Q16 ELLIPTICAL POLARIZATION. [SECT. xxn. 



lating curve lying wholly in that plane. If to this motion 

 there be superadded another similar and equal, but perpen- 

 dicular to the first, the cord will assume the form of an ellip- 

 tical helix ; its extremity will describe an ellipse, and every 

 molecule throughout its length will successively do the same. 

 But, if the second system of vibrations commence exactly a 

 quarter of an undulation later than the first, the cord will 

 take the form of a circular helix or cork-screw, the extremity 

 will move uniformly in a circle, and every molecule through- 

 out the cord will do the same in succession. It appears, 

 therefore, that both circular and elliptical polarization may 

 be produced, by the composition of the motions of two rays 

 in which the particles of ether vibrate in planes at right 

 angles to one another. 



Professor Airy, in a very profound and able paper published 

 in the Cambridge Transactions, has proved that all the dif- 

 ferent kinds of polarized light are obtained from rock crystal. 

 When polarized light is transmitted through the axis of a 

 crystal of quartz, in the emergent ray the particles of ether 

 move in a circular helix ; and when it is transmitted obliquely 

 so as to form an angle with the axis of the prism, the particles 

 of ether move in an elliptical helix, the ellipticity increasing 

 with the obliquity of the incident ray ; so that, when the in- 

 cident ray falls perpendicularly to the axis, the particles of 

 ether move in a straight line. Thus quartz exhibits every 

 variety of elliptical polarization, even including the extreme 

 cases where the excentricity is zero, or equal to the greater 

 axis of the ellipse (N. 210). In many crystals the two rays 

 are so little separated, that it is only from the nature of the 

 transmitted light that they are known to have the property 

 of double refraction. M. Fresnel discovered, by experiments 

 on the properties of light passing through the axis of quartz, 

 that it consists of two superposed rays, moving with different 

 velocities ; and Professor Airy has shown, that in these two 

 rays the molecules of ether vibrate in similar ellipses at right 

 angles to each other, but in different directions ; that their 



