 

 the point from which it started, when it has thus made 

 a complete vibration, it has around it a complete wave, 

 consisting of a crest and a depression. The wave it is, 

 and not the water, that spreads and runs in every direction. 



There is the upward and downward motion of the 

 molecules, and the forward advance of that motion. 

 Wave motion may also be illustrated by a rope fastened 

 at one end and drawn sharply up and down at the other. 

 In such a case it is easily seen that it is the wave motion, 

 not the first part acted on, that runs along. According 

 to the undulatory theory, light is propagated in this 

 manner. Wave motions in the ether are created by the 

 sun or other luminous body, are borne along through the 

 former, and advance with extraordinary velocity, till 

 they reach the pupil of the eye, pass through, strike 

 the retina, run along the nerves to the brain, and on the 

 perceiving nature associated with it produce the impres- 

 sion of light. The motions producing the impression are 

 the transverse, or to-and-fro, up-and-down vibrations. 



The exact constitution of the ether, whether it consist 

 of minute particles or not, science has not certainly 

 determined. We can therefore only speak of portions 

 of it, and affirm that its action at points innumerable and 

 infinitesimally small can be separated in thought, and 

 spoken of very much in the same manner as if there were 

 separate and independent particles. "A finite time is 

 required to generate in a finite portion of it, a finite 

 velocity, by means of a finite force." It is therefore 



