192 DISCOVERIES OF MM. FRESNEL AND AIRY. SECT. XXII. 



duction of circular and elliptical polarization by the internal 

 reflection of light from plate-glass. He has shown that, if light 

 polarized by any of the usual methods be twice reflected within 

 a glass rhomb (N. 169) 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 super- 

 added 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 corkscrew, the extremity will move uniformly in a 

 circle, and every molecule throughout the cord will do the same 

 in succession. It appears, therefore, that both circular and ellip- 

 tical 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 veiy 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 



