CHAPTEE XI. 



ELLIPTIC POLARIZATION. 



(192) WHEN an ethereal molecule is displaced from its 

 position of equilibrium, the forces of the neighbouring mole- 

 cules are no longer balanced, and their resultant tends to drive 

 the particle back to its position of rest.* The displacement 

 being supposed to be very small, in comparison with the in- 

 tervals between the molecules, the force thus excited will be 

 proportional to the displacement ; and from this it follows, 

 according to known mechanical principles, that the trajectory 

 described by the molecule will be an ellipse, whose centre 

 coincides with the position of equilibrium. Hence the vibra- 

 tion of the ethereal molecules is, in general, elliptic, and the 

 nature of the light depends on the direction and relative mag- 

 nitude of the axes. By the principle of transversal vibrations, 

 these elliptic vibrations are all in the plane of the wave ; their 

 axes, however, may either preserve constantly the same direc- 

 tion in that plane, or they may be continually shifting. In 

 the former case, the light is said to be polarized ; in the lat- 

 ter, it is unpolarized, or common light. 



The relative magnitude of the axes of the ellipse deter- 

 mines the nature of the polarization. When the axes are 

 equal, the ellipse becomes a circle, and the light is said to be 

 circularly polarized. On the other hand, when the lesser axis 

 vanishes, the ellipse becomes a right line, and the light is 



* This is not strictly true, except in homogeneous or uncrystallized 

 media. 



