ELLIPTIC POLAKIZATION. 193 



polarization, while they differ 180 in phase at all incidences 

 below it. According to this theory,- therefore, the difference 

 of phase changes abruptly, from TT to 2?r, at that critical inci- 

 dence. On the other hand, in reflexion from metals, the 

 difference of phase of the two components increases continuously 

 from TT to 2?r, as the incidence increases from to 90. 



Now M. Jamin has shown that the latter is generally 

 true for all bodies, whether opaque or transparent ; and that 

 the distinction of these bodies, as to their effects upon re- 

 flected light, consists only in this, that in transparent bodies 

 the variation of phase is insensible, except in the neighbour- 

 hood of the angle of maximum polarization. 



In transparent substances, accordingly, the difference of 

 phase is nearly constant, at low and at high incidences; and 

 passes from TT to 2-rr (not abruptly, as we are required to sup- 

 pose in Fresnel's theory, but) between two incidences, one 

 lower and the other higher than the angle of maximum pola- 

 rization. The elliptic polarization of the reflected light will 

 be sensible only within the same limits of incidence ; and 

 beyond them the light is (as to sense) plane-polarized. In 

 substances of low refractive power, these limiting incidences 

 differ from one another, and from the angle of maximum po- 

 larization, by a small amount ; and for these, therefore, the 

 change of phase (although not instantaneous) is very rapid, 

 and Fresnel's laws are approximately true. 



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When the difference of phase = T TT, the ellipticity of the 



reflected ray is greatest. The angle of incidence at which v 

 this occurs is the angle of maximum polarization in the case 

 of common light, and is called the principal incidence. It is 

 theoretically different from the angle given by Brewster's 

 law ; but the difference is in all cases small. 



(205) M. Jamin has shown, further, that transparent 



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