186 ELLIPTICAL POLARIZATION. SECT. XXII. 



be twice reflected within a glass rhomb (N. 1 G6) of a given 

 form, the vibrations of the ether that are perpendicular 

 to the plane of incidence will be retarded a quarter of a 

 vibration, which causes the vibrating particles to describe 

 circles, and the succession of such vibrating particles 

 throughout the extent of a wave to form altogether a 

 circular helix, or curve like a corkscrew. However, 

 that only happens when the plane of polarization is 

 inclined at an angle of 45 to the plane of incidence. 

 When these two planes form an angle either greater 

 or less, the succession of vibrating particles forms an 

 elliptical helix, which curve may be represented by 

 twisting a thread in a spiral about an oval rod. These 

 curves will turn to the right or left, according to the 

 position of the incident plane. 



The motion of the ethereal medium in elliptical and 

 circular polarization may be represented by the analogy 

 of a stretched cord ; for if the extremity of such a cord 

 be agitated at equal and regular intervals by a vibratory 

 motion entirely confined to one plane, the cord will be 

 thrown into an undulating curve lying wholly in that 

 plane. If to this motion there be superadded another 

 similar and equal, but perpendicular to the first, the 

 cord will assume the form of an elliptical helix ; its ex- 

 tremity 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 throughout the cord will do the same in suc- 

 cession. It appears, therefore, that both circular and 

 elliptical polarization may be produced, by the compo- 

 sition of the motions of two rays in which the particles 

 cf ether vibrate in places 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 different 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 



