SECT, xxii.] ELLIPTICAL POLARIZATION. 217 



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 rectili- 

 near, but confined to one plane ; in circular polarization the 

 vibrations are circular ; and in elliptical polarization the 

 molecules vibrate in ellipses. These vibrations are commu- 

 nicated from molecule to molecule, in straight lines when 

 they are rectilinear, in a circular helix when they are cir- 

 cular, and in an oval or elliptical helix when elliptical. 



Some fluids possess the property of circular polarization, 

 as oil of turpentine ; and elliptical polarization, or something 

 similar, seems to be produced by reflection from metallic 

 surfaces. 



The coloured images from polarized light arise from the 

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

 found that two rays of polarized light interfere and produce 

 coloured fringes if they be polarized in the same plane, but 

 that they do not interfere when polarized in different planes. 

 In all intermediate positions, fringes of intermediate bright- 

 ness are produced. The analogy of a stretched cord will 

 show how this happens. Suppose the cord to be moved 

 backwards and forwards horizontally at equal intervals ; it 

 will be thrown into an undulating curve lying all in one plane. 

 If to this motion there be superadded another similar and 

 equal, commencing exactly half an undulation later than the 

 first, it is evident that the direct motion every molecule will 

 assume, in consequence of the first system of waves, will at 

 every instant be exactly neutralized by the retrograde motion 

 it would take in virtue of the second ; and the cord itself will 

 be quiescent in consequence of the interference. But, if the 



