SECT. XXII. ELLIPTICAL POLARIZATION. 187 



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 incident 

 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 eccentricity 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 refrac- 

 tion. 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 dif- 

 ferent directions ; that their ellipticity varies with the 

 angle which the incident ray makes with the axis ; and 

 that, by the composition of their motions, they produce 

 all the phenomena of polarized light observed in quartz. 



It appears from what has been said, that the mole- 

 cules of ether always perform their vibrations at right 

 angles to the direction of the ray, but very differently in 

 the various kinds of light. In natural light the vibrations 

 are rectilinear, and in every plane. In ordinary polar- 

 ized light they are rectilinear, but confined to one plane ; 

 in circular polarization the vibrations are circular ; and 

 in elliptical polarization the molecules vibrate in ellipses. 

 These vibrations are communicated from molecule to 

 molecule, in straight lines when they are rectilinear, in 

 a circular helix when they are circular, and in an oval 

 or elliptical helix when elliptical. 



Some fluids possess the property of circular polar- 

 ization, as oil of turpentine ; and elliptical polarization, 

 or something similar, seems to be produced by reflection 

 from metallic surfaces. 



The colored images from polarized light arise from 

 the interference of the rays (N. 211). MM. Fresnel 

 and Arago found that two rays of polarized light inter- 

 fere and produce colored fringes if they be polarized in 

 the same plane, but that they do not interfere when 



