14 PROPAGATION OF LIGHT. 



the string ; and there will be a succession of similar undula- 

 tions as long as the original disturbance continues. The 

 particles a, a', a", or the par- 

 ticles 6, 6 X , b", &c.,, are said 

 to be in similar phases of vi- 

 bration. The wave, or undulation, consists of all the par- 

 ticles between two which are in similar phases, as between 

 a and a 7 , or between b and b' ; and the length of a wave is the 

 distancev between them, estimated in the direction in which 

 the motion is propagated. It is evident from this description 

 that a wave, or undulation, comprises particle^ in^wry phase 

 of their vibration. 



Now, instead of a single string, let us suppose an infinite 

 number, all diverging from the same centre ; and let us sup- 

 pose that they are each made to undulate by a disturbing 

 action at that centre, acting in a similar manner, and in the 

 same degree, on all. It is obvious, then, that an undulation 

 will be propagated along all the strings ; and that these undu- 

 lations will be equal in magnitude, and will be propagated 

 with the same velocity, provided the strings be equal in ten- 

 sion, elasticity, and other respects. In this case, then, simi- 

 lar w r aves will be propagated to points equally distant from 

 the origin of disturbance in the same time ; and all the points 

 which are in a similar phase of vibration will be situated on 

 the surface of a sphere, of which that origin is the centre. 



In place of the actual strings which we have been consi- 

 dering, let us imagine rows of ethereal particles connected 

 by their mutual actions, and all that has been said will 

 apply to the propagation of light, the luminous body being 

 the source of disturbance. The length of the wave is the 

 distance, estimated in any direction from the centre, of two 

 particles which are in similar phases of vibration ; and it is 

 therefore the space through which the vibratory movement 

 is propagated in the time of a single vibration. Accordingly, 



