194 ELLIPTIC POLARIZATION. 



bodies may be distinguished into two classes, with respect to 

 their action upon reflected light. In some of them, as in 

 opal, the phase of the component in the plane of incidence is 

 accelerated, relatively to the other component ; in others, as' 

 hyalite, it is retarded. The bodies of these classes are deno- 

 minated, by M. Jamin, substances of positive and of negative 

 reflexion, respectively. Intermediate to these two classes we 

 should expect to find a third, characterized by the property 

 that the phase is unaltered by reflexion, and for which, there- 

 fore, Fresnel's laws are accurately true. This class is very 

 small, the only bodies observed to belong to it being menilite 

 and alum. 



These distinctions appear to be connected with the refrac- 

 tive power. Thus all bodies, whose refractive index is greater 

 than 1*46, accelerate the phase of vibration in the plane of in- 

 cidence ; those whose refractive index is less than 1'46, retard 

 it ; while those bodies, for which JJL = 1'46, reflect according 

 to Fresnel's laws. 



(206) The elliptical vibration of the reflected light will 

 be completely known, when we know the difference of phase 

 of the two principal components, and the ratio of their inten- 

 sities. The difference of phase is determined experimentally 

 by M. Jamin, by the process which restores the light to the 

 condition of plane polarization ; while the azimuth of the 

 plane of polarization of the restored ray gives the ratio of 

 the intensities of the two components. The results obtained 

 have been compared with the formulas given by M. Cauchy 

 for the case of diamond ; and the agreement has been found 

 to be satisfactory. These formulas involve two constants, 

 the refractive index, and the coefficient of ellipticity ; and these 

 are determined, when we know the principal incidence, and 

 the ratio of the amplitudes of the two vibrations at that 

 incidence. 



