370 ON LIGHT. 



vibratory movement once set up and steadily maintained, 

 according to a regular law of periodicity in any one 

 molecule of such a fluid will, sooner or later (according 

 to its distance and situation), reach every other, wliich 

 from that moment' will be agitated by a vibratory move- 

 ment precisely similar in its phases (though of inferior 

 extent in its excursions) to the original movement, and 

 performed m the same period of time. It matters not 

 whether the medium be or be not equally elastic in all 

 directions. This will affect the rate of progress of a wave 

 through it in different directions, and by consequence 

 the form of the wave, and the length of time that has to 

 elapse before the molecule in question begins its vibra- 

 tory movement; but once set up in any molecule, that 

 movement will be maintained, so long as it is, so to 

 speak, fed from behind so long as successive waves 

 continue to pass through it. In the theory of light, the 

 eye being insensible to vibratory movements in the di- 

 rection of the ray, we have only to consider those com- 

 ponents of the motion which lie in a plane at right 

 angles to that direction, and which, for the present, we 

 will suppose to be that of the paper before us. 



(149.) Let us consider, then, the kind of motion which 

 an ethereal molecule will assume, under the influence of 

 two such vibratory movements simultaneously affecting 

 it, in directions transverse or otherwise inclined to each 

 other, but both directions lying in one common plane, 

 that of the paper, or of the wave-surface; each of which 

 will therefore represent the vibratory movement proper 

 to a ray polarized in a plane at right angles to our paper, 



