SECT. XXII. ELLIPTICAL POLAKIZATION. 193 



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 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 excen- 

 tricity is zero, or equal to the greater axis of the ellipse (N. 215). 

 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 

 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 molecules 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 polarized 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 polarization natur- 

 ally, as oil of turpentine, the essential oils of laurel and lemon, 

 sugar of grapes, and various liquids. 



Elliptical polarization is produced by reflection from metallic 

 surfaces. Mr. Baden Powell discovered it also in the light re- 

 flected from China ink, chromate of lead, plumbago, &c. Mr. 

 Airy observed that the light reflected from the diamond is ellip- 

 tically polarized ; and Mr. Jamin has shown that this kind of 

 polarization is generally produced by reflection from almost all 

 transparent bodies, whatever their refractive power may be, 



K 



